Santander UK (SANS+) 20-F 4 Mar 21 2020 FY Annual report (foreign) Financial data

Cover Page

Cover Page	12 Months Ended
	Dec. 31, 2020shares
Document Information [Line Items]
Document Type	20-F
Document Registration Statement	false
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Current Fiscal Year End Date	--12-31
Document Transition Report	false
Document Shell Company Report	false
Entity File Number	001-14928
Entity Registrant Name	Santander UK plc
Entity Incorporation, State or Country Code	X0
Entity Address, Address Line One	2 Triton Square
Entity Address, Address Line Two	Regent’s Place
Entity Address, City or Town	London
Entity Address, Postal Zip Code	NW1 3AN
Entity Address, Country	GB
Title Of 15(d) Security	7.95% Term Subordinated Securities due October 26, 2029
Entity Common Stock, Shares Outstanding	31,051,768,866
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Emerging Growth Company	false
Entity Shell Company	false
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Central Index Key	0001087711
Document Accounting Standard	International Financial Reporting Standards
Entity Filer Category	Non-accelerated Filer
Business Contact
Document Information [Line Items]
Contact Personnel Name	Julian Curtis
Entity Address, Address Line One	2 Triton Square
Entity Address, Address Line Two	Regent’s Place
Entity Address, City or Town	London
Entity Address, Postal Zip Code	NW1 3AN
Entity Address, Country	GB
City Area Code	+44
Local Phone Number	20 7756 4272
Contact Personnel Email Address	julian.curtis@santander.co.uk
2.500% Notes due January 5, 2021, issued by Santander UK plc
Document Information [Line Items]
Title of 12(b) Security	2.500% Notes due January 5, 2021, issued by Santander UK plc
Trading Symbol	SAN/21B
Security Exchange Name	NYSE
3.400% Notes due June 1, 2021, issued by Santander UK plc
Document Information [Line Items]
Title of 12(b) Security	3.400% Notes due June 1, 2021, issued by Santander UK plc
Trading Symbol	SAN/21C
Security Exchange Name	NYSE
Floating Rate Notes due June 1, 2021, issued by Santander UK plc
Document Information [Line Items]
Title of 12(b) Security	Floating Rate Notes due June 1, 2021, issued by Santander UK plc
Trading Symbol	SAN/21D
Security Exchange Name	NYSE
3.750% Notes due 2021 due November 15, 2021, issued by Santander UK plc
Document Information [Line Items]
Title of 12(b) Security	3.750% Notes due 2021 due November 15, 2021, issued by Santander UK plc
Trading Symbol	SAN/21F
Security Exchange Name	NYSE
Floating Rate Notes due November 15, 2021, issued by Santander UK plc
Document Information [Line Items]
Title of 12(b) Security	Floating Rate Notes due November 15, 2021, issued by Santander UK plc
Trading Symbol	SAN/21G
Security Exchange Name	NYSE
2.100% Notes due January 13, 2023, issued by Santander UK plc
Document Information [Line Items]
Title of 12(b) Security	2.100% Notes due January 13, 2023, issued by Santander UK plc
Trading Symbol	SAN/23B
Security Exchange Name	NYSE
4.000% Notes due March 13, 2024, issued by Abbey National Treasury Services plc
Document Information [Line Items]
Title of 12(b) Security	4.000% Notes due March 13, 2024, issued by Abbey National Treasury Services plc *
Trading Symbol	SAN/24
Security Exchange Name	NYSE
2.875% Notes due June 18, 2024, issued by Santander UK plc
Document Information [Line Items]
Title of 12(b) Security	2.875% Notes due June 18, 2024, issued by Santander UK plc
Trading Symbol	SAN/24D
Security Exchange Name	NYSE
10 3/8% Non-Cumulative Preference Shares
Document Information [Line Items]
Entity Common Stock, Shares Outstanding	200,000,000
8 5/8% Non-Cumulative Preference Shares
Document Information [Line Items]
Entity Common Stock, Shares Outstanding	125,000,000

Condensed Consolidated Income S

Condensed Consolidated Income Statement - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Profit (loss) [abstract]
Interest and similar income	£ 5,105	£ 5,917	£ 6,066
Interest expense and similar charges	(1,662)	(2,625)	(2,463)
Net interest income	3,443	3,292	3,603
Fee and commission income	756	1,112	1,170
Fee and commission expense	(371)	(426)	(421)
Net fee and commission income	385	686	749
Other operating income (expense)	147	195	182
Total operating income	3,975	4,173	4,534
Operating expenses before credit impairment losses, provisions and charges	(2,452)	(2,499)	(2,579)
Credit impairment losses	(645)	(221)	(153)
Provisions for other liabilities and charges	(273)	(441)	(257)
Total operating credit impairment losses, provisions and charges	(918)	(662)	(410)
Profit before tax	605	1,012	1,545
Tax on profit	(134)	(279)	(399)
Profit after tax	471	733	1,146
Attributable to:
Equity holders of the parent	452	714	1,124
Non-controlling interests	19	19	22
Profit after tax	£ 471	£ 733	£ 1,146

Condensed Consolidated Statemen

Condensed Consolidated Statement of Comprehensive Income - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of comprehensive income [abstract]
Profit after tax	£ 471	£ 733	£ 1,146
Movement in fair value reserve (debt instruments):
Change in fair value	114	147	(74)
Income statement transfers	(107)	(147)	21
Taxation	(2)	0	13
Other comprehensive income fair value reserve (debt instruments)	5	0	(40)
Cash flow hedges:
Effective portion of changes in fair value	971	(857)	793
Income statement transfers	(809)	1,013	(752)
Taxation	(52)	(41)	(13)
Other comprehensive income cash flow hedges, total	110	115	28
Currency translation on foreign operations	0	(4)	0
Net other comprehensive income that may be reclassified to profit or loss subsequently	115	111	(12)
Pension remeasurement:
Change in fair value	(505)	(522)	470
Taxation	133	131	(118)
Pension remeasurement, total	(372)	(391)	352
Own credit adjustment:
Change in fair value	(3)	(77)	84
Taxation	0	19	(21)
Own credit adjustment, total	(3)	(58)	63
Net other comprehensive expense that will not be reclassified to profit or loss subsequently	(375)	(449)	415
Total other comprehensive income/(expense) net of tax	(260)	(338)	403
Total comprehensive income	211	395	1,549
Attributable to:
Equity holders of the parent	194	374	1,528
Non-controlling interests	17	21	21
Total comprehensive income	£ 211	£ 395	£ 1,549

Condensed Consolidated Balance

Condensed Consolidated Balance Sheet - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets
Cash and balances at central banks	£ 41,250	£ 21,180
Financial assets at fair value through profit or loss:
Derivative financial instruments	3,406	3,316
Other financial assets at fair value through profit or loss	208	386
Financial assets at amortised cost:
Loans and advances to customers	208,750	207,287
Loans and advances to banks	1,682	1,855
Reverse repurchase agreements – non trading	19,599	23,636
Other financial assets at amortised cost	1,163	7,056
Financial assets at fair value through other comprehensive income	8,950	9,747
Interests in other entities	172	117
Intangible assets	1,646	1,766
Property, plant and equipment	1,734	1,967
Current tax assets	264	200
Retirement benefit assets	495	669
Other assets	3,013	2,520
Total assets	292,332	281,702
Financial liabilities at fair value through profit or loss:
Derivative financial instruments	1,584	1,448
Other financial liabilities at fair value through profit or loss	1,434	1,713
Financial liabilities at amortised cost:
Deposits by customers	195,135	181,883
Deposits by banks	20,958	14,353
Repurchase agreements – non trading	15,848	18,286
Debt securities in issue	35,566	41,129
Subordinated liabilities	2,556	3,528
Other liabilities	2,337	2,344
Provisions	464	572
Deferred tax liabilities	111	149
Retirement benefit obligations	403	280
Total liabilities	276,396	265,685
Equity
Share capital	3,105	3,105
Share premium	5,620	5,620
Other equity instruments	2,191	2,191
Retained earnings	4,348	4,546
Other reserves	510	395
Total shareholders’ equity	15,774	15,857
Non-controlling interests	162	160
Total equity	15,936	16,017
Total liabilities and equity	£ 292,332	£ 281,702

Condensed Consolidated Cash Flo

Condensed Consolidated Cash Flow Statement - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Profit after tax	£ 471	£ 733	£ 1,146
Adjustments for:
– Depreciation and amortisation	562	543	375
– Provisions for other liabilities and charges	273	441	257
– Impairment losses	672	239	189
– Corporation tax charge	134	279	399
– Other non-cash items	(267)	(439)	238
– Pension charge/(credit) for defined benefit pension schemes	38	35	79
Adjustments to reconcile profit (loss)	1,412	1,098	1,537
Net change in operating assets and liabilities:
– Cash and balances at central banks	(147)	(71)	(255)
– Trading assets	0	0	24,528
– Derivative assets	(90)	1,943	14,683
– Other financial assets at fair value through profit or loss	1,603	1,664	(3,635)
– Loans and advances to banks and customers	(2,654)	170	(9,129)
– Other assets	(340)	247	(246)
– Deposits by banks and customers	19,977	641	926
– Derivative liabilities	136	79	(16,244)
– Trading liabilities	0	0	(31,101)
– Other financial liabilities at fair value through profit or loss	(1,618)	(959)	4,106
– Debt securities in issue	1,201	(529)	(2,524)
Other liabilities	(966)	(568)	(556)
Net change in operating assets and liabilities	17,102	2,617	(19,447)
Corporation taxes paid	(159)	(292)	(391)
Effects of exchange rate differences	410	(1,079)	1,750
Net cash flows from operating activities	19,236	3,077	(15,405)
Cash flows from investing activities
Investments in other entities	0	0	(66)
Proceeds from disposal of subsidiaries	0	0	348
Purchase of property, plant and equipment and intangible assets	(373)	(505)	(696)
Proceeds from sale of property, plant and equipment and intangible assets	166	108	26
Purchase of financial assets at amortised cost and financial assets at fair value through other comprehensive income	(3,015)	(5,013)	(7,002)
Proceeds from sale and redemption of financial assets at amortised cost and financial assets at fair value through other comprehensive income	9,858	8,300	3,708
Net cash flows from investing activities	6,636	2,890	(3,682)
Cash flows from financing activities
Issue of other equity instruments	0	500	0
Issue of debt securities and subordinated notes	4,190	4,145	10,642
Issuance costs of debt securities and subordinated notes	(13)	(15)	(23)
Repayment of debt securities and subordinated notes	(12,037)	(7,969)	(6,281)
Repurchase of preference shares and other equity instruments	0	(318)	(290)
Dividends paid on ordinary shares	(129)	(315)	(1,139)
Dividends paid on preference shares and other equity instruments	(148)	(142)	(157)
Dividends paid on non-controlling interests	(15)	(12)	(22)
Net cash flows from financing activities	(8,152)	(4,126)	2,730
Change in cash and cash equivalents	17,720	1,841	(16,357)
Cash and cash equivalents at beginning of the year	27,817	26,029	42,226
Effects of exchange rate changes on cash and cash equivalents	45	(53)	160
Cash and cash equivalents at the end of the year	45,582	27,817	26,029
Cash and cash equivalents consist of:
Cash and balances at central banks	41,250	21,180	19,747
Less: regulatory minimum cash balances	(854)	(707)	(636)
Cash and bank balances at central banks less regulatory minimum cash balances	40,396	20,473	19,111
Other cash equivalents	5,186	7,344	6,918
Cash and cash equivalents at the end of the year	£ 45,582	£ 27,817	£ 26,029

Condensed Consolidated Cash F_2

Condensed Consolidated Cash Flow Statement (Parenthetical) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Statement [Line Items]
Cash consideration received on sale of subsidiaries	£ 0	£ 0
Cash outflow for leases	48	58
Cash Outflow For Leases, Principal Amount, Classified As Operating Activities	£ 45	£ 54

Condensed Consolidated Statem_2

Condensed Consolidated Statement of Changes in Equity - GBP (£) £ in Millions	Total	Share capital	Share premium	Other equity instruments	Available for sale	Fair value	Cash flow hedging	Currency translation	Retained earnings	Total	Non-controlling interests
Beginning balance at Dec. 31, 2017	£ 16,205	£ 3,119	£ 5,620	£ 2,281	£ 68		£ 228	£ 5	£ 4,732	£ 16,053	£ 152
Adjustment For Adoption Of IFRS9	(192)				(68)	63			(187)	(192)
Ending balance at Jan. 01, 2018	16,013	3,119	5,620	2,281	0	63	228	5	4,545	15,861	152
Beginning balance at Dec. 31, 2017	16,205	3,119	5,620	2,281	68		228	5	4,732	16,053	152
Profit for the year	1,146								1,124	1,124	22
Other comprehensive income, net of tax:
- Fair value reserve (debt instruments)	(40)					(40)				(40)
- Cash flow hedges	28						28			28
- Pension remeasurement	352								353	353	(1)
- Own credit adjustment	63								63	63
Total comprehensive income	1,549					(40)	28		1,540	1,528	21
Other	(45)								(45)	(45)
Repurchase of other equity instruments	(290)			(290)						(290)
Dividends on ordinary shares	(1,139)								(1,139)	(1,139)
Dividends on preference shares and other equity instruments	(157)								(157)	(157)
Dividends on non-controlling interests	(22)										(22)
Ending balance at Dec. 31, 2018	15,909	3,119	5,620	1,991		23	256	5	4,744	15,758	151
Profit for the year	733								714	714	19
Other comprehensive income, net of tax:
- Fair value reserve (debt instruments)	0
- Cash flow hedges	115						115			115
- Pension remeasurement	(391)								(393)	(393)	2
- Own credit adjustment	(58)								(58)	(58)
- Currency translation on foreign operations	(4)							(4)		(4)
Total comprehensive income	395					0	115	(4)	263	374	21
Issue of other equity instruments	500			500						500
Repurchase of other equity instruments	318	14		300					4	318
Dividends on ordinary shares	(315)								(315)	(315)
Dividends on preference shares and other equity instruments	(142)								(142)	(142)
Dividends on non-controlling interests	(12)										(12)
Ending balance at Dec. 31, 2019	16,017	3,105	5,620	2,191		23	371	1	4,546	15,857	160
Profit for the year	471								452	452	19
Other comprehensive income, net of tax:
- Fair value reserve (debt instruments)	5					5				5
- Cash flow hedges	110						110			110
- Pension remeasurement	(372)								(370)	(370)	(2)
- Own credit adjustment	(3)								(3)	(3)
- Currency translation on foreign operations	0							0		0
Total comprehensive income	211					5	110	0	79	194	17
Dividends on ordinary shares	(129)								(129)	(129)
Dividends on preference shares and other equity instruments	(148)								(148)	(148)
Dividends on non-controlling interests	(15)	0	0	0		0	0	0	0	0	(15)
Ending balance at Dec. 31, 2020	£ 15,936	£ 3,105	£ 5,620	£ 2,191		£ 28	£ 481	£ 1	£ 4,348	£ 15,774	£ 162

Consolidated Statement of Chang

Consolidated Statement of Changes in Equity (Parenthetical) £ in Millions	Jan. 01, 2018GBP (£)
Statement [Line Items]
Adoption of IFRS 9	£ 192
Decrease in financial assets arising from change in measurement attribute, initial application of IFRS 9	49
Change in financial assets from application of ECL impairment methodology	211
Recognition of deferred tax asset from application of IFRS 9	£ 68

Capital risk

Capital risk	12 Months Ended
	Dec. 31, 2020
Text block [abstract]
Capital risk	Capital risk Overview Capital risk is the risk that we do not have an adequate amount or quality of capital to meet our internal business needs, regulatory requirements and market expectations. In this section, we set out how we are regulated. We explain how we manage capital on a standalone basis as a subsidiary in the Banco Santander group. We then analyse our capital resources and key capital ratios including our RWAs. Key metrics CET1 capital ratio of 15.4% (2019: 14.3%) Total qualifying regulatory capital of £15.2bn (2019: £15.8bn) THE SCOPE OF OUR CAPITAL ADEQUACY Regulatory supervision For capital purposes, we are subject to prudential supervision by the PRA, as a UK banking group, and by the European Central Bank (ECB) as part of the Banco Santander group. The ECB supervises Banco Santander as part of the Single Supervisory Mechanism (SSM). Although we are part of the Banco Santander group, we do not have a guarantee from our ultimate parent Banco Santander SA and we operate as a standalone subsidiary. As we are part of the UK sub-group that is regulated by the PRA, we have to meet the PRA capital requirements on a standalone basis. We also have to show the PRA that we can withstand capital stress tests without the support of our parent. Reinforcing our corporate governance framework, the PRA exercises oversight through its rules and regulations on the Board and senior management appointments. Santander UK Group Holdings plc is the holding company of Santander UK plc and is the head of the Santander UK group for regulatory capital and leverage purposes. Santander UK plc is the head of the ring-fenced bank sub-group and is subject to regulatory capital and leverage rules in relation to that sub-group. Our basis of consolidation for our capital disclosures is substantially the same as for our Consolidated Financial Statements. CAPITAL RISK MANAGEMENT The Board is responsible for capital management strategy and policy and ensuring that we monitor and control our capital resources within regulatory and internal limits. We manage our funding and maintain capital adequacy on a standalone basis. We operate within the capital risk framework and appetite approved by our Board. This reflects the business environment we operate in, our strategy for each material risk and the potential impact of any adverse scenarios or stresses on our capital position. Management of capital requirements (audited) Our capital risk appetite aims to maintain capital levels appropriate to the level of stress applied, and the expected regulatory response. In: – An adverse economic stress, which we might expect to occur once in 20 years, the firm should remain profitable and exceed all regulatory capital minimums at all times. – A very severe economic stress, which we might expect to occur once in 100 years, and which has been designed to test any specific weaknesses of a firm’s business model, the firm should meet all regulatory capital minimums at all times. This is subject to the use of regulatory buffers designed to absorb losses in such a stress. Management of capital resources (audited) We use a mix of regulatory and EC ratios and limits, internal buffers and restrictions to manage our capital resources. We also take account of the costs of differing capital instruments and capital management techniques. We also use these to shape the best structure for our capital needs. We decide how to allocate our capital resources as part of our strategic planning process. We base this in part on the relative returns on capital using both EC and regulatory capital measures. We plan for severe stresses and we set out what action we would take if an extremely severe stress threatened our viability and solvency. This could include not paying dividends, selling assets, reducing our business and issuing more capital. Risk measurement We apply Banco Santander’s approach to capital measurement and risk management for CRD IV. Santander UK plc is classified as a significant subsidiary of Banco Santander SA. Key metrics The main metrics we use to measure capital risk are CET1 capital ratio and total capital ratio. We continue to be in excess of overall capital requirements, minimum leverage requirements and minimum requirements for own funds and eligible liabilities (MREL). Stress testing Each year we create a capital plan, as part of our ICAAP. We share our ICAAP with the PRA. The PRA then tells us how much capital (Pillar 2A), and of what quality, it thinks we should hold on top of our Pillar 1 requirements and buffer levels. We also develop a series of economic scenarios to stress test our capital needs and confirm that we have enough regulatory capital to meet our projected and stressed capital needs and to meet our obligations as they fall due. We augment our regulatory minimum capital with internal buffers. We hold buffers to ensure we have enough time to take action against unexpected movements. Risk mitigation We have designed our capital risk framework, policies and procedures to ensure that we operate within our Risk Appetite. We manage capital transferability between our subsidiaries in line with our business strategy, our risk and capital management policies, and UK laws and regulations. There are no legal restrictions on us moving capital resources promptly, or repaying liabilities, between the Company and its subsidiaries except for distributions between Santander UK entities in the ring-fenced bank sub-group and Santander UK entities that are not members of the ring-fenced bank sub-group, where the PRA is required to assess the impact of proposed distribution prior to payment. For details on our Recovery framework in the event of a capital stress, see the risk mitigation section in the ‘Liquidity risk’ section. Santander UK plc, Cater Allen Limited and certain other non-regulated subsidiaries within the ring-fenced bank entered into a capital support deed dated 13 November 2018 (the RFB Sub-Group Capital Support Deed). The parties to the RFB Sub-Group Capital Support Deed are permitted by the PRA to form a core UK group, as defined in the PRA Rulebook, a permission which will expire on 31 December 2021. Exposures of each of the regulated entities to other members of the core UK group are exempt from large exposure limits that would otherwise apply. The purpose of the RFB Sub-Group Capital Support Deed is to facilitate the prompt transfer of available capital resources from, or repayment of liabilities by, the non-regulated parties to any of the regulated parties in the event that one of the regulated parties breaches or is at risk of breaching its capital resources requirements or risk concentrations requirements. Risk monitoring and reporting We monitor and report regularly against our capital plan. We do this to identify any change in our business performance that might affect our capital. Each month, we also review the economic assumptions we use to create and stress test our capital plan. We do this to identify any potential reduction in our capital. Impact of IFRS 9 on regulatory capital We expect ECL-based provisions continue to be more volatile than our IAS 39 incurred loss-based provisions used prior to 2018 as our ECL methodology takes account of forward-looking data and covers a range of possible economic outcomes. This is likely to impact our CET1 capital levels, and result in increased pro-cyclicality of risk-based capital and leverage ratios. However, the impact is currently mitigated by our surplus of ECL over provisions for exposures using the IRB approach. For such exposures (which include residential mortgages) the adverse impact on CET1 capital of provision increases from reserve movements is offset by the related reduction of the negative CET1 capital adjustment for regulatory expected loss amounts. Furthermore, the EU transition arrangements for the capital impact of IFRS 9 mean that adverse CET1 effects from increases in ECL-based provisions from the level of such provisions at 1 January 2018 are partly reduced until the end of 2024. We reflect projections of ECL provisions in our capital position forecasting under base case and stress scenarios for ICAAP and capital management purposes. We also consider the dynamics of ECL in how we assess, monitor and manage capital risk. The greater volatility from IFRS 9 ECL could result in material favourable and unfavourable swings to our Income Statement. Whilst the initial impacts of IFRS 9 were based on estimates prepared in a supportive economic environment, a period of economic instability (such as was seen in 2020 due to the impacts of the Covid-19 pandemic) could significantly impact our results and our financial assets. It could also impact the amount of capital we have to hold. We take into account the volatility of ECL in our capital planning strategy. Meeting evolving capital requirements We target a CET1 management buffer of sufficient size to absorb volatility in CET1 deductions, capital supply and capital demand whilst remaining above the regulatory CET1 requirement. Distribution restrictions would be expected to be applied if we were unable to meet both our minimum requirement, which consists of the Pillar 1 minimum plus Pillar 2A, the CRD IV buffers consisting of the Capital Conservation Buffer (CCB), the Countercyclical Capital Buffer (CCyB) ,and from 28 December 2020 the Other Systemically Important Institutions Buffer (O-SII) (previously the Systemic Risk Buffer (SRB)). The Bank of England's Financial Policy Committee (FPC) reduced UK CCyB to 0% in March 2020 in response to the Covid-19 pandemic, and the FPC stated that it expected to maintain the UK CCyB at this rate for at least 12 months. As there is a one year time lag for any announced CCyB increases to apply, any subsequent increase would not take effect until March 2022 at the earliest. We review the resilience of our capital position via internal stress tests conducted as part of our ICAAP exercise, and we participate in the annual UK Banking Stress Test. In the last exercise in 2019, our projected CET1 ratio exceeded the hurdle rates after ‘strategic’ management actions and the Prudential Regulation Committee judged that the stress test did not reveal capital inadequacies in our balance sheet. Headroom of our CET1 capital ratio to our current MDA trigger level at 31 December 2020 At 31 December 2020, the headroom of our CET1 capital ratio of 15.4% to our 7% AT1 permanent write down (PWD) securities trigger was 8.4% of total RWAs or £6.0bn (2019: 7.3% of total RWAs or £5.3bn). The headroom of our CET1 capital ratio to our current maximum distributable amount (MDA) trigger level at 31 December 2020 was: Current MDA % Pillar 1 4.5 Pillar 2A (1) 2.8 CCB 2.5 CCyB (2) — O-SII 1.0 Current MDA trigger 10.8 Headroom to current MDA 4.6 Total CET1 capital ratio 15.4 (1) Santander UK’s (i.e. the Ring-Fenced Bank’s) Pillar 2 CET1 requirement was 4.94% at 31 December 2020, Pillar 2A guidance is a point in time assessment. (2) The current applicable UK CCyB rate is 0% (2019: 1%). MREL recapitalisation We have made major progress to meet MREL requirements. To date, we have down streamed £7.5bn of senior unsecured bonds from Santander UK Group Holdings plc as Internal MREL compliant, secondary non-preferential debt to Santander UK plc as the ring-fenced bank. Key capital ratios 2020 2019 % % CET1 capital ratio 15.4 14.3 AT1 2.7 2.7 Grandfathered Tier 1 0.4 0.7 Tier 2 2.7 4.0 Total capital ratio 21.2 21.7 The total subordination available to Santander UK plc bondholders was 21.2% (2019: 21.7%) of RWAs. Regulatory capital resources (audited) This table shows our qualifying regulatory capital. 2020 2019 £m £m CET1 capital 11,057 10,419 AT1 capital 2,281 2,443 Tier 1 capital 13,338 12,862 Tier 2 capital 1,909 2,925 Total regulatory capital (1) 15,247 15,787 (1) Capital resources include a transitional IFRS 9 benefit at 31 December 2020 of £73m (2019: £16m). AT1 capital These are preference shares and innovative/hybrid Tier 1 securities. None of the instruments we issued before 1 January 2014 fully meet the CRD IV AT1 capital rules, which apply from that date. The instruments contribution to Tier 1 capital will be phased out by CRD IV rules until the start of 2022. The £750m Fixed Rate Reset Perpetual AT1 Capital Securities (net of issuance costs), the £800m Perpetual Capital Securities and the £500m Perpetual Capital Securities we issued since then fully meet the CRD IV AT1 capital rules. In August 2019, as part of a capital management exercise, the Company purchased and redeemed the £300m Fixed Rate Reset Perpetual AT1 Capital Securities, and issued a further £500m Fixed Rate Reset Perpetual AT1 Capital Securities to Santander UK Group Holdings plc. Tier 2 capital These are fully CRD IV eligible Tier 2 instruments and grandfathered Tier 2 instruments whose recognition as capital is being phased out under CRD IV until the start of 2022. Risk-weighted assets The tables below are consistent with our regulatory filings for 31 December 2020 and 31 December 2019. 2020 2019 £bn £bn Total RWAs 71.9 72.6

Capital risk (Tables)

Capital risk (Tables)	12 Months Ended
	Dec. 31, 2020
Text block [abstract]
Summary of Detailed Information About Capital Ratio In Current MDA Explanatory	The headroom of our CET1 capital ratio to our current maximum distributable amount (MDA) trigger level at 31 December 2020 was: Current MDA % Pillar 1 4.5 Pillar 2A (1) 2.8 CCB 2.5 CCyB (2) — O-SII 1.0 Current MDA trigger 10.8 Headroom to current MDA 4.6 Total CET1 capital ratio 15.4 (1) Santander UK’s (i.e. the Ring-Fenced Bank’s) Pillar 2 CET1 requirement was 4.94% at 31 December 2020, Pillar 2A guidance is a point in time assessment. (2) The current applicable UK CCyB rate is 0% (2019: 1%).
Summary of Detailed Information About Recapitalisation For Holdings Group Explanatory	2020 2019 % % CET1 capital ratio 15.4 14.3 AT1 2.7 2.7 Grandfathered Tier 1 0.4 0.7 Tier 2 2.7 4.0 Total capital ratio 21.2 21.7
Summary of Regulatory Capital Resources	Regulatory capital resources (audited) This table shows our qualifying regulatory capital. 2020 2019 £m £m CET1 capital 11,057 10,419 AT1 capital 2,281 2,443 Tier 1 capital 13,338 12,862 Tier 2 capital 1,909 2,925 Total regulatory capital (1) 15,247 15,787 (1) Capital resources include a transitional IFRS 9 benefit at 31 December 2020 of £73m (2019: £16m).
Summary of Detailed Information About For Additional Capital And Risk Management Explanatory	The tables below are consistent with our regulatory filings for 31 December 2020 and 31 December 2019. 2020 2019 £bn £bn Total RWAs 71.9 72.6

Capital Risk - Summary of Regul

Capital Risk - Summary of Regulatory Capital Resources (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of Regulatory Capital Resources [Line Items]
CET1 capital	£ 11,057	£ 10,419
AT1 capital	2,281	2,443
Tier 1 capital	13,338	12,862
Tier 2 capital	1,909	2,925
Total regulatory capital	£ 15,247	£ 15,787

Risk Framework

Risk Framework	12 Months Ended
	Dec. 31, 2020
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Risk Framework	Our risk governance structure We are committed to the highest standards of corporate governance in every part of our business, including risk management. For details of our governance, including the Board and its Committees, see the ‘Governance’ section of this Annual Report. The Board delegates certain responsibilities to Board Level Committees as needed and where appropriate. Our risk governance structure strengthens our ability to identify, assess, manage and report risks, as follows: – Committees: A number of Board and Executive committees are responsible for specific parts of our Risk Framework – Key senior management roles: A number of senior roles have specific responsibilities for risk management – Risk organisational structure: We have the ‘three lines of defence’ model built into the way we run our business. Committees The Board Level Committee responsibilities for risk are: Board Level Committee Main risk responsibilities The Board – Has overall responsibility for business execution and for managing risk – Reviews and approves the Risk Framework and Risk Appetite. Board Risk Committee (BRC) – Assesses the Risk Framework and recommends it to the Board for approval – Advises the Board on our overall Risk Appetite, tolerance and strategy – Oversees our exposure to risk and our strategy and advises the Board on both – Reviews the effectiveness of our risk management systems and internal controls. Board Responsible Banking Committee – Responsible for culture and operational risk from conduct, compliance, competition, financial crime & legal matters – Reviews reports from the CLRO on the adequacy and effectiveness of the compliance function – Ensures that adequate and effective control processes are in place to identify and manage reputational risks – Oversees our Corporate Social Responsibility programme and how it impacts on employees, communities, the environment including sustainability and climate change, reputation, brand and market positioning. Board Audit Committee – Monitors and reviews the financial statements integrity, and any formal announcements on financial performance – Reviews the adequacy and effectiveness of the internal financial controls and whistleblowing arrangements – Monitors and reviews the effectiveness of the internal audit function. – Oversees the independence and performance of our auditors. Board Remuneration Committee – Oversees implementation of remuneration policies, ensuring they promote sound and effective risk management. The Executive Level Committee responsibilities for risk are: Executive Level Committee Main risk responsibilities Executive Committee – Reviews business plans in line with our Risk Framework and Risk Appetite before they are sent to the Board to approve. – Receives updates on key risk issues managed by CEO-level committees and monitors the actions taken. Senior Management Committee – Focuses on the responsibilities of the Executive Committee Senior Management Function holders and how they are discharged – Reviews updates on key risk issues, customer, reputational and conduct matters. Executive Risk Control Committee (ERCC) – Reviews Risk Appetite proposals before they are sent to the Board Risk Committee and the Board to approve – Ensures that we comply with our Risk Framework, Risk Appetite and risk policies – Reviews and monitors our risk exposures and approves any corrective steps we need to take. Asset and Liability Committee (ALCO) – Reviews liquidity risk appetite (LRA) proposals – Ensures we measure and control structural balance sheet risks, including capital, funding and liquidity, in line with the policies, strategies and plans set by the Board – Reviews and monitors key asset and liability management activities to ensure we keep our exposures within our Risk Appetite. Pensions Committee – Reviews pension risk appetite proposals – Approves actuarial valuations and reviews the impact they may have on our contributions, capital and funding – Consults with the pension scheme trustees on the scheme’s investment strategy. Capital Committee – Puts in place reporting systems and risk control processes to make sure capital risks are managed within our Risk Framework – Reviews capital adequacy and capital plans, including the ICAAP, before they are sent to the Board to approve. Incident Accountability Committee – Considers, calibrates, challenges and agrees any appropriate individual remuneration adjustments – Presents recommendations to the Board Remuneration Committee. Credit Approval Committee – Approves corporate and wholesale credit transactions which exceed levels delegated to lower level forums or individuals. Investment Approval Committee – Approves equity type investment transactions which exceed levels delegated to lower level approval forums or individuals. Financial Crime Committee – Ensures due reporting, consideration, oversight and informed decision making regarding compliance by the Company and its subsidiaries with financial crime laws and regulations, and best industry practice aligned to the Company’s stated risk appetite. Key senior management roles Senior roles with specific responsibilities for risk management are: Role Main risk responsibilities Chief Executive Officer The Board delegates responsibility for our business activities and managing risk on a day-to-day basis to the CEO. The CEO proposes our strategy and business plan, puts them into practice and manages the risks involved. The CEO must also ensure we have a suitable system of controls to manage risks and report to the Board on it. Chief Risk Officer (CRO) As Risk Division leader, oversees and challenges risk activities, and ensures lending decisions are made within our Risk Appetite. Accountable for control and oversight of credit, market, liquidity, capital, pension, strategic & business, operational and model risk. Chief Legal and Regulatory Officer (CLRO) Accountable for the control and oversight of legal, conduct and regulatory, reputational and financial crime risk, and is responsible for reporting on these risks to the CRO, to provide the CRO with a holistic enterprise wide view of all risks. Chief Financial Officer Responsible for developing strategy, leadership and management of the CFO Division. In supporting our corporate goals within our risk appetite, the CFO is responsible for managing interest rate, liquidity, pension and capital risks. The CFO aims to maximise the return on Regulatory and Economic Capital, ensuring transactions create value with the right risk-based profile. Chief Internal Auditor (CIA) Designs and uses an audit system that identifies key risks and evaluates controls. The CIA also develops an audit plan to assess existing risks that involve producing audit, assurance and monitoring reports. Money Laundering Reporting Officer (MLRO) Responsible to the CLRO for control and oversight of financial crime risk but has regulatory responsibility to report on this risk type to Executive and Board Committees and the FCA.

Credit risk

Credit risk	12 Months Ended
	Dec. 31, 2020
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Credit risk	Credit risk Overview Credit risk is the risk of financial loss due to the default or credit quality deterioration of a customer or counterparty to which we provided credit, or for which we have assumed a financial obligation. Santander UK group level We start by discussing credit risk at a Santander UK group level. We set out how our exposures arise, our types of customer and how we manage them, and our approach to credit risk across the credit risk lifecycle. We discuss our ECL approach and the key inputs to our ECL model. We also summarise various Covid-19 support measures provided to our customers and their impact on ECL. We then analyse our key metrics, credit performance and forbearance, and highlight how Covid-19 affected them where relevant. Business segments Then we cover Retail Banking separately from our other business segments – Corporate & Commercial Banking, Corporate & Investment Banking and Corporate Centre – in more detail. Key metrics Stage 3 ratio increased to 1.45% (2019: 1.15%). Loss allowances increased to £1,377m (2019: £863m). Average LTV of 64% (2019: 65%) on new mortgage lending. SANTANDER UK GROUP LEVEL – CREDIT RISK MANAGEMENT Exposures ( audited ) Exposures to credit risk arise in our business segments from: Retail Banking Corporate & Commercial Banking Corporate & Investment Banking Corporate Centre – Residential mortgages, business banking, consumer (auto) finance and other unsecured lending (credit cards, personal loans and overdrafts). – We provide these to individuals and small businesses. – Loans, bank accounts, treasury services, invoice discounting, cash transmission, trade finance and asset finance. – We provide these to SMEs and mid corporates, Commercial Real Estate and Social Housing associations. – Loans, bank accounts, treasury services, treasury markets activities, trade finance, receivables discounting and cash transmission. – We provide these to large corporates and financial institutions. – Asset and liability management of our balance sheet, as well as our non-core and Legacy Portfolios being run down. – Exposures include sovereign and other international organisation assets that we hold for liquidity. The segmental basis of presentation in this Annual Report has been changed, and the prior periods restated, to report some customer assets in Corporate & Commercial Banking rather than in Business Banking (in Retail Banking), some non-core corporate mortgages in Corporate & Commercial Banking rather than in Corporate Centre, and a number of smaller business lines in Corporate Centre rather than in Corporate & Investment Banking. See Note 2 for more information. Our types of customers and how we manage them We manage credit risk across all our business segments in line with the credit risk lifecycle that we show in the next section. We tailor the way we manage risk to the type of customer. We classify our customers as standardised or non-standardised: Standardised Non-standardised – Mainly individuals and small businesses. Their transactions are for relatively small amounts of money and share similar credit characteristics. – Mainly medium and large corporate customers. Their transactions are for larger values and have more diverse credit characteristics. – We manage risk using automated decision-making tools. These are backed by teams of expert analysts. – We manage risk through expert analysis with support from internal risk assessment models. For many of our business customers, the impact of the Covid-19 pandemic has been extreme with, in some cases, revenue streams disappearing overnight and cash flows becoming very challenged. Many businesses that had been performing well were suddenly faced with considerable unforeseen challenges. Given the scale of these issues and their impact on the wider economy, it is understandable that the UK Government wanted to provide financial help and recognised that one of the most efficient ways to do so quickly was through the banking sector. Covid-19 loan schemes Our approach to credit risk We manage our portfolios across the credit risk lifecycle (above), from drawing up our risk strategy, plans, budgets and limits to making sure the actual risk profile of our exposures stays in line with our business plans and within our Risk Appetite. We further tailor the way we manage risk across the lifecycle to the type of product. We say more on this in the Credit risk – Retail Banking and the Credit risk – Other business segments sections. 1. Risk strategy and planning (audited) All relevant areas of the business work together to create our business plans. We aim to balance our strategy, goals, and financial and technical resources with our Risk Appetite. To do this, we focus on economic and market conditions and forecasts, regulations, conduct matters, profitability, returns and market share. The result is an agreed set of targets and limits that help us direct our business. 2. Assessment and origination (audited) Managing credit risk begins with lending responsibly. That means only lending to customers who can afford to pay us back, even if things get tighter for them, and are committed to paying us back. We perform a thorough risk assessment to make sure customers can meet their obligations before we approve a credit application. These decisions are made with authority from the Board and consider: – The credit quality of the customer – The underlying risk and how we can mitigate it, such as through netting or collateral, or participation in Government-sponsored programs, such as the coronavirus support loan schemes introduced in 2020 – Our risk policy, limits and appetite – Whether we can balance the amount of risk we face with the returns we expect, and – Assessment of customer affordability. We lend responsibly by having a number of prohibitions and restrictions on a range of activities. For details, see the 'Other business segments - credit risk management' section. 3. Monitoring (audited) We measure and monitor changes in our credit risk profile on a regular and systematic basis against our budgets, limits and benchmarks. We monitor credit performance by portfolio, segment, customer or transaction. If our portfolios do not perform as we expect, we investigate to understand the reasons. Then we take action to mitigate it as far as possible and bring performance back on track. We monitor and review our risk profile through formal governance forums and committees across our business. These agree and track any steps we need to take to manage our portfolios, to make sure the impact is prompt and effective. This structure is a vital feedback tool to coordinate issues, trends and developments across each part of the credit risk lifecycle. Credit concentrations A core part of our monitoring and management is a focus on credit concentrations, such as the proportion of our lending that goes to specific borrowers, groups or industries. We set concentration limits in line with our Risk Appetite and review them on a regular basis. We track how concentrated our portfolios are using a range of criteria, these include geographies, economic sectors, products and groups of customers. Geographical concentrations We set exposure limits to countries and geographies, with reference to the country limits set by Banco Santander. These are determined according to how the country is classified (whether it is a developed OECD country or not), its credit rating, its gross domestic product, and the products and services we or Banco Santander want to offer in that country. For more geographical information, see ‘Country risk exposures’. Industry concentrations We also set exposure limits by industry sector. We set these limits based on the industry outlook, our strategic aims and desired level of concentration, and relevant limits set by Banco Santander. We analyse committed exposures in the ‘Credit risk review’. 4. Arrears management (audited) Sometimes our customers face financial difficulty and may fall into payment arrears or breach the conditions of their credit facility. If this happens, we work with them to get their account back on track. We aim to support our customers and keep our relationship with them. To do this, we: – Find affordable and sustainable ways of repaying to fit their circumstances – Monitor their finances and use models to predict how they will cope. This helps us put in place the right strategy to manage their debt – Work with them to get their account back on track as soon as possible in a way that works for them and us – Monitor agreements we make to manage their debt, so we know they are working. For more, see the Forbearance section on the next page. 5. Debt recovery (audited) Sometimes, even when we have taken all reasonable and responsible steps we can to manage arrears, they are not effective. If this happens, we have to end our agreement with the customer and try to recover the whole debt, or as much of it as we can. Loan modifications (audited) We sometimes change the terms of a loan when a customer gets into financial difficulty (this is known as forbearance), or for other commercial reasons. Forbearance When a customer gets into financial difficulties, we can change the terms of their loan, either temporarily or permanently. We do this to help customers through temporary periods of difficulty so they can get back on to sustainable terms and fully pay off the loan over its lifetime, with support if needed. We try to do this before the customer defaults. Whatever we offer, we assess it to make sure the customer can afford the repayments. Forbearance improves our customer relationships and our credit risk profile. We review our approach regularly to make sure it is still effective. In a few cases, we can help a customer in this way more than once. This can happen if the plan to repay their debt doesn’t work and we have to draw up another one. When this happens more than once in a year, or more than three times in five years, we call it multiple forbearance. We only use foreclosure or repossession as a last resort. When we agree to forbearance, we consider that the account has suffered a Significant Increase in Credit Risk (SICR), as we explain later on. We review our loss allowance for it and report the account as forborne. For retail accounts, if an account is in Stage 1 (a 12-month ECL) when we agree forbearance, we transfer it to Stage 2 (a lifetime ECL). For all accounts, if an account is already in Stage 2 when we agree forbearance, we keep it in Stage 2 unless the forbearance arrangement involves an account that is deemed unlikely to pay (defined through a number of events listed in our Classification Policy), the forgiveness of fees and interest or debt, or is being granted multiple forbearances which would put the case into Stage 3 (a lifetime ECL). If an account is already in Stage 3 when we agree forbearance, we keep it in Stage 3. We monitor the performance of all forborne loans. A loan moves from a lifetime ECL to a 12-month ECL once the criteria to exit forbearance have been met, as set out below. Exit from forbearance or cure For an account in Stage 3 to exit forbearance, all the following conditions must be met: – The account has been classed as Stage 3 for at least one year since the end of the latest forbearance strategy – The account is not deemed unlikely to pay – The account is no longer in arrears, and the customer has no other material debts with us which are more than 90 days in arrears. If all the conditions are met, the account is re-classed as Stage 2 forbearance until the Stage 2 forbearance exit conditions set out below are also met. For an account in Stage 2 to exit forbearance, all the following conditions must be met: – The account has been classed as Stage 2 for at least two years since the end of the latest forbearance strategy – The account has been performing, i.e. the customer is no longer in financial difficulty – Meaningful capital and interest repayments have been made for at least 50% of the two year period – The account is no longer in arrears, and the customer has no other material debts with us which are more than 30 days in arrears. Other modifications When a customer is not showing any signs of financial difficulties, we can also change the terms of their loan. We do this to assist them to manage their financial liabilities. In addition, since March 2020, we have provided mortgage customers with payment holiday terms in line with UK Government and FCA guidance. Similar payment holidays have also been granted in respect of consumer (auto) finance, personal loans, credit cards, businesses and corporates. For more on this, see 'Covid-19 Support measures'. Loans for customers who were provided with payment holidays were considered to have the contract terms modified. The granting of a payment holiday on its own was not considered to be a Significant Increase in Credit Risk (SICR) event, nor was it considered a default under regulatory definitions. Neither were they considered to have been granted forbearance. For customers who have needed further financial support after the payment holiday period, we help them by offering assistance in line with our policies. We measure and control credit risk at all stages across the credit risk lifecycle. We have a range of tools, processes and approaches, but we rely mainly on: – Credit control: as a core part of risk management we generate, extract and store accurate, comprehensive and timely data to track credit limits. We use internal data and data from third parties like credit bureaux. – Models: we use models widely to measure credit risk and capital needs. They range from statistical and expert models to benchmarks. – Review: we use formal and informal forums to approve, validate, review and challenge our risk management. We do this to help predict if our credit risk will worsen. Key metrics (audited) We use a number of key metrics to measure and control credit risk, as follows: Metric Description Expected Credit Loss (ECL) ECL tells us what credit risk is likely to cost us either over the next 12 months on qualifying exposures, or defaults over the lifetime of the exposure where there is evidence of a SICR since origination. We explain how we calculate ECL below. Stages 1, 2 and 3 We assess each facility’s credit risk profile to determine which stage to allocate them to, and we monitor where there is a SICR and transfers between the Stages including monitoring of coverage ratios for each stage. We explain how we allocate a facility to Stage 1, 2 or 3 below. Stage 3 ratio The Stage 3 ratio is total Stage 3 exposure as a percentage of customer loans plus undrawn Stage 3 exposures. The Stage 3 ratio is the main indicator of credit quality performance. Expected Loss (EL) EL is based on the regulatory capital rules of CRD IV and gives us another view of credit risk. It is the product of the probability of default, exposure at default and loss given default. We calculate each factor in accordance with CRD IV and include direct and indirect costs. We base them on our risk models and our assessment of each customer’s credit quality. There are differences between regulatory EL and IFRS 9 ECL, which we set out below. The rest of our Risk review, impairments, losses and loss allowances refer to calculations in accordance with IFRS, unless we specifically say they relate to CRD IV. For our IFRS accounting policy on impairment, see Note 1 to the Consolidated Financial Statements. We also assess risks from other perspectives, such as geography, business area, product and process to identify areas we need to focus on. We also use stress testing to establish vulnerabilities to economic deterioration. Our business segments tailor their approach to credit risk to their own customers, as we explain later on. Key differences between regulatory EL and IFRS 9 ECL models There are differences between the regulatory EL and the IFRS 9 ECL approaches. Although our IFRS 9 models use the existing Basel advanced IRB risk components, we need to make several adjustments to ensure the outcome is in line with the IFRS 9 requirements, i.e. the financial reporting standard we use, as follows. Basel advanced IRB EL IFRS 9 ECL Rating philosophy Mix of point-in-time, through-the-cycle or hybrid Point-in-time, forward-looking. Considers a range of economic scenarios Parameters calibration Contains regulatory floors and downturn calibration Unbiased estimate, based on conditions known at the balance sheet date Calculation timing Considers aggregation of possible default events in the next 12 months Considers monthly calculation of parameters, for all possible future default dates. First 12 months are used for Stage 1, full lifetime for Stages 2 and 3 Probability of Default (PD) PD in the next 12 months Includes forward-looking economic data and removes conservatism. PD in next 12 months for Stage 1, lifetime for Stages 2 and 3 Loss Given Default (LGD) Lifetime LGD for defaults in the next 12 months Modelled without regulatory floors and exclusion of indirect costs Exposure at Default (EAD) Exposure at the point of default if the customer defaults in the next 12 months Floored at amount owed, except on some revolving facilities. Recognises ability for exposure to reduce from the balance sheet date to default date SICR Does not include SICR concept Includes SICR concept Discounting applied At the weighted average cost of capital to the default date At the effective interest rate (EIR) to the balance sheet date Recognising ECL (audited) The ECL approach estimates the credit losses arising from defaults in the next 12 months on qualifying exposures, or defaults over the lifetime of the exposure where there is evidence of a SICR since the origination date. The ECL approach takes into account forward-looking data, including a range of possible outcomes, which should be unbiased and probability-weighted in order to reflect the risk of a loss being incurred even when it is considered unlikely. Multiple economic scenarios and probability weights (audited) For all our portfolios, except CIB, we use five forward-looking economic scenarios. For 2020, they consist of a central base case, one upside scenario and three downside scenarios. We use five scenarios to reflect a wide range of possible outcomes for the UK economy. (i) For all our portfolios, except CIB Our forecasting approach We derive our scenarios in part by using a set of parameters in GDP fan charts published by the Office for Budget Responsibility (OBR). To avoid major changes to the scenarios due to changes in the OBR fan charts, we place more weight on the long-run outlook of the fan charts rather than relying solely on each individual release as this can create large swings in the scenarios which may not be appropriate. We use the OBR fan charts to calculate our GDP paths for each scenario. For 2020 this applied to the Upside 1, Downside 1 and Downside 2 scenarios. These fan charts reflect the probability distribution of a deviation from the OBR’s central forecast to illustrate the uncertainty regarding the outcome of a variable, in this case GDP. We use the 0.6 fan chart path for our Upside 1 scenario and the 0.3 path for Downside 1. For Downside 2 we use a blend of the Downside 1 scenario and the base case rather than the 2008/09 recession, which is used under BAU. This is because the fall in GDP in the base case is markedly higher than the one seen in 2008/09, due to the lockdown restrictions imposed due to Covid-19. This means that in the longer run the GDP levels in our Downside 1 and 2 scenarios converge. To ensure that Downside 2 is kept consistent with any changes to the OBR fan charts, we calculate the Downside 2 GDP by taking the percentage difference between Downside 2 and Downside 1 GDP in the original forecast and applying this difference to the new Downside 1. Once we have established the GDP paths for each scenario, we run them through the Oxford Global Economic Model (OGEM) to derive the other macroeconomic variables, such as unemployment and house prices. These variables are the product of the GDP growth paths we have forecast and the output of the OGEM for these growth paths. We then impose a Bank Rate profile for each scenario using expert judgement. We determine the Bank of England Bank Rate (Bank Rate) by using the base case Bank Rate profile and adjusting this for each of the four other scenarios. To do this, we firstly consider what each of the scenarios is trying to achieve. For the upside scenario, which has a higher growth path and rising productivity growth, we allow for a managed tightening of the monetary stance, so we assume small increases in Bank Rate. In contrast, for Downside 2 the scenario shows monetary policy forced into a reactive stance to contain CPI inflation at a time of weakening output growth, so we assume the Bank of England would raise rates in this scenario to bring inflation back to its target rate. The rising Bank Rate profiles are based on forward guidance from the Bank of England, where increases are assumed to be gradual and incremental. For the Downside 1 scenario, this is aligned to the base case forecast as inflation is similar to that of the base case, and for Downside 3, this shows a negative interest rate profile which the Bank of England follows to try and boost growth with inflation remaining low. In this way, our scenarios reflect a range of possible outcomes that the Bank of England may follow for different growth paths. Our use of five scenarios is designed to reflect different possible outcomes to the base case forecast highlighting the upside and downside risks associated with the central scenario. The downside risks for the UK economy include further waves of Covid-19 leading to restrictions on economic activity, a further and sharper downturn in global growth, a continuation of the very low productivity growth seen in the UK, and a move to a more protectionist agenda for trade. The upside risks are more muted at present and include the smooth implementation of a new free trade agreement with the EU with limited trade frictions caused by customs checks and a recovery in global growth, coupled with a move to more open trade. We update the baseline in our economic scenarios at least twice a year in line with our annual budgeting and three-year planning processes, or sooner if there is a material change in current or expected economic conditions (as was the case in Q4 2020 when a second national lockdown was imposed). We refresh all our economic scenarios each quarter to reflect the latest data and OBR fan charts if these have changed, which are then reviewed and approved by the Credit Risk Provisions Forum (CRPF). The CRPF also assesses the probability weights at least once a quarter. We do not use consensus forecasts as inputs to our models, but we do compare the outputs of our models against consensus views for the base case, to make sure that we understand any significant differences and address them where needed. At the end of 2020, there were no significant differences between our base case forecasts and the consensus views. In 2020, we were also able to do further peer benchmarking analysis of the economic scenarios using the data the PRA provided, which for Q4 2020 included the mean weighted analysis for a selection of economic variables, including GDP, unemployment rate and HPI. This meant that we could compare our weighted scenarios against the average of our peers to understand what differences there may be. The conclusion of this analysis demonstrated that our economic scenarios were in line with our peers. Key changes to our forecasting approach in 2020 In 2020, although the number of alternative scenarios remained the same, we removed the best upside scenario and replaced it with a further downside scenario that reflects the key risks associated with Covid-19. This scenario was developed internally by the Economic Analysis team with the Enterprise Risk team, rather than using a different path from the OBR fan charts, in order to reflect the specific circumstances of Covid-19 and further lockdowns which are unprecedented in the economic history of the UK. In addition there was a slight change to the Downside 1 Bank Rate profile, which in 2019 had Bank Rate rising. In the 2020 scenarios it was decided that in an environment which has seen Bank Rate remain at record lows, there should a be a downside scenario which reflects this status quo i.e. Bank Rate held flat at 10 bps. Base case Two key assumptions underpin the base case. Firstly it assumed a trade agreement was negotiated with the EU and that further discussions would take place post 2020 on aspects not covered. However despite the agreement, disruption would still occur in Q1 2021 as firms factor in the additional requirements they need to meet in order to trade. It further assumed that the second national lockdown was followed by the UK entering tier restrictions again and that this system would continue through Q1 2021 and into Q2 2021. However, it did not assume that there would be a third lockdown in Q1 2021. It is normal practice to review the scenarios and associated weights every quarter to ensure they appropriately reflect the current economic circumstances and we will continue to follow that approach particularly as the advice the UK Government issues is subject to change in this fluid environment. In order to factor in this further lockdown the Q4 2020 weights were updated with 5% removed from both Upside 1 and the base case and put onto Downside 2 and Downside 3. The reason for moving weight to both Downside 2 and Downside 3 is that Downside 3 incorporates a double dip recession, which is now considered more likely given the second lockdown in November 2020 and now a third in January 2021, although it is noted that this is still a very severe scenario and not a replacement for a base case. In terms of Downside 2, the longer restrictions remain in place, the greater the risk of longer-term effects which are reflected in this scenario. Base case key macroeconomic assumptions – House price growth: As we move into 2021, with the end of the stamp duty holiday in March 2021, the rise in unemployment, and negative real wage growth, there is likely to be a reduced demand and so lower house prices. As such we expect to see some negative growth towards the end of 2021 but, with the supply side still weak, this will be limited. We are projecting a fall of 2% by the end of 2021. – GDP: The outlook assumes that the recovery is curtailed by the return of the virus and the second national lockdown which leaves Q4 2020 growth in negative territory with GDP expected to contract by 11.5% in 2020. For 2021, it assumes the gradual easing of restrictions towards the end of Q2 2021 will support a modest rebound in growth. – Unemployment rate: Unemployment is expected to peak in Q2 2021 at just under 8% as the UK Government’s job support schemes come to an end. With support ending for firms and limited savings left to keep trading, rising insolvency rates trigger a further increase in unemployment. As consumers and businesses become more confident about efforts to contain the coronavirus in the second half of 2021, unemployment falls back gradually. However, given the need for the economy to restructure, unemployment remains at elevated levels compared to recent figures over the remaining forecast period. – Bank Rate: For the Bank Rate forecast, the base case assumes a flat profile of 10bps with a rise to 25bps at the beginning of 2024. This was based on the view that we have a limited trade agreement by the end of 2020, with inflation expected to remain near target over the five year forecast period. The Monetary Policy Committee will wait to understand how the economy responds to the new economic environment before changing the Bank Rate. In the medium-term, the projections assume that current demographic and productivity trends will continue, causing a reduction in the UK’s growth potential. This is reflected in an average growth expectation of 1.6% pa, the OBR’s latest estimate of the UK’s long run average growth rate. CPI inflation is forecast to be below the 2% target rate in the initial forecast period but returning to target by the end. Nominal earnings growth will fall in 2020 before starting to recover in 2021. This will then support household spending power as we move into 2022. However, the effect of limited business investment on growth will continue as firms look to repay debt that they have taken on due to the lockdown. In summary, the base case assumes that activity starts to recover as the restrictions are lifted, but that progress will be tempered by the uncertainty around the UK’s new trading relationship with the EU. Key changes to our base case in 2020 The key changes to our base case assumptions in 2020 were: (i) lower GDP projections in 2020 and 2021 to reflect the impact of the ongoing restrictions on economic activity due to Covid-19; (ii) the unemployment rate, whilst lower than expected given the government furlough scheme, peaks in Q2 2021 and only slowly recovers as firms adapt to the new economic environment; (iii) there is negative house price growth for 2021 as the stamp duty holiday ends and unemployment rises; and (iv) the Bank Rate profile is held flat at 0.10% until Q1 2024, when there is a rise to 0.25% and remains flat over the rest of the forecast period. Other scenarios Based on this revised base case, we have reviewed our suite of scenarios to ensure that they capture the wide range of potential outcomes for the UK economy. These include (i) a significant rise in Covid-19 cases and further lockdown measures being imposed; (ii) a slower recovery that is more akin to the ‘U’ shape of past recessions; (iii) higher inflation; (iv) the long-term effects caused by higher and longer unemployment rates or higher and longer unemployment rate persisting, increasing the natural rate of unemployment; (v) a vaccine or treatments being developed at a quicker pace; and (vi) the global economy bouncing back more swiftly than expected. In order to reflect these potential outcomes, we decided to continue to use the base case and four additional scenarios, which management considers to provide a range wide enough to reflect all of the above potential outcomes. However, as the risks remain skewed to the downside, to reflect these outcomes sufficiently, we concluded that only one upside scenario would be needed to reflect the upside risks to the base case. As with the base case, the scenarios are forecast over a five-year period and then mean revert over the next three years to the OBR's latest estimate of the UK's long run average growth rate. The four scenarios are as follows: One upside scenario All our Q4 2020 scenarios reflect a sharp recession in 2020, although a modest upside scenario remains appropriate based on vaccines being distributed quickly and effectively to the population, with a faster global recovery and the UK quickly concluding trade agreements with a number of countries after leaving the EU, along with minimum effective tariffs. It is also based on productivity growth recovering. HPI for Upside 1 is less positive than for the base case and is based on the HPI equations built into the OGEM and the particular GDP profile used, whereas our base case reflects our planning view which allows for flexibility to align what is currently seen in the market to the outlook of the economic variable forecast. Three downside scenarios Downside 1 assumes further local/regional lockdowns as we move through 2021 than in the base case as a means of controlling increases in infection rates, which in turn impact economic growth as the vaccine(s) is not as effective in reducing the virus outbreak as hoped. The scenario also reflects a fall in demand for housing leading to significant downward price corrections over the next five years with a peak to trough of negative 10%. It assumes trade agreements with other countries being negotiated over the forecast period, but fewer than in the base case. Downside 2 reflects a severe downturn with a longer recovery needed (U shape) capturing even more conservatism and lack of confidence in terms of spending by consumers with the higher levels of unemployment. For businesses it reflects a slower return to profitability and more insolvencies as the rollout of the vaccine progresses at a slower pace than needed to keep infection rates under control. It retains a rising bank rate profile to ensure there is a scenario which encapsulates rising inflation. However, the rise i

Market risk

Market risk	12 Months Ended
	Dec. 31, 2020
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Market risk	Market risk Overview Market risk comprises banking market risk and trading market risk. Banking market risk is the risk of loss of income or economic value due to changes to interest rates in the banking book or to changes in other market risk factors (e.g. credit spread and inflation risk), where such changes would affect our net worth through an adjustment to revenues, assets, liabilities and off-balance sheet exposures in the banking book. Trading market risk is the risk of changes in market factors that affect the value of the positions in the trading book. In this section, we set out which of our assets and liabilities are exposed to banking and trading market risk. Then we explain how we manage these risks and discuss our key market risk metrics. We also provide an update on the process of replacing LIBOR and other Interbank Offered Rates. Key metrics Net Interest Margin (NIM) sensitivity to +50bps was £225m and to ‑50bps was £(15)m (2019: £99m and £56m) Economic Value of Equity (EVE) sensitivity to +50bps was £367m and to ‑50bps was £(585)m (2019: £10m and £(88)m) BALANCE SHEET ALLOCATION BY MARKET RISK CLASSIFICATION (AUDITED) We manage our assets and liabilities exposed to market risk as either banking or trading market risk. We classify all our assets and liabilities exposed to market risk as banking market risk, except for certain derivatives that we manage on a trading intent basis. For accounting purposes, we classify all derivatives as held for trading unless they are designated as being in a hedging relationship. The derivatives that we manage on a trading intent basis are a small proportion of the derivatives that we classify as held for trading for accounting purposes. For more, see Note 11 to the Consolidated Financial Statements. BANKING MARKET RISK OUR KEY BANKING MARKET RISKS (AUDITED) Banking market risk mainly comes from providing banking products and services to our customers, as well as our structural balance sheet exposures. It arises in all our business segments. In Retail Banking and Corporate & Commercial Banking, it is a by-product of us writing customer business and we transfer most of these risks to Corporate Centre to manage. The only types of banking market risk that we keep in Retail Banking and Corporate & Commercial Banking are short-term mismatches due to forecasting variances in prepayment and launch risk. This is where customers repay their loans earlier than their expected maturity date or do not take the expected volume of new products. In Corporate & Investment Banking, it arises from lending to corporates, which we also transfer to Corporate Centre to manage. Corporate Centre also manages our structural balance sheet exposures, such as foreign exchange and Income Statement volatility risk. Our key banking market risks are: Key risks Description Interest rate risk Yield curve risk: comes from timing mismatches in repricing fixed and variable rate assets, liabilities and off-balance sheet instruments. It also comes from investing non-rate sensitive liabilities in interest-earning assets. We mainly measure yield curve risk with NIM and EVE sensitivities, which are measures that are commonly used in the financial services industry. We also use other risk measures, such as Value at Risk (VaR) which is a statistical measure based on a historical simulation of events, and stress testing. Our NIM and EVE sensitivities cover all the material yield curve risk in our banking book balance sheet. Basis risk: comes from pricing assets using a different rate index to the liabilities that fund them. We are exposed to basis risks associated with Bank of England bank rate, reserve rate linked assets we deposit with central banks, the Sterling Overnight Index Average (SONIA) rate, and LIBOR rates of different terms. As LIBOR and other Interbank Offered Rates are being replaced, we continue to engage with stakeholders across the business to ensure we capture and understand new risks as they emerge. Spread risks Spread risk arises when the value of assets or liabilities which are accounted for at fair value (either through Other Comprehensive Income or through Profit and Loss) are affected by changes in the spread. We measure these spreads as the difference between the discount rate we use to value the asset or liability, and an underlying interest rate curve. Foreign exchange risk Our banking businesses operate mainly in sterling markets, so we do not create significant foreign exchange exposures. The only exception to this is money we raise in foreign currencies. For more on this, see ‘Wholesale funding’ in the ‘Liquidity risk’ section. Income statement We measure most of the assets and liabilities in our banking book balance sheet at amortised cost. We sometimes manage their risk profile by using derivatives. As all derivatives are accounted for at fair value, the mismatch in their accounting treatment can lead to volatility in our Income Statement. This happens even if the derivative is an economic hedge of the asset or liability. BANKING MARKET RISK MANAGEMENT Risk appetite Our framework for dealing with market risk is part of our overall Risk Framework. The Structural and Banking Market Risk framework sets out our high-level arrangements and standards to manage, control and oversee banking market risk. Our Risk Appetite sets the controls, risk limits and key risk metrics for banking market risk. We articulate risk appetite by the income and value sensitivity limits we set in our Risk Appetite, at both Santander UK and Banco Santander group levels. Risk measurement For banking market risk, we mainly measure our exposures with NIM and EVE sensitivity analysis. We support this with VaR risk measures and stress testing. We also monitor our interest rate repricing gap. NIM and EVE sensitivities The calculations for NIM and EVE sensitivities involve many assumptions, including expected customer behaviour (such as early repayment of loans) and how interest rates may move. These assumptions are a key part of our overall control framework, so we update and review them regularly. Our NIM and EVE sensitivities include the interest rate risk from all our banking book positions. Our banking book positions generate almost all our reported net interest income. NIM sensitivity – NIM sensitivity is an income-based measure we use to forecast the changes to interest income and interest expense in different scenarios. It gives us a combined impact on net interest income over a given period – usually 12 or 36 months. – We calculate NIM sensitivity by simulating the NIM using two yield curves. The difference between the two NIM totals is the NIM sensitivity. – Our main model assumptions are that: – The balance sheet is dynamic. This means that it includes the run-off of current assets and liabilities as well as retained and new business – We use a behavioural balance sheet rather than contractual one. This means that we adjust balances for behavioural or assumed profile. We do this with most retail products whose behavioural maturity is different to the contractual maturity. This is usually because customers are exercising the option to withdraw or prepay early, or there is no contractual maturity, EVE sensitivity – We calculate EVE as the change in the net present value of all the interest rate sensitive items in the banking book balance sheet for a defined set of instantaneous parallel and non-parallel shifts in the yield curve. – We use a static balance sheet. This means that all balance sheet items run-off according to their contractual, behavioural or assumed run-off behaviour (whichever is appropriate), and there is no retained or new business. The limitations of sensitivities We use sensitivities to measure the impact of standard, instantaneous, parallel shifts in relevant yield curves. The advantage of using standard parallel shifts is they generally give us a constant measure of the size of our market risk exposure, with a simple and consistent stress. This compares to specific scenarios like ‘flat rates’. The magnitude of flat rates depends on the shape of the current curve and the shift required to reach the flat rate scenario. There is one exception to the relative simplicity of parallel shifts. In order to limit negative interest rates, the yield curve may be ‘floored’. Using material parallel shocks does not always seem realistic, or it might not necessarily test the scenarios that have the most impact on us. So we run non-parallel stress tests too, to calculate the impact of some plausible non-parallel scenarios, and over various time periods for income stresses, usually one or three years. VaR (audited) VaR – VaR indicates the losses that we might suffer because of unfavourable changes in the markets under normal (non-stressed) market conditions. – We run a historical simulation using the past two years of daily price moves, at a 99% confidence level, to find how much we might lose – the VaR. – For any given day’s position, we expect to suffer losses greater than the VaR estimate 1% of the time – once every 100 trading days, or two to three times a year. – This gives us a consistent way of assessing risk for all relevant market risk factors in our portfolios. The limitations of VaR VaR is a useful and important market standard measure of risk, but it does have some limitations. These include: – VaR assumes what happened in the past is a reliable way to predict what will happen in the future. This may not always be the case – VaR is based on positions at the end of the business day so it doesn’t include intra-day positions – VaR does not predict how big the loss could be on the 1% of trading days that it is greater than the VaR – Using a time horizon of one day means VaR does not tell us everything about exposures that we cannot liquidate or hedge within a day, or products with infrequent pricing. Back-testing – comparing VaR estimates with reality To check that the way we estimate VaR is reasonable, we back-test our VaR by comparing it against both actual and hypothetical profits and losses, using a one-day time horizon. Back-testing allows us to identify exceptions – times when the predictions were out of line with what happened. We can then look for trends in these exceptions, which can help us decide whether we need to recalibrate our VaR model. Other ways of measuring risk As well as using sensitivities and stress tests, we can measure banking market risk using net notional positions. This can give us a simple view of our exposure, although we generally need to combine it with other risk measures to cover all aspects of a risk profile, such as projected changes over time. Other metrics we can use include Earnings at Risk (EaR). Although VaR can be useful as it captures changes in economic values, as we describe above, VaR will not reflect the actual Income Statement impact of most of our banking book positions. This is because we account for them at amortised cost rather than fair value. EaR is like VaR but captures changes in income rather than value. We use this approach mainly to generate a one-year EaR measure to assess Basis risk. Stress testing Stress testing is an essential part of our risk management. It helps us to measure and evaluate the potential impact on portfolio values of more extreme, although plausible, events or market moves. We express limits as on how much we could lose in a stress event, and this restricts how much risk we take. Stress testing scenarios Simple stress tests (like parallel shifts in relevant curves) give us clear measures of risk control and a consistent starting point for setting limits. More complex, multi-factor and multi-time period stress tests can give us information about specific potential events. They can also test outcomes that we might not capture through parallel stresses or VaR-type measures because of data or model limitations. We can also use stress tests to estimate losses in extreme market events beyond the confidence level used in VaR models. We can adapt our stress tests to reflect current concerns such as Brexit, the Covid-19 pandemic and other macroeconomic events or changing market conditions quicker than we can with other risk measures, like VaR. We can include both individual business area stresses and Santander UK-wide scenarios. We can produce stress tests using either income or value measures. They cover one or more categories of exposures on an accruals basis or at fair value. We use expert judgement to define appropriate hypothetical stress tests and any adjusting assumptions based on the balance sheet, management actions and customer behaviour. How we use stress testing We discuss stress testing results at senior management committees. They affect Corporate Centre’s decisions by highlighting possible risks in the banking book and the effectiveness of remedial actions we could take. We compare stress test results with stress limits and triggers set by our internal committees, or against metrics set by the PRA. If the results are over our limits or triggers, we take remedial actions and follow an escalation process. Risk mitigation (audited) We mitigate Income Statement volatility mainly through hedge accounting. We monitor any hedge accounting ineffectiveness that might lead to Income Statement volatility with a VaR measure and trigger, reported monthly. For our accounting policies for derivatives and hedge accounting, see Note 1 to the Consolidated Financial Statements. We typically hedge the interest rate risk of the securities we hold for liquidity and investment purposes with interest rate swaps, retaining spread exposures. These retained exposures are the key drivers of the VaR and stress tests we use to assess the risk of the portfolio. We hedge our foreign currency funding positions back to sterling, so our foreign exchange positions tend to be residual exposures that remain after hedging. These positions could be, for example, to ‘spot’ foreign exchange rates on to cross currency basis. We monitor foreign exchange risk against absolute net exposures and VaR-based limits and triggers. For more on this, see ‘Funding strategy‘ and ‘Term issuance’ in the ‘Liquidity risk’ section. Risk monitoring and reporting (audited) We monitor the banking market risks of the securities we hold for liquidity and investment purposes using sensitivities, VaR and stress tests. We report them against limits and triggers to senior management daily and to ALCO and ERCC each month. The VaR we report captures all key sources of volatility (including interest rate and spread risks) to fully reflect the potential volatility. BANKING MARKET RISK REVIEW Interest rate risk Yield curve risk The table below shows how our base case income and valuation would be affected by a 50 basis point parallel shift (both up and down) applied instantaneously to the yield curve at 31 December 2020 and 31 December 2019. Sensitivity to parallel shifts represents the amount of risk in a way that we think is both simple and scalable. 50 basis points is the stress we typically focus on for banking market risk controls, although we also monitor sensitivities to other parallel and non-parallel shifts as well as scenarios. 2020 2019 +50bps -50bps +50bps -50bps £m £m £m £m NIM sensitivity (audited) 225 (15) 99 56 EVE sensitivity 367 (585) 10 (88) Spread risks The table below shows the risk metrics covering the portfolios of securities we hold for liquidity and investment purposes. 2020 2019 £m £m VaR 7 3 Worst three month stressed loss 93 102 TRADING MARKET RISK OUR KEY TRADING MARKET RISKS (AUDITED) Our main trading market risks are in Corporate & Investment Banking. They come from providing financial services to our customers. Our exposures are affected by market movements in interest rates, credit spreads, and foreign exchange. We do not have trading market risk in Retail Banking or Corporate & Commercial Banking. Trading market risk can reduce our net income. We only have a very small amount of trading market risk. This is from permitted products and permitted customers. We hedge risks from client trades and our books are as close to back-to-back as possible. Market risk is hedged with Banco Santander SA or CCPs. This is required by Banking Reform legislation. We have two trading desks. The Link Desk sells ring-fenced bank permissible products to clients. The Retail Structured Products desk (RSP) sells investments (Santander UK plc issued notes) to retail investors, through our UK branches and other channels. The Link Desk is exposed to the credit quality of our clients. We adjust valuations for this - Credit Valuation Adjustment (CVA), which feeds our valuations and hence income and expenses. The low market risk in our trading business means that CVA is the main driver of income movements, along with similar factors – Debt Valuation Adjustment (DVA) driven by our own credit, and Liquidity Valuation Adjustment (LVA) driven by the market price of liquidity. These valuation adjustments are collectively referred to as XVAs. We calculate market risk capital using standard rules. TRADING MARKET RISK MANAGEMENT Risk appetite Market risk is managed within our overall Risk Framework. The market risk framework sets our high level arrangements and standards for managing, controlling, and overseeing trading market risk. Our Risk Appetite for trading market risk is low. We only need to report a qualitative measure to the Board. We monitor trading market risk using stress measures. Risk measurement We have a range of ways of measuring trading market risk, including stress testing (explained in the Banking market risk management section above) and detailed sensitivity measures. Stress testing This is an essential part of our risk management. It helps us measure and evaluate the possible results of extreme, although plausible, events or market moves. We set limits on what we could lose in a stress event. This restricts how much risk we take. Stress testing scenarios We calculate the impact of 100 scenarios on our trading books every month. The scenarios we create may be inspired by past events, like the global financial crisis. They may include ways that unusual market conditions could happen. They include interest rates, equity prices and exchange rates. Most are reported against limits, and so could lead to our front office being asked to reduce risk. Our scenarios are not all calibrated to the same severity; some may be for a much longer holding period or a completely artificial and unrealistic scenario. We therefore do not limit all of them in the same way. How we use stress testing We use limits to manage how much we can lose in a crisis. This limits the risk we take. We make sure that plausible losses are below the Risk Appetite set by the Board. We report to senior management regularly at the Market & Structural Risk Control Forum. Risk mitigation (audited) We manage and control trading market risk within clear limits. There are specific levels that need escalation or action. This means we limit the impact of negative market movements. We keep the areas that create trading market risk separate from areas which control and oversee risk. Risk monitoring and reporting (audited) We maintain a complete set of written policies, procedures, and processes. These make sure we identify, assess, manage, and report trading market risk.

Liquidity risk

Liquidity risk	12 Months Ended
	Dec. 31, 2020
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Liquidity risk	Liquidity risk Overview Liquidity risk is the risk that we do not have the liquid financial resources to meet our obligations when they fall due, or we can only secure such resources at excessive cost. In this section, we describe our sources and uses of liquidity and how we manage liquidity risk. We also analyse our key liquidity metrics, including our LCRs and our eligible liquidity pools. We then explain our funding strategy and structure and we analyse our wholesale funding. Finally, we analyse how we have encumbered some of our assets to support our funding activities. Key metrics RFB DoLSub LCR of 150% (2019: 142%) Wholesale funding and AT1 with maturity <1 year £21.1bn (2019: £22.5bn) RFB DoLSub LCR eligible liquidity pool of £51.5bn (2019: £42.0bn) OUR KEY LIQUIDITY RISKS (AUDITED) Through our LRA framework, we manage our funding or structural contingent and market liquidity risks wherever they arise. This can be in retail and corporate deposit outflows, wholesale secured and unsecured liquidity outflows and off-balance sheet activities. Other risks our framework covers include funding concentrations, intra-day cash flows, intra-group commitments and support, and franchise retention. Our main sources of liquidity Customer deposits finance most of our customer lending. Although these funds are mostly callable, in practice they give us a stable and predictable core of funding. This is due to the nature of retail accounts and the breadth of our retail customer relationships. We have a strong wholesale funding investor base, diversified across product types and geographies. Through the wholesale markets, we have active relationships in many sectors including banks, other financial institutions, corporates and investment funds. We access the wholesale funding markets through the issuance of capital, senior unsecured debt, covered bonds, structured notes and short-term funding. We also access these markets through securitisations of certain assets of Santander UK plc and our operating subsidiaries. For more on our programmes, see Notes 14, 22 and 26 in the Consolidated Financial Statements. We generate funding on the strength of our own balance sheet, our own profitability and our own network of investors. In addition, we have access to UK Government funding schemes. We comply with rules set by the PRA, other regulators, and Banco Santander standards. While we manage, consolidate and monitor liquidity risk centrally, we also manage and monitor it in the business area it comes from. For more on our structural relationship with Banco Santander and how that impacts our liquidity management, see the Directors’ report. Our main uses of liquidity Our main uses of liquidity are to fund our lending in Retail Banking, Corporate & Commercial Banking and Corporate & Investment Banking, to pay interest and dividends, and to repay debt. Our ability to pay dividends depends on various factors. These include our regulatory capital needs, the level of our distributable reserves, and our financial performance. We also use liquidity to pay for business combinations. LIQUIDITY RISK MANAGEMENT Introduction We manage liquidity risk on a consolidated basis in our CFO division, which is our centralised function for managing funding, liquidity and capital. We created our governance, oversight and control frameworks, and our LRA, on the same consolidated basis. Under the PRA’s liquidity rules, Santander UK plc and its subsidiary Cater Allen Limited form the RFB Domestic Liquidity Sub-group (the RFB DoLSub), which allows the entities to collectively meet regulatory requirements for the purpose of managing liquidity risk. Each member of the RFB DoLSub will support the other by transferring surplus liquidity in times of stress. Risk appetite Our LRA statement is based on the principles of liquidity management we use to manage our balance sheet. It also supports our need to meet or exceed the rules of our regulators. In line with our liquidity management principles, we avoid an over-reliance on funding from a single product, customer or counterparty. We also maintain enough unencumbered customer assets to support current and future funding and collateral requirements and maintain enough capacity to monetise liquid assets and other counterbalancing capacity within an appropriate timeframe. Our LRA is proposed to the Risk division and the Board, which is then approved under advice from the Board Risk Committee. Our LRA, in the context of our overall Risk Appetite, is reviewed and approved by the Board each year, or more often if needed. Risk measurement We use a number of metrics to manage liquidity risk. These include metrics that show the difference between cash and collateral inflows and outflows in different periods. They also include structural metrics, such as our level of encumbered assets. Ongoing business management Within our framework of prudent funding and liquidity management, we manage our activities to minimise our liquidity risk. We have clear responsibilities for short-term funding, medium-term funding, encumbrance, collateral and liquid asset management. This ensures we manage liquidity risks as part of our daily operations, strategy and planning. Our liquidity management framework is split between short-term and strategic activities. Our short-term activities focus on intra-day collateral; management and maintaining liquid assets to cover unexpected demands on cash in a stress scenario (such as large and unexpected deposit withdrawals by customers and loss of wholesale funding). Our strategic activities focus on ensuring we are not over reliant on any one source for funding and that we avoid excessive concentrations in the maturity of our funding. We regularly test the liquidity of our eligible liquidity pool, in line with PRA and Basel rules. We do this by realising some of the assets through repurchase or outright sale to the market. We make sure that over any 12-month period we realise a significant part of our eligible liquidity pool. As well as our eligible liquidity pool, we always hold a portfolio of unencumbered liquid assets. Our LRA and PRA requirements determine the size and composition of this portfolio. These assets give us a source of contingent liquidity, as we can realise some of them in a time of stress to create liquidity through repurchase or outright sale to the market. Stress testing We have a liquidity stress test framework in place which is central to our LRA measurement and monitoring. It includes three severe but plausible stress test scenarios. To fit with our risk appetite, the liquidity outflows that come from these stress tests must be fully covered with high-quality liquid assets, other liquid assets and management actions sanctioned at the right level of governance. A funding plan disruption stress scenario also forms part of our LRA monitoring. Our Risk division runs a range of stress tests. Our LRA stress test is a combination of three tests that cover idiosyncratic, market-wide and combined scenarios. Our other tests consider scenarios such as a global economic slowdown that results in reduced confidence in the banking industry, a slowdown in one of the major economies or a deterioration in the availability of liquidity. These are considered on both an acute and protracted basis. We also run severe combined stress tests which look at both a deep and prolonged UK recession that results in a reduction in wholesale funding availability and a simultaneous idiosyncratic shock that would lead to retail and commercial outflows. In 2020, we added a new Covid-19 pandemic stress which excludes any UK government support. We also conduct sensitivity analysis and reverse stress testing for instant liquidity shocks by each key liquidity risk. We do this to understand the impacts they would have on our LRA and our regulatory liquidity metrics. We monitor our LCR to ensure we continue to meet the requirements. We also monitor the Net Stable Funding Ratio (NSFR), which is due to be implemented on 1 January 2022, and we expect to exceed any future requirements. Risk mitigation (audited) The Board aims to make our balance sheet resilient at all times and for it to be perceived as such by stakeholders. This preserves our short and long-term viability. The Board recognises that as we are involved in maturity transformation, we cannot hold enough liquidity to cover all possible stress scenarios. The Board requires us to hold enough liquidity to make sure we will survive three plausible but severe stress scenarios (our LRA stress). We do this by maintaining a prudent balance sheet structure and approved liquid resources. Recovery framework Our recovery plan sets out the risks we face, the indicators we use to monitor these risks and the actions available to mitigate a stress to liquidity or capital. We can therefore respond to a wide variety of stresses, from mild to severe, in a coordinated, effective and timely manner. We are mindful of our recovery capacity and monitor the headroom to recovery triggers. Recovery indicators are both qualitative and quantitative and are embedded into our management processes. If needed, recovery would be invoked early, in order to mitigate the effects of a stress and restore our financial position and balance sheet strength. Our recovery plan is approved by the Board under advice from the Board Audit Committee and is subject to ongoing review and enhancement. It is owned by the CFO and managed by the CFO division. Risk monitoring and reporting (audited) We monitor liquidity risk daily, weekly and monthly. We do this through different committees and levels of management, including ALCO and the Board Risk Committee. LIQUIDITY RISK REVIEW Liquidity Coverage Ratio This table shows our LCR and LRA at 31 December 2020 and 31 December 2019. The LRA data reflect the stress testing methodology in place at that time. LCR RFB DoLSub (1) LRA RFB (2) 2020 2019 2020 2019 £bn £bn £bn £bn Eligible liquidity pool (liquidity value) (3) 51.2 41.6 47.2 40.6 Net stress outflows (34.1) (29.3) (34.4) (31.7) Surplus 17.1 12.3 12.8 8.9 Eligible liquidity pool as a percentage of anticipated net cash flows 150 % 142 % 137 % 128 % (1) The RFB LCR was 152% (2019:146%). (2) The LRA is calculated for the Santander UK plc group (the RFB Group) and is a three-month Santander UK specific requirement. (3) The liquidity value is calculated as applying an applicable haircut to the carrying value. LCR eligible liquidity pool This table shows the carrying value of our eligible liquidity pool assets at 31 December 2020 and 31 December 2019. It also shows the weighted average carrying value in the year. RFB DoLSub Carrying value Weighted average carrying 2020 2019 2020 2019 Level 1 Level 2 Total Level 1 Level 2 Total Total Total £bn £bn £bn £bn £bn £bn £bn £bn Cash and balances at central banks 39.4 — 39.4 19.3 — 19.3 26.8 19.1 Government bonds 8.9 0.1 9.0 16.7 1.2 17.9 15.5 20.8 Supranational bonds and multilateral development banks 1.5 — 1.5 2.9 — 2.9 2.9 2.9 Covered bonds 1.0 — 1.0 1.4 0.1 1.5 1.2 2.4 Asset-backed securities — 0.6 0.6 — 0.4 0.4 0.6 1.4 50.8 0.7 51.5 40.3 1.7 42.0 47.0 46.6 Currency analysis This table shows the carrying value of our eligible liquidity pool by major currencies at 31 December 2020 and 31 December 2019. The composition of the pool is consistent with the currency profile of our net liquidity outflows. RFB DoLSub US Dollar Euro Sterling Other Total £bn £bn £bn £bn £bn 2020 2.0 1.2 48.1 0.2 51.5 2019 3.6 1.2 36.1 1.1 42.0 2020 compared to 2019 RFB DoLSub LCR at 150% reflects our prudent approach in an uncertain operating environment and is significantly above regulatory requirements. Although the key aim of the UK Government financial support measures introduced in 2020 (including the TFS and TFSME) was to limit damage to the wider economy from Covid-19, they had the side-effect of reducing any potential liquidity risks arising due to Covid-19. Although Covid-19 did not trigger a liquidity stress, its immediate negative impacts on liquidity, such as the drawing of committed credit and liquidity facilities, were largely offset by deposits from those same drawings as corporates reduced their spending. Similarly, the impact of the initial effective shutdown of the mortgage market and payment holidays granted to customers was offset by better than expected retail deposit balances as customers reduced their spending. FUNDING RISK MANAGEMENT Funding strategy Our funding strategy continues to be based on maintaining a conservatively structured balance sheet and diverse sources of funding to meet the needs of our business strategy and plans. The CFO Division maintains a funding plan and ensures it is compliant with the LRA and regulatory liquidity and capital requirements. Most of our funding comes from customer deposits. We source the rest from a mix of secured and unsecured funding in the wholesale markets. Overall, this means that we do not rely too heavily on wholesale funds. We manage funding requirements by targeting a specific Liquidity Coverage Ratio, we ensure maturities are prefunded and capital/TLAC requirements are prioritised. We also have checks and controls to limit our asset encumbrance from our secured funding operations. As part of maintaining a diverse funding base, we raise funding in a number of currencies, including euro and USD, and convert it into sterling through currency swaps to fund our commercial assets which are largely sterling denominated. Our base of stable retail and corporate deposits is a key funding source for us. We leverage our large and diverse customer base to offer products that give us a long-term sustainable source of funding. We do this by focusing on building long-term relationships. Over 85% of our total core retail customer liabilities are covered by the Financial Services Compensation Scheme (the FSCS). Behavioural maturities The contractual maturity of our balance sheet assets and liabilities highlights the maturity transformation that underpins the role of banks to lend long term, but to fund themselves mainly with shorter-term liabilities, like customer deposits. We do this by diversifying our funding operations across a wide customer base, both in numbers and by type of depositor. In practice, the behavioural profiles of many liabilities show more stability and longer maturity than their contractual maturity. This is especially true of many types of retail and corporate deposits that, while they may be repayable on demand or at short notice, have shown good stability even in times of stress. We model behaviour profiles using our experience of customer behaviour. We use this data to determine the funds transfer pricing interest rates at which we reward and charge our business units for sources and uses of funds. We apply this rate until a customer changes to a different product or service offered by us or by one of our competitors. We continue to maintain the quality of our retail, commercial and wholesale deposits. We aim to deepen our customer relationships across all customer segments. We do this to lengthen the contractual and behavioural profile of our liability base. Deposit funding We mainly fund our Retail Banking, Corporate & Commercial Banking and Corporate & Investment Banking activities by customer deposits. We fund the rest through wholesale markets. Wholesale funding Wholesale funding and issuance model Banco Santander is a multiple point of entry resolution group. This means that should it fail, it would be split up into parts. Healthy parts might be sold or be kept as a residual group without their distressed sister companies. The resolution or recapitalisation of the distressed parts might be effected via ‘bail in’ of bonds that had been issued to the market by a regional intermediate holding company. Santander UK is a single point of entry resolution group. This means that resolution would work downwards from the group’s holding company i.e. Santander UK Group Holdings plc. Losses in subsidiaries would first be transferred up to Santander UK Group Holdings plc. If the holding company is bankrupt as a result, the group is deemed to be failing or likely to fail, it will be put into resolution. The ‘bail in’ tool is applied to the holding company, with the equity being written off and bonds written off or converted into equity as needed to recapitalise the group. Those bondholders would become the new owners, and the group would stay together. Santander UK Group Holdings plc is the immediate holding company of Santander UK plc but does not guarantee its debts or other obligations. This structure is a Bank of England recommended configuration which aims to ensure the activities of the operating company are not disrupted as the Santander UK group goes through resolution, thereby maintaining continuity of services for customers. Composition of wholesale funding We are active in the wholesale markets and we have direct access to both money market and long-term investors through our funding programmes. This makes our wholesale funding well diversified by product, maturity, geography and currency. This includes currencies available across a range of channels from money markets, repo markets, senior unsecured, secured, medium-term and capital. For details of our main programmes, see the Funding Information section of our website www.santander.co.uk/uk/about-santander-uk/investor-relations/funding-information. Santander UK plc is our main operating company issuer of senior unsecured debt, structured notes, short-term funding and covered bonds. Our immediate parent Santander UK Group Holdings plc is the issuer of capital and MREL/Total Loss Absorbing Capacity (TLAC) eligible senior unsecured debt. Since the implementation of CRR II in June 2019, G-SIBs have been subject to the MREL standard. Since 1 January 2019, UK resolution entities that are G-SIBs or are part of a G-SIB, including our immediate parent Santander UK Group Holdings plc, have been required to meet the MREL minimum requirements, implemented through the Bank of England Statement of Policy on MREL in the UK. From 1 January 2020 the MREL requirement is the higher of (i) two times the Pillar 1 capital requirements and one times their Pillar 2A add-ons; (ii) 6% of CRR leverage exposures or (iii) two times the minimum leverage ratio requirement. The MREL requirements will be fully implemented from 1 January 2022 and G-SIBs will be required to meet to the higher of (i) two times the sum of Pillar 1 capital requirements and their Pillar 2A add-ons; (ii) 6.75% of CRR leverage exposures or (iii) two times the minimum leverage ratio requirement. The Companyis subject to internal MREL as it meets the requirements of a material subsidiary of our ultimate parent Banco Santander SA. We also access the wholesale markets through securitisations of certain assets of our operating subsidiaries. In addition, we have access to UK Government funding schemes. Eligible collateral for these schemes includes all collateral that is eligible in the Bank of England’s Discount Window Facility. We ensure that enough collateral is placed and available at the Discount Window. FUNDING RISK REVIEW Our funding strategy continues to be based on maintaining a conservatively structured balance sheet and diverse sources of funding to meet the needs of our business strategy and plans. The CFO Division maintains a funding plan and ensures it is compliant with the LRA and regulatory liquidity and capital requirements. 2020 compared to 2019 – Together with our immediate parent, Santander UK Group Holdings plc, our overall funding strategy remains to develop and sustain a diversified funding base. We also need to fulfil regulatory requirements as well as support our credit ratings. – 2020 presented unforeseen challenges in the debt capital markets with the Covid-19 pandemic causing significant volatility in March and April. However, despite the ongoing effect of Covid-19 on global economies and the growing concerns around a Brexit deal, the credit markets remained open, although at wider credit spreads. Following a rapid response from central banks to provide liquidity, notably the BoE’s Term Funding Scheme with additional incentives for SMEs (TFSME) in the UK, credit spreads retraced towards the end of the year. – In January 2020 it was estimated that the 2020 funding requirement was £10bn to £12bn. At the end of Q1 2020, and before the impact of Covid-19 on global markets, we had issued £4.6bn of medium-term funding. We issued a further £0.8bn in August 2020 as we continued on our plan to achieve the required end state MREL requirement in January 2022. The introduction of the TFSME scheme in March 2020 meant there was no further need to access markets for funding, as any residual requirement would be met by drawing down on our initial borrowing allowance from the BoE scheme. – In 2020 the total term funding was £5.4bn (2019: £4.5bn), of which £3.0bn was covered bonds and £2.4bn of senior unsecured notes. – We have £6.3bn outstanding under the TFS and £11.7bn outstanding under the TFSME. – Maturities in 2020 were £16.5bn (2019: £8.1bn). At 31 December 2020, 68% (2019: 67%) of wholesale funding had a maturity of greater than one year, with an overall residual duration of 38 months (2019: 33 months). – In 2020, c£4.3bn of medium-term funding was left in US Dollars and a further £0.7bn in Euro. These balances were used to fund customer assets and our HQLA portfolio. – Our level of encumbrance from external and internal issuance of securitisations and covered bonds was broadly static in 2020, as planned. Reconciliation of wholesale funding to the balance sheet (audited) This table reconciles our wholesale funding to our balance sheet at 31 December 2020 and 31 December 2019. . Balance sheet line item Funding Deposits by banks (3) Deposits by customers (1) Repurchase agreements - non trading Financial Debt Subordinated Other equity instruments (2) 2020 £bn £bn £bn £bn £bn £bn £bn £bn Deposits by banks — — — — — — — — Certificates of deposit and commercial paper 5.7 — — — — 5.7 — — Senior unsecured – public benchmark 15.4 — 7.8 — — 7.6 — — – privately placed 1.1 — 0.1 — 0.9 0.1 — — Covered bonds 17.9 — — — — 17.9 — — Securitisation and structured issuance 2.8 — — — 0.5 2.3 — — Term Funding Scheme 6.3 6.3 — — — — — — TFSME 11.7 11.7 — — — — — — Subordinated liabilities and equity 4.4 — — — — — 2.2 2.2 Total wholesale funding 65.3 18.0 7.9 — 1.4 33.6 2.2 2.2 Repos 15.8 — — 15.8 — — — — Foreign exchange and hedge accounting 2.5 — 0.2 — — 2.0 0.3 — Other 3.0 3.0 — — — — — — Balance sheet total 86.6 21.0 8.1 15.8 1.4 35.6 2.5 2.2 2019 Deposits by banks 0.3 0.3 — — — — — — Certificates of deposit and commercial paper 5.8 — — — — 5.8 — — Senior unsecured – public benchmark 18.8 — 8.6 — — 10.2 — – privately placed 2.7 — 0.1 — 1.0 1.6 — — Covered bonds 18.2 — — — — 18.2 — — Securitisation and structured issuance 5.6 — — 1.4 0.5 3.7 — — Term Funding Scheme 10.8 10.8 — — — — — — TFSME — — — — — — — — Subordinated liabilities and equity 5.2 — — — — — 3.0 2.2 Total wholesale funding 67.4 11.1 8.7 1.4 1.5 39.5 3.0 2.2 Repos 16.9 — — 16.9 — — — — Foreign exchange and hedge accounting 2.5 — 0.4 — — 1.6 0.5 — Other 3.5 3.3 — — 0.2 — — — Balance sheet total 90.3 14.4 9.1 18.3 1.7 41.1 3.5 2.2 (1) This is included in our balance sheet total of £195,135m (2019: £181,883m). (2) Consists of £nil (2019: £nil ) fixed/floating rate non-cumulative callable preference shares, £235m (2019: £235m) Step-up Callable Perpetual Reserve Capital Instruments and £1,956m (2019: £1,956m) Perpetual Capital Securities . See Notes 33 and 34 to the Consolidated Financial Statements. (3) Other consists of items in the course of transmission and other deposits, excluding the TFS. See Note 24 to the Consolidated Financial Statements. Maturity profile of wholesale funding (audited) This table shows our main sources of wholesale funding. It does not include securities finance agreements. The table is based on exchange rates at issue and scheduled repayments and call dates. It does not reflect the final contractual maturity of the funding. ≤ 1 >1 and ≤ 3 months >3 and ≤ 6 months >6 and ≤ 9 months >9 and ≤ 12 months Sub-total >1 and >2 and >5 years Total 2020 £bn £bn £bn £bn £bn £bn £bn £bn £bn £bn Downstreamed from Santander UK Group Holdings plc to Santander UK plc (1) Senior unsecured – public benchmark 0.7 — — 1.1 — 1.8 1.2 4.8 1.3 9.1 – privately placed — — — — — — — — 0.1 0.1 Subordinated liabilities and equity (incl. AT1) — — — — — — 0.8 1.5 0.4 2.7 0.7 — — 1.1 — 1.8 2.0 6.3 1.8 11.9 Other Santander UK plc Certificates of deposit and commercial paper 0.7 3.7 1.2 — 0.1 5.7 — — — 5.7 Senior unsecured – public benchmark 0.4 — 1.6 — 0.8 2.8 0.6 2.6 0.3 6.3 – privately placed — 0.3 — — 0.1 0.4 — 0.2 0.4 1.0 Covered bonds — — 3.7 1.7 — 5.4 1.7 6.4 4.4 17.9 Securitisation & structured issuance (2) 0.2 — — 0.2 — 0.4 0.9 0.3 — 1.6 Term Funding Scheme — 1.5 1.5 1.0 — 4.0 2.3 — — 6.3 TFSME — — — — — — — 11.7 — 11.7 Subordinated liabilities — — — — — — — 0.5 1.2 1.7 1.3 5.5 8.0 2.9 1.0 18.7 5.5 21.7 6.3 52.2 Other group entities Securitisation & structured issuance (3) 0.2 0.1 — 0.1 0.2 0.6 0.4 0.2 — 1.2 Total at 31 December 2020 2.2 5.6 8.0 4.1 1.2 21.1 7.9 28.2 8.1 65.3 Of which: – Secured 0.4 1.6 5.2 3.0 0.2 10.4 5.3 18.6 4.4 38.7 – Unsecured 1.8 4.0 2.8 1.1 1.0 10.7 2.6 9.6 3.7 26.6 2019 Total at 31 December 2019 1.7 5.5 4.7 2.1 8.5 22.5 16.6 18.5 9.8 67.4 Of which: – Secured 0.2 0.1 2.7 0.3 5.8 9.1 11.5 10.4 3.6 34.6 – Unsecured 1.5 5.4 2.0 1.8 2.7 13.4 5.1 8.1 6.2 32.8 (1) 95% of Senior Unsecured debt issued from Santander UK Group Holdings plc has been downstreamed to Santander UK plc as ‘secondary non-preferential debt’ in line with the guidelines from the Bank of England for Internal MREL. (2) Includes funding from mortgage-backed securitisation vehicles where Santander UK plc is the asset originator. (3) Includes funding from asset-backed securitisation vehicles where entities other than Santander UK plc are the asset originator. Currency composition of wholesale funds (audited) This table shows our wholesale funding by major currency at 31 December 2020 and 31 December 2019. 2020 2019 Sterling US Dollar Euro Other Sterling US Dollar Euro Other % % % % % % % % Downstreamed from Santander UK Group Holdings plc to Santander UK plc Senior unsecured – public benchmark 10 62 28 — 11 65 22 2 – privately placed — — — 100 — — — 100 Subordinated liabilities and equity (incl. AT1) 73 27 — — 67 33 — — 24 53 22 1 25 56 16 3 Other Santander UK plc Deposits by banks — — — — 3 97 — — Certificates of deposit and commercial paper 51 44 4 1 45 54 1 — Senior unsecured – public benchmark 10 73 17 — 14 54 32 — – privately placed 41 37 10 12 21 15 59 5 Covered bonds 48 5 46 1 54 — 45 1 Securitisation & structured issuance 77 23 — — 72 28 — — Term Funding Scheme 100 — — — 100 — — — TFSME 100 — — — — — — — Subordinated liabilities 63 37 — — 49 51 — — 63 18 19 — 54 22 24 — Other group entities Securitisation & structured issuance 100 — — — 95 5 — — Total 57 24 19 — 50 27 22 1 Term issuance (audited) In 2020, our external term issuance (sterling equivalent) was: Sterling US Dollar Euro Total 2020 Total 2019 £bn £bn £bn £bn £bn Downstreamed from Santander UK Group Holdings plc to Santander UK plc Senior unsecured – public benchmark — 0.8 0.6 1.4 — Subordinated debt and equity (inc. AT1) — — — — 0.5 — 0.8 0.6 1.4 0.5 Other Santander UK plc Covered bonds 1.0 0.9 1.1 3.0 2.9 Senior unsecured – public benchmark — 1.0 — 1.0 0.9 TFSME 11.7 — — 11.7 — 12.7 1.9 1.1 15.7 3.8 Other group entities Securitisations — — — — 0.2 Total gross issuances 12.7 2.7 1.7 17.1 4.5 We have encumbered an asset if we have pledged or transferred it as collateral against an existing liability. This means it is no longer available to secure funding, meet our collateral needs or be sold to reduce future funding needs. Being able to pledge or transfer assets as collateral is an integral part of a financial institution’s operations. We do various things that lead to asset encumbrance. These include where we: – Enter into securitisation, covered bonds, and repurchase agreements (including central bank programmes) to access medium and long-term funding – Enter into short-term funding transactions. These include repurchase agreements and stock borrowing transactions as part of our operational liquidity management – Pledge collateral as part of participating in payment and settlement systems – Post collateral as part of derivatives activity. We monitor our mix of secured and unsecured funding sources in our funding plan. We aim to use our available collateral efficiently to raise secured funding and to meet our other collateralised obligations. Our biggest source of encumbrance is where we use our mortgage portfolio to raise funds through securitisation, covered bonds or other structured borrowing. We control our levels of encumbrance from these by setting a minimum level of unencumbered assets that must be available after we factor in our future funding plans, whether we can use our assets for our future collateral needs, the impact of a possible stress and our current level of encumbrance. Assets classified as readily available for encumbrance include cash and securities we hold in our eligible liquidity pool. They also include other unencumbered assets that give us a source of contingent liquidity. We do not rely on these extra unencumbered assets in our LRA, but we might use some of them in a time of stress. We can create liquidity by using them as collateral for secured funding or through outright sale. Loans and advances to customers are only classified as readily available for encumbrance if they are already in a form we can use to raise funding without any other actions on our part. This includes excess collateral that is already in a secured funding structure. It also includes collateral that is pre-positioned at central banks and is available for use in secured funding. All other loans and advances are classified as not readily available for encumbrance, however, may still be suitable for use in secured funding structures. Encumbrance of customer loans and advances We have issued prime retail mortgage-backed and other asset-backed securitised products to a diverse investor base through our mortgage-backed and other asset-backed funding programmes. We have raised funding with mortgage-backed notes, both issued to third parties and retained – the latter being central bank eligible collateral for funding purposes in other Bank of England facilities. We also have a covered bond programme, under which we issue securities to investors secured by a pool of residential mortgages. For more on how we have issued notes from our secured programmes externally and also retained them, and what we have used them for, see Notes 14 and 26 to the Consolidated Financial Statements in the 2020 Annual Report. On-balance sheet encumbered and unencumbered assets (audited) Encumbered with counterparties other than central banks Unencumbered assets not pre-positioned with central banks Covered Securitis- Other Total Assets positioned at central banks (3) Readily Other Cannot be Total Total 2020 £m £m £m £m £m £m £m £m £m £m Cash and balances at central banks (1)(2) — — 985 985 854 39,411 — — 40,265 41,250 Financial assets at FVTPL: – Derivative financial instruments — — — — — — — 3,406 3,406 3,406 – Other financial assets at FVTPL — — — — — — — 208 208 208 Financial assets at amortised cost: – Loans and advances to customers 23,669 7,469 184 31,322 61,292 74,758 19,801 21,577 177,428 208,750 – Loans and advances to banks — — 804 804 — — — 878 878 1,682 – Repurchase agreements – non trading — — — — — — — 19,599 19,599 19,599 – Other financial assets at amortised cost — — 648 648 — 515 — — 515 1,163 Financial assets at FVOCI — — 5,581 5,581 — 3,369 — — 3,369 8,950 Interests in other entities — — — — — — — 172 172 172 Intangible assets — — — — — — — 1,646 1,646 1,646 Property, plant and equipment — — — — — — 1,734 — 1,734 1,734 Current tax assets — — — — — — — 264 264 264 Retirement benefit assets — — — — — — — 495 495 495 Other assets — — — — — — — 3,013 3,013 3,013 Total assets 23,669 7,469 8,202 39,340 62,146 118,053 21,535 51,258 252,992 292,332 2019 Cash and balances at central banks (1)(2) — — 1,080 1,080 707 19,393 — — 20,100 21,180 Financial assets at FVTPL: – Derivative financial instruments — — — — — — — 3,316 3,316 3,316 – Other financial assets at FVTPL — — — — — — — 386 386 386 Financial assets at amortised cost: – Loans and advances to customers 23,

Accounting Policies

Accounting Policies	12 Months Ended
	Dec. 31, 2020
Disclosure of significant accounting policies [Abstract]
Accounting Policies	1. ACCOUNTING POLICIES These financial statements are prepared for Santander UK plc (the Company) and the Santander UK plc group (the Santander UK group) under the UK Companies Act 2006. The principal activity of the Santander UK group is the provision of a wide range of banking and financial services to personal, business and corporate customers. Santander UK plc is a public company, limited by shares and incorporated in England and Wales having a registered office at 2 Triton Square, Regent’s Place, London, NW1 3AN, phone number 0870-607-6000. It is an operating company undertaking banking and financial services transactions Basis of preparation These financial statements incorporate the financial statements of the Company and entities controlled by the Company (its subsidiaries) made up to 31 December each year. The consolidated financial statements have been prepared on the going concern basis using the historical cost convention, except for financial assets and liabilities that have been measured at fair value. An assessment of the appropriateness of the adoption of the going concern basis of accounting is disclosed in the statement of going concern in the Directors’ Report. Compliance with International Financial Reporting Standards The consolidated financial statements of the Santander UK group and the separate financial statements of the Company comply with international accounting standards in conformity with the requirements of the Companies Act 2006 and have also applied international financial reporting standards adopted pursuant to Regulation (EC) No 1606/2002 as it applies in the European Union. The financial statements are also prepared in accordance with International Financial Reporting Standards (IFRSs) as issued by the International Accounting Standards Board (IASB), including interpretations issued by the IFRS Interpretations Committee, as there are no applicable differences from IFRSs as issued by the IASB for the periods presented. Disclosures required by IFRS 7 ‘Financial Instruments: Disclosure’ relating to the nature and extent of risks arising from financial instruments, and IAS 1 ‘Presentation of Financial Statements’ relating to objectives, policies and processes for managing capital, can be found in the Risk review. Those disclosures form an integral part of these financial statements. Recent accounting developments Interest Rate Benchmark Reform In September 2019, the IASB issued ‘Interest Rate Benchmark Reform: Amendments to IFRS 9, IAS 39 and IFRS 7’. The Santander UK group applies IAS 39 hedge accounting so the amendments to IFRS 9 do not apply. The IAS 39 amendments apply to all hedging relationships directly affected by uncertainties related to interbank offered rate (IBOR) reform and must be applied for annual periods beginning on or after 1 January 2020. Following their endorsement for use in the European Union, the Santander UK group adopted the IAS 39 and IFRS 7 amendments in the preparation of the financial statements for the year ended 31 December 2019. The exceptions given by the IAS 39 amendments mean that IBOR reform had no impact on hedge relationships for affected hedges. In August 2020, the IASB issued ‘Interest Rate Benchmark Reform – Phase 2 - Amendments to IFRS 9, IAS 39, IFRS 7, IFRS 4 and IFRS 16’. These amendments apply only to changes required by IBOR reform to financial instruments and hedging relationships. The amendments are effective from 1 January 2021 and must be applied retrospectively without restating comparative information. Following their endorsement for use in the European Union and the UK, the Santander UK group has elected to apply the amendments in the preparation of these financial statements. The amendments address the accounting issues for financial instruments when IBOR reform is implemented including providing a practical expedient for changes to contractual cash flows, giving relief from specific hedge accounting requirements, and specifying a number of additional disclosures to enable users of financial statements to understand the effect of IBOR reform on an entity’s financial instruments and risk management strategy. Further details of the impact of these amendments on the financial statements for the year ended 31 December 2020 and the additional disclosures required are provided in Note 43. Other changes The Santander UK group adopted IFRS 16 and amendments to IAS 12 in 2019 and adopted IFRS 9 in 2018, with the impact included in the statement of changes in equity for that year end. Future accounting developments At 31 December 2020, for the Santander UK group, there were no significant new or revised standards and interpretations, and amendments thereto, which have been issued but which are not yet effective or which have otherwise not been early adopted where permitted. Comparative information As required by US public company reporting requirements, these financial statements include two years of comparative information for the consolidated income statement, consolidated statement of comprehensive income, consolidated statement of changes in equity, consolidated statement of cash flows and related Notes. Consolidation a) Subsidiaries The Consolidated Financial Statements incorporate the financial statements of the Company and entities (including structured entities) controlled by it and its subsidiaries. Control is achieved where the Company (i) has power over the investee; (ii) is exposed, or has rights, to variable returns from its involvement with the investee; and (iii) has the ability to use its power to affect its returns. The Company reassesses whether or not it controls an investee if facts and circumstances indicate that there are changes to one or more of the three elements of control listed above. When the Company has less than a majority of the voting rights of an investee, it has power over the investee when the voting rights are sufficient to give it the practical ability to direct the relevant activities of the investee unilaterally. The Company considers all relevant facts and circumstances in assessing whether or not the Company's voting rights in an investee are sufficient to give it power, including: – The size of the Company's holding of voting rights relative to the size and dispersion of holdings of the other vote holders – Potential voting rights held by the Company, other vote holders or other parties – Rights arising from other contractual arrangements – Any additional facts and circumstances that indicate that the Company has, or does not have, the current ability to direct the relevant activities at the time that decisions need to be made, including voting patterns at previous shareholders' meetings. Consolidation of a subsidiary begins when the Company obtains control over the subsidiary and ceases when the Company loses control of the subsidiary. Specifically, the results of a subsidiary acquired or disposed of during the year are included in the consolidated income statement and the consolidated statement of comprehensive income from the date the Company gains control until the date when the Company ceases to control the subsidiary. Inter-company transactions, balances and unrealised gains on transactions between Santander UK group companies are eliminated; unrealised losses are also eliminated unless the cost cannot be recovered. The acquisition method of accounting is used to account for the acquisition of subsidiaries which meet the definition of a business. The cost of an acquisition is measured at the fair value of the assets given up, shares issued or liabilities undertaken at the date of acquisition. Acquisition-related costs are expensed as incurred. The excess of the cost of acquisition, as well as the fair value of any interest previously held, over the fair value of the Santander UK group’s share of the identifiable net assets of the subsidiary at the date of acquisition is recorded as goodwill. When the Santander UK group loses control of a subsidiary, the profit or loss on disposal is calculated as the difference between (i) the aggregate of the fair value of the consideration received and the fair value of any retained interest and (ii) the previous carrying amount of the assets (including goodwill), less liabilities of the subsidiary and any non-controlling interests. Amounts previously recognised in other comprehensive income in relation to the subsidiary are accounted for (i.e. reclassified to profit or loss or transferred directly to retained earnings) in the same manner as would be required if the relevant assets or liabilities are disposed of. The fair value of any investment retained in a former subsidiary at the date when control is lost is regarded as the fair value on initial recognition for subsequent accounting under IFRS 9 or, when applicable, the costs on initial recognition of an investment in an associate or joint venture. Business combinations between entities under common control (i.e. fellow subsidiaries of Banco Santander SA, the ultimate parent) are outside the scope of IFRS 3 – ‘Business Combinations’, and there is no other guidance for such transactions under IFRS. The Santander UK group elects to account for business combinations between entities under common control at their book values in the acquired entity by including the acquired entity’s results from the date of the business combination and not restating comparatives. Reorganisations of entities within the Santander UK group are also accounted for at their book values. Interests in subsidiaries are eliminated during the preparation of the Consolidated Financial Statements. Interests in subsidiaries in the Company unconsolidated financial statements are held at cost subject to impairment. Credit protection entities established as part of significant risk transfer (SRT) transactions are not consolidated by the Santander UK group in cases where third party investors have the exposure, or rights, to all of the variability of returns from the performance of the entities. b) Joint ventures Joint ventures are joint arrangements whereby the parties that have joint control of the arrangement have rights to its net assets. Joint control is the contractually agreed sharing of control of an arrangement, which exists only when decisions about the relevant activities require unanimous consent of the parties sharing control. Accounting policies of joint ventures have been aligned to the extent there are differences from the Santander UK group’s policies. Investments in joint ventures are accounted for by the equity method of accounting and are initially recorded at cost and adjusted each year to reflect the Santander UK group’s share of their post-acquisition results. When the Santander UK group's share of losses of a joint venture exceed its interest in that joint venture, the Santander UK group discontinues recognising its share of further losses. Further losses are recognised only to the extent that the Santander UK group has incurred legal or constructive obligations or made payments on behalf of the joint venture. . Foreign currency translation Items included in the financial statements of each entity in the Santander UK group are measured using the currency that best reflects the economic substance of the underlying events and circumstances relevant to that entity (the functional currency). The Consolidated Financial Statements are presented in sterling, which is the functional currency of the Company. Income statements and cash flows of foreign entities are translated into the Santander UK group’s presentation currency at average exchange rates for the year and their balance sheets are translated at the exchange rates ruling on 31 December. Exchange differences on the translation of the net investment in foreign entities are recognised in other comprehensive income. When a foreign entity is sold, such exchange differences are recognised in the income statement as part of the gain or loss on sale. Foreign currency transactions are translated into the functional currency of the entity involved at the exchange rates prevailing at the dates of the transactions. Foreign exchange gains and losses resulting from the settlement of such transactions and from the translation of monetary assets and liabilities denominated in foreign currencies are recognised in the income statement unless recognised in other comprehensive income in connection with a cash flow hedge. Non-monetary items denominated in a foreign currency measured at historical cost are not retranslated. Exchange rate differences arising on non-monetary items measured at fair value are recognised in the consolidated income statement except for differences arising on equity securities measured at fair value through other comprehensive income (FVOCI), which are recognised in other comprehensive income. Revenue recognition a) Interest income and expense Interest and similar income comprises interest income on financial assets measured at amortised cost, investments in debt instruments measured at FVOCI and interest income on hedging derivatives. Interest expense and similar charges comprises interest expense on financial liabilities measured at amortised cost, and interest expense on hedging derivatives. Interest income on financial assets measured at amortised cost, investments in debt instruments measured at FVOCI and interest expense on financial liabilities other than those at fair value through profit or loss (FVTPL) is determined using the effective interest rate method. The effective interest rate is the rate that discounts the estimated future cash payments or receipts over the expected life of the instrument or, when appropriate, a shorter period, to the gross carrying amount of the financial asset (i.e. its amortised cost before any impairment allowance) or to the amortised cost of a financial liability. When calculating the effective interest rate, the future cash flows are estimated after considering all the contractual terms of the instrument excluding expected credit losses. The calculation includes all amounts paid or received by the Santander UK group that are an integral part of the overall return, direct incremental transaction costs related to the acquisition, issue or disposal of the financial instrument and all other premiums or discounts. Interest income is calculated by applying the effective interest rate to the gross carrying amount of financial assets, except for financial assets that have subsequently become credit-impaired (or ‘Stage 3’), for which interest revenue is calculated by applying the effective interest rate to their amortised cost (i.e. net of the ECL provision). For more information on stage allocations of credit risk exposures, see ‘Significant increase in credit risk’ in the ‘Santander UK group level – credit risk management’ section of the Risk Review. During Q4 2020, we revised the accounting treatment for certain items of mortgage income. Mortgage account fees, which are normally paid at the end of the mortgage and were previously recognised as received in fee income, are now recognised in interest income as part of the effective interest rate method throughout the life of the mortgage to better reflect the requirements of IFRS. In addition, we no longer accrue interest income relating to the period after mortgages revert to the standard variable rate (or equivalent) beyond the incentive period. This better aligns our policy to current practice. These changes resulted in an increase in net interest income of £44m for 2020. The net impact of these changes is not material and comparatives have not been restated. b) Fee and commission income and expense Fees and commissions that are not an integral part of the effective interest rate are recognised when the service is performed. Most fee and commission income is recognised at a point in time. Certain commitment, upfront and management fees are recognised over time but are not material. For retail and corporate products, fee and commission income consists principally of collection services fees, commission on foreign currencies, commission and other fees received from retailers for processing credit card transactions, fees received from other credit card issuers for providing cash advances for their customers through the Santander UK group’s branch and ATM networks, annual fees payable by credit card holders and fees for non-banking financial products. For insurance products, fee and commission income consists principally of commissions and profit share arising from the sale of building and contents insurance and life protection insurance. Commissions arising from the sale of buildings and contents insurance are recognised over the period of insurance cover, adjusted to take account of cancelled policies. Profit share income from the sale of buildings and contents insurance which is not subject to any adjustment is recognised when the profit share income is earned. Commissions and profit share arising from the sale of life protection insurance is subject to adjustment for cancellations of policies within 3 years from inception. Fee and commission income which forms an integral part of the effective interest rate of a financial instrument (for example certain loan commitment fees) is recognised as an adjustment to the effective interest rate and recorded in ‘Interest income’. c) Dividend income Except for equity securities classified as trading assets or financial assets held at fair value through profit or loss, described below, dividend income is recognised when the right to receive payment is established. This is the ex-dividend date for equity securities. d) Other operating income Other operating income includes all gains and losses from changes in the fair value of financial assets and liabilities held at fair value through profit or loss (comprising financial assets and liabilities held for trading, trading derivatives and other financial assets and liabilities at fair value through profit or loss), together with related interest income, expense, dividends and changes in fair value of any derivatives managed in conjunction with these assets and liabilities. Changes in fair value of derivatives in a fair value hedging relationship are also recognised in other operating income. Other operating income also includes income from operating lease assets, and profits and losses arising on the sales of property, plant and equipment and subsidiary undertakings. Borrowing costs Borrowing costs directly attributable to the acquisition, construction or production of qualifying assets, including computer software, which are assets that necessarily take a substantial period of time to develop for their intended use, are added to the cost of those assets, until the assets are substantially ready for their intended use. All other borrowing costs are recognised in profit or loss in the period in which they occur. Pensions and other post-retirement benefits a) Defined benefit schemes A defined benefit scheme is a pension scheme that guarantees an amount of pension benefit to be provided, usually as a function of one or more factors such as age, years of service or compensation. Pension costs are charged to ‘Administration expenses’, within the line item ‘Operating expenses before impairment losses, provisions and charges’ with the net interest on the defined benefit asset or liability included within ‘Net interest income’ in the income statement. The asset or liability recognised in respect of defined benefit pension schemes is the present value of the defined benefit obligation at the balance sheet date, less the fair value of scheme assets. The defined benefit obligation is calculated annually by independent actuaries using the projected unit credit method. The assets of the schemes are measured at their fair values at the balance sheet date. The present value of the defined benefit obligation is estimated by projecting forward the growth in current accrued pension benefits to reflect inflation and salary growth to the date of pension payment, then discounted to present value using the yield applicable to high-quality AA rated corporate bonds of the same currency and which have terms to maturity closest to the terms of the scheme liabilities, adjusted where necessary to match those terms. In determining the value of scheme liabilities, demographic and financial assumptions are made by management about life expectancy, inflation, discount rates, pension increases and earnings growth, based on past experience and future expectations. Financial assumptions are based on market conditions at the balance sheet date and can generally be derived objectively. Demographic assumptions require a greater degree of estimation and judgement to be applied to externally derived data. Any surplus or deficit of scheme assets over liabilities is recognised in the balance sheet as an asset (surplus) or liability (deficit). An asset is only recognised to the extent that the surplus can be recovered through reduced contributions in the future or through refunds from the scheme. The income statement includes the net interest income/expense on the net defined benefit liability/asset, current service cost and any past service cost and gain or loss on settlement. Remeasurement of defined benefit pension schemes, including return on scheme assets (excludes amounts included in net interest), actuarial gains and losses (arising from changes in demographic assumptions, the impact of scheme experience and changes in financial assumptions) and the effect of the changes to the asset ceiling (if applicable), are recognised in other comprehensive income. Remeasurement recognised in other comprehensive income will not be reclassified to the income statement. Past-service costs are recognised as an expense in the income statement at the earlier of when the scheme amendment or curtailment occurs and when the related restructuring costs or termination benefits are recognised. Curtailments include the impact of significant reductions in the number of employees covered by a scheme, or amendments to the terms of the scheme so that a significant element of future service will no longer qualify for benefits or will qualify only for reduced benefits. Curtailment gains and losses on businesses that meet the definition of discontinued operations are included in profit or loss for the year from discontinued operations. Gains and losses on settlements are recognised when the settlement occurs. b) Defined contribution plans A defined contribution plan is a pension scheme under which the Santander UK group pays fixed contributions as they fall due into a separate entity (a fund). The pension paid to the member at retirement is based on the amount in the separate fund for each member. The Santander UK group has no legal or constructive obligations to pay further contributions into the fund to ‘top up’ benefits to a certain guaranteed level. The regular contributions constitute net periodic costs for the year in which they are due and are included in staff costs within Operating expenses in the income statement. c) Post-retirement medical benefit plans Post-retirement medical benefit liabilities are determined using the projected unit credit method, with actuarial valuations updated at each year-end. The expected benefit costs are accrued over the period of employment using an accounting methodology similar to that for the defined benefit pension scheme. Share-based payments The Santander UK group engages in cash-settled and equity-settled share-based payment transactions in respect of services received from certain of its employees. Shares of the Santander UK group’s parent, Banco Santander SA are purchased in the open market by the Santander UK group (for the Employee Sharesave scheme) or are purchased by Banco Santander SA or another Banco Santander subsidiary (including awards granted under the Long-Term Incentive Plan and the Deferred Shares Bonus Plan) to satisfy share options or awards as they vest. Options granted under the Employee Sharesave scheme are accounted for as cash-settled share-based payment transactions. Awards granted under the Long-Term Incentive Plan and Deferred Shares Bonus Plan are accounted for as equity-settled share-based payment transactions. The fair value of the services received is measured by reference to the fair value of the shares or share options initially on the date of the grant for both the cash and equity settled share-based payments and then subsequently at each reporting date for the cash-settled share-based payments. The cost of the employee services received in respect of the shares or share options granted is recognised in the income statement in administration expenses over the period that the services are received i.e. the vesting period. A liability equal to the portion of the services received is recognised at the fair value determined at each balance sheet date for cash-settled share-based payments. A liability equal to the amount to be reimbursed to Banco Santander SA is recognised at the fair value determined at the grant date for equity-settled share-based payments. The fair value of the options granted under the Employee Sharesave scheme is determined using an option pricing model, which takes into account the exercise price of the option, the current share price, the risk free interest rate, the expected volatility of the Banco Santander SA share price over the life of the option and the dividend growth rate. The fair value of the awards granted for the Long-Term Incentive Plan was determined at the grant date using an option pricing model, which takes into account the share price at grant date, the risk free interest rate, the expected volatility of the Banco Santander SA share price over the life of the award and the dividend growth rate. Vesting conditions included in the terms of the grant are not taken into account in estimating fair value, except for those that include terms related to market conditions. Non-market vesting conditions are taken into account by adjusting the number of shares or share options included in the measurement of the cost of employee service so that, ultimately, the amount recognised in the income statement reflects the number of vested shares or share options. Where vesting conditions are related to market conditions, the charges for the services received are recognised regardless of whether or not the market–related vesting conditions are met, provided that the non-market vesting conditions are met. Where an award has been modified, as a minimum, the expense of the original award continues to be recognised as if it had not been modified. Where the effect of a modification is to increase the fair value of an award or increase the number of equity instruments, the incremental fair value of the award or incremental fair value of the modification of the award is recognised in addition to the expense of the original grant, measured at the date of modification, over the modified vesting period. Cancellations in the vesting period are treated as an acceleration of vesting, and recognised immediately for the amount that would otherwise have been recognised for services over the vesting period. Goodwill and other intangible assets Goodwill represents the excess of the cost of an acquisition, as well as the fair value of any interest previously held, over the fair value of the share of the identifiable net assets of the acquired subsidiary, associate, or business at the date of acquisition. Goodwill on the acquisition of subsidiaries and businesses is included in intangible assets. Goodwill on acquisitions of associates is included as part of investment in associates. Goodwill is tested for impairment annually, or more frequently when events or changes in circumstances dictate, and carried at cost less accumulated impairment losses. Gains and losses on the disposal of an entity or business include the carrying amount of goodwill relating to the entity or business sold. Other intangible assets are recognised if they arise from contractual or other legal rights or if they are capable of being separated or divided from Santander UK and sold, transferred, licensed, rented or exchanged. The value of such intangible assets is amortised on a straight-line basis over their useful economic life of three to seven years. Other intangible assets are reviewed annually for impairment indicators and tested for impairment where indicators are present. Software development costs are capitalised when they are direct costs associated with identifiable and unique software products that are expected to provide future economic benefits and the cost of those products can be measured reliably. These costs include payroll, materials, services and directly attributable overheads. Internally developed software meeting these criteria and externally purchased software are classified in intangible assets on the balance sheet and amortised on a straight-line basis over their useful life of three to seven years, unless the software is an integral part of the related computer hardware, in which case it is treated as property, plant and equipment as described below. Capitalisation of costs ceases when the software is capable of operating as intended. Costs of maintaining software are expensed as incurred. Property, plant and equipment Property, plant and equipment include owner-occupied properties (including leasehold properties), office fixtures and equipment and computer software. Property, plant and equipment also includes operating leases where the Santander UK group is the lessor and right-of-use assets where the Santander UK group is the lessee, as described further in ‘Leases’ below. Property, plant and equipment are carried at cost less accumulated depreciation and accumulated impairment losses. A review for indications of impairment is carried out at each reporting date. Gains and losses on disposal are determined by reference to the carrying amount and are reported in other operating income. Repairs and renewals are charged to the income statement when the expenditure is incurred. Internally developed software meeting the criteria set out in ‘Goodwill and other intangible assets’ above and externally purchased software are classified in property, plant and equipment where the software is an integral part of the related computer hardware (for example operating system of a computer). Classes of property, plant and equipment are depreciated on a straight-line basis over their useful life, as follows: Owner-occupied properties Not exceeding 50 years Office fixtures and equipment 3 to 15 years Computer software 3 to 7 years Right-of-use assets (see ‘Leases – The Santander UK group as lessee’ below) Shorter of the lease term or the useful life of the underlying asset Depreciation is not charged on freehold land and assets under construction. Depreciation on operating lease assets where the Santander UK group is the lessor is described in 'Leases' below. Financial instruments a) Initial recognition and measurement Financial assets and liabilities are initially recognised when the Santander UK group becomes a party to the contractual terms of the instrument. The Santander UK group determines the classification of its financial assets and liabilities at initial recognition and measures a financial asset or financial liability at its fair value plus or minus, in the case of a financial asset or financial liability not at FVTPL, transaction costs that are incremental and directly attributable to the acquisition or issue of the financial asset or financial liability. Transaction costs of financial assets and financial liabilities carried at FVTPL are expensed in profit or loss. Immediately after initial recognition, an expected credit loss (ECL) allowance is recognised for financial assets measured at amortised cost and investments in debt instruments measured at FVOCI. A regular way purchase is a purchase of a financial asset under a contract whose terms require delivery of the asset within the timeframe established generally by regulation or convention in the market place concerned. Regular way purchases of financial assets classified as loans and receivables, issues of equity or financial liabilities measured at amortised cost are recognised on settlement date; all other regular way purchases and issues are recognised on trade date. b) Financial assets and liabilities i) Classification and subsequent measurement The Santander UK group classifies its financial assets in the measurement categories of amortised cost

Segments

Segments	12 Months Ended
	Dec. 31, 2020
Disclosure of operating segments [abstract]
Segments	2. SEGMENTS Santander UK’s principal activity is financial services, mainly in the UK. The business is managed and reported on the basis of four segments, which are strategic business units that offer different products and services, have different customers and require different technology and marketing strategies. Corporate & Commercial Bankin g offers a wide range of financial services and solutions to more complex businesses across multiple sectors, typically with annual turnovers of between £2m and £500m. Service and expertise are provided by relationship managers, product specialists and through digital and telephony channels, and cover clients’ needs both in the UK and overseas. – Corporate & Investment Banking services corporate clients with an annual turnover of £500m and above. CIB clients require specially tailored solutions and value-added services due to their size, complexity and sophistication. We provide these clients with products to manage currency fluctuations, protect against interest rate risk, and arrange capital markets finance and specialist trade finance solutions, as well as providing support to the rest of Santander UK’s business segments. – Corporate Centre mainly includes the treasury, non-core corporate and legacy portfolios Corporate Centre is also responsible for managing capital and funding, balance sheet composition, structure, pension and strategic liquidity risk. To enable a more targeted and strategically aligned apportionment of capital and other resources, revenues and costs incurred in Corporate Centre are allocated to the three business segments. The non-core corporate and legacy portfolios are being run-down and/or managed for value. The segmental basis of presentation in this Annual Report has changed following a management review of our structure. As a result, customer assets of £2.0bn and customer deposits of £3.1bn have been transferred from Business Banking (in Retail Banking) to CCB, non-core corporate mortgages of £0.4bn have been transferred from Corporate Centre to CCB, and a number of smaller business lines have been transferred from CIB to Corporate Centre. This resulted in an increase in profit before tax in Retail Banking of £18m (2019: decrease of £36m), a decrease in CCB of £9m (2019: increase of £24m), a decrease in CIB of £10m (2019: decrease of £1m), and an increase of £1m in Corporate Centre (2019: increase of £13m). The net impact for Santander UK was nil. The segmental data below is presented in a manner consistent with the internal reporting to the committee which is responsible for allocating resources and assessing performance of the segments and has been identified as the chief operating decision maker. The segmental data is prepared on a statutory basis of accounting, in line with the accounting policies set out in Note 1. Transactions between segments are on normal commercial terms and conditions. Internal charges and internal UK transfer pricing adjustments are reflected in the results of each segment. Revenue sharing agreements are used to allocate external customer revenues to a segment on a reasonable basis. Funds are ordinarily reallocated between segments, resulting in funding cost transfers disclosed in operating income. Interest charged for these funds is based on Santander UK’s cost of wholesale funding. Interest income and interest expense have not been reported separately. The majority of segment revenues are interest income in nature and net interest income is relied on primarily to assess segment performance and to make decisions on the allocation of segment resources. Results by segment Retail Banking Corporate & Commercial Banking Corporate & Investment Banking Corporate Centre Total 2020 £m £m £m £m £m Net interest income/(expense) 3,100 364 55 (76) 3,443 Non-interest income 375 93 68 (4) 532 Total operating income/(expense) 3,475 457 123 (80) 3,975 Operating expenses before credit impairment losses, provisions and charges (1,913) (316) (114) (109) (2,452) Credit impairment losses (308) (294) (7) (36) (645) Provisions for other liabilities and (charges)/release (173) (12) (10) (78) (273) Total operating credit impairment losses, provisions and charges (481) (306) (17) (114) (918) Profit/(loss) before tax 1,081 (165) (8) (303) 605 Revenue from external customers 4,160 549 123 (857) 3,975 Inter-segment revenue (685) (92) — 777 — Total operating income/(expense) 3,475 457 123 (80) 3,975 Revenue from external customers includes the following fee and commission income disaggregated by income type: (1) – Current account and debit card fees 441 41 10 — 492 – Insurance, protection and investments 65 — — — 65 – Credit cards 67 — — — 67 – Non-banking and other fees (2) 15 47 66 4 132 Total fee and commission income 588 88 76 4 756 Fee and commission expense (335) (19) (10) (7) (371) Net fee and commission income/(expense) 253 69 66 (3) 385 Customer loans 183,404 17,626 2,784 3,196 207,010 Total assets(3) 192,070 17,626 2,784 79,852 292,332 Customer deposits 152,167 24,985 6,506 2,049 185,707 Total liabilities 152,715 25,011 6,517 92,153 276,396 Average number of full-time equivalent staff 19,151 2,092 716 39 21,998 (1) The disaggregation of fees and commission income as shown above is not included in reports provided to the chief operating decision maker but is provided to show the split by reportable segments. (2) Non-banking and other fees include mortgages (except mortgage account fees), consumer finance, commitment commission, asset finance, invoice finance and trade finance. (3) Includes customer loans, net of credit impairment loss allowances. Retail Banking Corporate & Commercial Banking Corporate & Investment Banking Corporate Centre Total 2019 £m £m £m £m £m Net interest income/(expense) 2,827 422 62 (19) 3,292 Non-interest income 691 109 70 11 881 Total operating income/(expense) 3,518 531 132 (8) 4,173 Operating expenses before credit impairment losses, provisions and charges (1,980) (324) (130) (65) (2,499) Credit impairment (losses)/releases (156) (45) (22) 2 (221) Provisions for other liabilities and charges (290) (22) (16) (113) (441) Total operating credit impairment losses, provisions and charges (446) (67) (38) (111) (662) Profit/(loss) before tax 1,092 140 (36) (184) 1,012 Revenue from external customers 4,255 633 138 (853) 4,173 Inter-segment revenue (737) (102) (6) 845 — Total operating income/(expense) 3,518 531 132 (8) 4,173 Revenue from external customers includes the following fee and commission income disaggregated by income type: (1) – Current account and debit card fees 696 49 13 — 758 – Insurance, protection and investments 76 — — 1 77 – Credit cards 86 — — — 86 – Non-banking and other fees (2) 61 58 67 5 191 Total fee and commission income 919 107 80 6 1,112 Fee and commission expense (373) (23) (17) (13) (426) Net fee and commission income/(expense) 546 84 63 (7) 686 Customer loans 178,762 18,391 4,041 3,814 205,008 Total assets (3) 185,920 18,391 4,046 73,345 281,702 Customer deposits 142,735 20,546 6,102 2,332 171,715 Total liabilities 143,602 20,572 6,233 95,278 265,685 Average number of full-time equivalent staff 20,594 2,151 804 21 23,570 2018 Net interest income 3,076 470 69 (12) 3,603 Non-interest income 632 115 132 52 931 Total operating income 3,708 585 201 40 4,534 Operating expenses before credit impairment losses, provisions and charges (1,914) (318) (203) (144) (2,579) Credit impairment (losses)/ releases (135) (8) (14) 4 (153) Provisions for other liabilities and charges (228) (16) (8) (5) (257) Total operating credit impairment losses, provisions and (charges)/releases (363) (24) (22) (1) (410) Profit/(loss) before tax 1,431 243 (24) (105) 1,545 Revenue from external customers 4,361 750 245 (822) 4,534 Inter-segment revenue (653) (165) (44) 862 — Total operating income 3,708 585 201 40 4,534 Revenue from external customers includes the following fee and commission income disaggregated by income type: (1) – Current account and debit card fees 690 49 14 — 753 – Insurance, protection and investments 105 — — — 105 – Credit card fees 85 — — — 85 – Non-banking and other fees (2) 76 65 81 5 227 Total fee and commission income 956 114 95 5 1,170 Fee and commission expense (382) (25) (14) — (421) Net fee and commission income 574 89 81 5 749 Customer loans 171,134 19,858 4,543 4,051 199,586 Total assets (3) 177,959 19,858 3,244 82,311 283,372 Customer deposits 139,976 19,695 4,853 2,791 167,315 Total liabilities 140,750 19,723 4,400 102,590 267,463 Average number of full-time equivalent staff 20,964 2,102 985 154 24,205 (1) The disaggregation of fees and commission income as shown above is not included in reports provided to the chief operating decision maker but is provided to show the split by reportable segments. (2) Non-banking and other fees include mortgages (except mortgage account fees), consumer finance, commitment commission, asset finance, invoice finance and trade finance. (3) Includes customer loans, net of credit impairment loss allowances. Geographical information is not provided, as substantially all of Santander UK’s activities are in the UK.

Net Interest Income

Net Interest Income	12 Months Ended
	Dec. 31, 2020
Analysis of income and expense [abstract]
Net Interest Income	3. NET INTEREST INCOME Group 2020 2019 2018 £m £m £m Interest and similar income: Loans and advances to customers (3) 4,819 5,230 5,458 Loans and advances to banks 49 137 202 Reverse repurchase agreements – non trading 118 244 124 Other 119 306 282 Total interest and similar income (1) 5,105 5,917 6,066 Interest expense and similar charges: Deposits by customers (1,011) (1,540) (1,433) Deposits by banks (47) (134) (117) Repurchase agreements – non trading (43) (126) (42) Debt securities in issue (440) (678) (721) Subordinated liabilities (111) (137) (142) Other (10) (10) (8) Total interest expense and similar charges (2) (1,662) (2,625) (2,463) Net interest income 3,443 3,292 3,603 (1) This includes £38m (2019: £155m) of interest income on financial assets at fair value through other comprehensive income. (2) This includes £451m (2019: £310m) of interest expense on derivatives hedging debt issuances and £3m (2019: £4m) of interest expense on lease liabilities. (3) During 2020, we revised the accounting treatment for certain items of mortgage income to better align our policy to current practice. The net impact of these changes is not material and comparatives have not been restated. Further details are provided in Note 1 Accounting Policies.

Net Fee and Commission Income

Net Fee and Commission Income	12 Months Ended
	Dec. 31, 2020
Fee and commission income (expense) [abstract]
Net Fee and Commission Income	4. NET FEE AND COMMISSION INCOME Group 2020 2019 2018 £m £m £m Fee and commission income: Current account and debit card fees 492 758 753 Insurance, protection and investments 65 77 105 Credit cards 67 86 85 Non-banking and other fees (1) 132 191 227 Total fee and commission income 756 1,112 1,170 Total fee and commission expense (371) (426) (421) Net fee and commission income 385 686 749 (1) Non-banking and other fees include mortgages (except mortgage account fees), consumer finance, commitment commission, asset finance, invoice finance and trade finance.

Other Operating Income

Other Operating Income	12 Months Ended
	Dec. 31, 2020
Net Trading And Other Income [abstract]
Net Trading and Other Income	5. OTHER OPERATING INCOME Group 2020 2019 2018 £m £m £m Net losses on financial instruments designated at fair value through profit or loss (75) (134) (86) Net gains on financial instruments mandatorily at fair value through profit or loss 46 70 36 Hedge ineffectiveness 20 8 34 Net profit on sale of financial assets at fair value through other comprehensive income 17 15 19 Income from operating lease assets 126 124 86 Other 13 112 93 147 195 182 Following the implementation of our ring-fencing plans in 2018, assets and liabilities held at fair value through profit or loss, including derivatives, are predominantly used to provide customers with risk management solutions, and to manage and hedge the Santander UK group’s own risks, and do not give rise to significant overall net gains/(losses) in the income statement. 'Net gains on financial instruments mandatorily at fair value through profit or loss' includes fair value gains of £89m (2019: losses of £42m, 2018: gains of £22m) on embedded derivatives bifurcated from certain equity index-linked deposits, as described in the derivatives accounting policy in Note 1. The embedded derivatives are economically hedged, the results of which are also included in this line item, and amounted to losses of £88m (2019: gains of £43m, 2018: losses of £21m). As a result, the net fair value movements recognised on the equity index-linked deposits and the related economic hedges were net gains of £1m (2019: £1m, 2018: £1m). In 2019, ‘net profit on sale of financial assets at fair value through other comprehensive income’ included additional consideration of £15m in connection with the 2017 Vocalink Holdings Limited shareholding sale. Exchange rate differences recognised in the Consolidated Income Statement on items not at fair value through profit or loss were £751m expense (2019: £1,102m income, 2018: £689m expense) and are presented in the line ‘Other'. These are principally offset by related releases from the cash flow hedge reserve of £809m income (2019: £1,013m expense, 2018: £752m income) as set out in the Consolidated Statement of Comprehensive Income, which are also presented in 'Other’. Exchange rate differences on items measured at fair value through profit or loss are included in the line items relating to changes in fair value. In 2020, Santander UK repurchased certain securities as part of its ongoing liability management exercises, resulting in a loss of £24m.

Operating Expenses Before Credi

Operating Expenses Before Credit Impairment Losses, Provisions and Charges	12 Months Ended
	Dec. 31, 2020
Expenses by nature [abstract]
Operating Expenses Before Credit Impairment Losses, Provisions and Charges	6. OPERATING EXPENSES BEFORE CREDIT IMPAIRMENT LOSSES, PROVISIONS AND CHARGES Group 2020 2019 2018 £m £m £m Staff costs: Wages and salaries 831 852 898 Performance-related payments 97 159 159 Social security costs 101 111 111 Pensions costs: – defined contribution plans 66 66 67 defined benefit plans 38 35 79 Other share-based payments — — 3 Other personnel costs 33 40 52 1,166 1,263 1,369 Other administration expenses 724 693 835 Depreciation, amortisation and impairment 562 543 375 Total 2,452 2,499 2,579 Staff costs ’Performance-related payments’ include bonuses paid in cash and share awards granted under the Long-Term Incentive Plan and the Deferred shares bonus plan, as described in Note 37. Included in this are equity-settled share-based payments, none of which related to option-based schemes. These are disclosed in the table below as ‘Share awards’. Performance-related payments above include amounts related to deferred performance awards as follows: Costs recognised in 2020 Costs expected to be recognised in 2021 or later Arising from awards in current year Arising from awards in prior year Total Arising from awards in current year Arising from awards in prior year Total £m £m £m £m £m £m Cash 2 7 9 2 9 11 Shares 2 6 8 2 9 11 4 13 17 4 18 22 The following table shows the amount of bonus awarded to employees for the performance year 2020. In the case of deferred cash and share awards, the final amount paid to an employee is influenced by forfeiture provisions and any performance conditions to which these awards are subject. The deferred share award amount is based on the fair value of these awards at the date of grant. Expenses charged in the year Expenses deferred to future periods Total 2020 2019 2020 2019 2020 2019 £m £m £m £m £m £m Cash award – not deferred 77 130 — — 77 130 – deferred 9 10 11 17 20 27 Shares award – not deferred 3 10 — — 3 10 – deferred 8 9 11 15 19 24 Total discretionary bonus 97 159 22 32 119 191 On 26 October 2018, the High Court handed down a judgement concluding that defined benefit schemes should equalise pension benefits for men and women in relation to GMP and concluded on the methods that were appropriate. The estimated increase in liabilities at the date of the judgement was £40m and was based on a number of assumptions and the actual impact may be different. This was reflected in the income statement and in the closing net accounting surplus of the Scheme in 2018. The allowance included in the Scheme liabilities at 31 December 2020 increased by £5m (2019: £5m) to £50m (2019: £45m) to reflect the latest assumptions. This change was recognised in other comprehensive income. We work is being undertaken to consider and agree how to implement GMP equalisation. ‘Other share-based payments’ consist of options granted under the Employee Sharesave scheme which comprise the Santander UK group’s cash-settled share-based payments. For more, see Note 37. The average number of full-time equivalent staff was 21,998 (2019: 23,570, 2018: 24,205). The increase in staff numbers in 2018 reflected Santander UK plc’s acquisition of Santander UK Operations Ltd (formerly Geoban UK Ltd, a subsidiary of Geoban SA) and Santander UK Technology Ltd (formerly Isban UK Ltd, a subsidiary of Ingenieria de Software Bancario SL). Depreciation, amortisation and impairment In 2020, depreciation, amortisation and impairment was impacted by operating lease depreciation of £92m (2019: £103m) on operating lease assets (where the Santander UK group is the lessor) with a net book value of £542m at 31 December 2020 (2019: £574m). It was also impacted by depreciation of £58m (2019: £60m) on right-of-use assets with a net book value of £100m at 31 December 2020 (2019: £152m), following the adoption of IFRS 16 on 1 January 2019. Other administration expenses 'Other administration expenses' includes £10m (2019: £13m) expenses related to short-term leases.

Audit and Other Services

Audit and Other Services	12 Months Ended
	Dec. 31, 2020
Audit And Other Services [Abstract]
Audit and Other Services	7. AUDIT AND OTHER SERVICES Group 2020 2019 2018 £m £m £m Audit fees: Fees payable to the Company's auditor and its associates for the audit of the Santander UK group's annual accounts 10.0 8.0 7.2 Fees payable to the Company’s auditor and its associates for other services to the Santander UK group: – Audit of the Santander UK group's subsidiaries 1.4 1.3 1.1 Total audit fees (1) 11.4 9.3 8.3 Non-audit fees: Audit-related assurance services (1) 0.8 0.8 0.7 Other assurance services 0.3 0.2 0.1 Other non-audit services 0.0 0.2 1.0 Total non-audit fees 1.1 1.2 1.8 (1) 2020 audit fees included £0.8m (2019: £0.1m) which related to the prior year. (2) 2020 audit-related assurance services included £nil (2019: £0.1m) which related to the prior year. Audit-related assurance services mainly comprises services performed in connection with review of the interim financial information of the Company and reporting to the Company's UK regulators. Of the total non-audit fees, £0.4m (2019: £0.6m, 2018: £0.1m) accords with the definition of "Audit Fees" per US Securities and Exchange Commission (SEC) guidance, £0.7m (2019: £0.4m, 2018: £0.6m) accords with the definition of "Audit related fees" per that guidance and £nil (2019: £0.2m, 2018: £1.1m) accords with the definition of "All other fees" per that guidance. In 2020, the Company’s auditors earned £24,000 fees (2019: no fees, 2018: £150,000) payable by entities outside the Santander UK group for the review of the financial position of corporate and other borrowers.

Credit Impairment Losses and Pr

Credit Impairment Losses and Provisions	12 Months Ended
	Dec. 31, 2020
Disclosure of credit impairment loss and provisions [abstract]
Credit Impairment Losses and Provisions	8. CREDIT IMPAIRMENT LOSSES AND PROVISIONS Group 2020 2019 2018 £m £m £m Credit impairment losses: Loans and advances to customers 672 239 189 Recoveries of loans and advances, net of collection costs (24) (40) (42) Off-balance sheet exposures (See Note 29) (3) 22 6 645 221 153 Provisions for other liabilities and charges (excluding off-balance sheet credit exposures) (See Note 29) 267 435 257 Provisions for residual value and voluntary termination 6 6 — 273 441 257 918 662 410 In 2020 and 2019 there were no material credit impairment losses on loans and advances to banks, non-trading reverse repurchase agreements, other financial assets at amortised cost and financial assets at fair value through other comprehensive income.

Taxation

Taxation	12 Months Ended
	Dec. 31, 2020
Major components of tax expense (income) [abstract]
Taxation	9. TAXATION Group 2020 2019 2018 £m £m £m Current tax: UK corporation tax on profit for the year 120 265 408 Adjustments in respect of prior years (24) (25) (20) Total current tax 96 240 388 Deferred tax: Charge for the year 34 46 16 Adjustments in respect of prior years 4 (7) (5) Total deferred tax 38 39 11 Tax on profit 134 279 399 The standard rate of UK corporation tax was 27% for banking entities and 19% for non-banking entities (2019: 27% for banking entities and 19% for non-banking entities; 2018: 27% for banking entities and 19% for non-banking entities) following the introduction of an 8% surcharge to be applied to banking companies from 1 January 2016. Taxation for other jurisdictions is calculated at the rates prevailing in the relevant jurisdictions. Finance Act 2016 introduced a reduction in the standard rate of corporation tax rate to 17% from 2020 but this was reversed in the UK Budget in March 2020. As a result, the standard rate of corporation tax remains at 19% and the effect of the increase of 2% over that expected at 31 December 2019 has been reflected in the opening deferred tax balance at 1 January 2020. The Santander UK group’s effective tax rate for 2020 was 22.1% (2019: 27.6%, 2018: 25.8%). The tax on profit before tax differs from the theoretical amount that would arise using the basic corporation tax rate of the Company as follows: Group 2020 2019 2018 £m £m £m Profit before tax 605 1,012 1,545 Tax calculated at a tax rate of 19% (2019: 19%, 2018: 19%) 115 192 294 Bank surcharge on profits 31 65 109 Non-deductible preference dividends paid 8 8 8 Non-deductible UK Bank Levy 19 24 20 Non-deductible conduct remediation, fines and penalties (4) 44 6 Other non-deductible costs and non-taxable income 25 31 30 Effect of change in tax rate on deferred tax provision 6 (14) (1) Tax relief on dividends in respect of other equity instruments (40) (39) (42) Adjustment to prior year provisions (26) (32) (25) Tax charge 134 279 399 Current tax assets and liabilities Movements in current tax assets and liabilities during the year were as follows: Group 2020 2019 £m £m Assets 200 153 Liabilities — — At 1 January 200 153 Income statement charge (96) (240) Other comprehensive income (charge)/credit 1 (4) Corporate income tax paid 159 292 Other movements — (1) 264 200 Assets 264 200 Liabilities — — At 31 December 264 200 The amount of corporation income tax paid differs from the tax charge for the period as a result of the timing of payments due to the tax authorities together with the effects of movements in deferred tax, adjustments to prior period current tax provisions and current tax recognised directly in other comprehensive income. Santander UK proactively engages with HM Revenue & Customs to resolve tax matters relating to prior years. The accounting policy for recognising provisions for such matters are described in Note 1. It is not expected that there will be any material movement in such provisions within the next 12 months. Santander UK adopted the Code of Practice on Taxation for Banks in 2010. For more on this, see our Tax Strategy at www.santander.co.uk/about-santander/sustainability/taxation-strategy. Deferred tax The table below shows the deferred tax assets and liabilities including the movement in the deferred tax account during the year. Deferred tax balances are presented in the balance sheet after offsetting assets and liabilities where the Santander UK group has the legal right to offset and intends to settle on a net basis. Group Fair value of financial instruments Pension remeasurement Cash flow hedges Fair value Tax losses carried forward Accelerated tax depreciation Other temporary differences Total £m £m £m £m £m £m £m £m At 1 January 2020 (52) (96) (58) (8) 13 17 35 (149) Income statement (charge)/ credit (13) (63) — — 2 21 15 (38) Transfers/reclassifications — — 12 (1) — — (13) (2) Credited/(charged) to other comprehensive income — 133 (53) (2) — — — 78 At 31 December 2020 (65) (26) (99) (11) 15 38 37 (111) At 1 January 2019 (51) (183) (43) (13) 20 (6) 53 (223) Income statement (charge)/credit (1) (44) — — (7) 23 (10) (39) Transfers/reclassifications — — 22 5 — — (27) — Credited/(charged) to other comprehensive income — 131 (37) — — — 19 113 At 31 December 2019 (52) (96) (58) (8) 13 17 35 (149)

Dividends on Ordinary Shares

Dividends on Ordinary Shares	12 Months Ended
	Dec. 31, 2020
Dividends On Ordinary Shares [abstract]
Dividends on Ordinary Shares	10. DIVIDENDS ON ORDINARY SHARES Dividends on ordinary shares declared and paid in the year were as follows: Group Group 2020 2019 2018 2020 2019 2018 Pence per share Pence per share Pence per share £m £m £m In respect of current year – first interim 0.42 0.53 0.81 129 164 250 – second interim — 0.49 2.15 — 151 668 – third interim — — 0.71 — — 221 0.42 1.02 3.67 129 315 1,139 Following the PRA's announcement regarding the resumption of dividend payments, an interim dividend of £129m for 2020 was paid in December 2020. In 2018, in addition to the interim dividends of £250m and £221m, we also paid a dividend of £668m that related to the ring-fencing transfers to Banco Santander, London Branch.

Derivative Financial Instrument

Derivative Financial Instruments	12 Months Ended
	Dec. 31, 2020
Disclosure of nature and extent of risks arising from financial instruments [abstract]
Derivative Financial Instruments	11. DERIVATIVE FINANCIAL INSTRUMENTS a) Use of derivatives The Santander UK group undertakes derivative activities primarily to provide customers with risk management solutions and to manage and hedge the Santander UK group’s own risks. The Santander UK group’s derivative activities do not give rise to significant open positions in portfolios of derivatives. Any residual position is managed to ensure that it remains within acceptable risk levels, with matching transactions being used to achieve this where necessary. When entering into derivatives, the Santander UK group employs the same credit risk management procedures to assess and approve potential credit exposures that are used for traditional lending. For information on how the Santander UK group is managing the transition to alternative benchmark interest rates, see ‘Managing LIBOR transition’ in the Banking market risk section of the Risk review and Note 43 Interest Rate Benchmark Reform. b) Analysis of derivatives The table below includes the notional amounts of transactions outstanding at the balance sheet date; they do not represent actual exposures. Group 2020 2019 Fair value Fair value Notional amount Assets Liabilities Notional amount Assets Liabilities £m £m £m £m £m £m Derivatives held for trading: Exchange rate contracts 14,951 395 418 14,149 134 200 Interest rate contracts 40,160 888 542 46,564 718 315 Equity and credit contracts 1,140 123 55 2,474 283 160 Total derivatives held for trading 56,251 1,406 1,015 63,187 1,135 675 Derivatives held for hedging Designated as fair value hedges: Exchange rate contracts 789 84 6 1,482 166 2 Interest rate contracts 93,748 1,225 1,885 94,550 1,022 1,488 Equity derivative contracts — — — — — — 94,537 1,309 1,891 96,032 1,188 1,490 Designated as cash flow hedges: Exchange rate contracts 27,020 1,978 409 28,502 2,023 462 Interest rate contracts 19,407 467 23 17,451 184 35 Equity derivative contracts — — — — — — 46,427 2,445 432 45,953 2,207 497 Total derivatives held for hedging 140,964 3,754 2,323 141,985 3,395 1,987 Derivative netting (1) (1,754) (1,754) (1,214) (1,214) Total derivatives 197,215 3,406 1,584 205,172 3,316 1,448 (1) Derivative netting excludes the effect of cash collateral, which is offset against the gross derivative position. The amount of cash collateral received that had been offset against the gross derivative assets was £330m (2019: £222m) and the amount of cash collateral paid that had been offset against the gross derivative liabilities was £651m (2019: £629m). For information about the impact of netting arrangements on derivative assets and liabilities in the table above, see Note 41. The reduction in the notional value of interest rate derivatives held for trading reflected the completion of a series of derivative trade compressions to reduce our gross LIBOR exposure. The table below analyses the notional and fair values of derivatives by trading and settlement method. Notional Traded over the counter Asset Liability Traded on recognised exchanges Settled by central counterparties Not settled by central counterparties Total Traded on recognised exchanges Traded over the counter Traded on recognised exchanges Traded over the counter 2020 £m £m £m £m £m £m £m £m Exchange rate contracts — — 42,760 42,760 — 2,457 — 833 Interest rate contracts — 144,343 8,972 153,315 — 826 — 696 Equity and credit contracts — — 1,140 1,140 — 123 — 55 — 144,343 52,872 197,215 — 3,406 — 1,584 2019 Exchange rate contracts — — 44,133 44,133 — 2,324 — 663 Interest rate contracts — 131,538 27,027 158,565 — 709 — 625 Equity and credit contracts — — 2,474 2,474 — 283 — 160 — 131,538 73,634 205,172 — 3,316 — 1,448 c) Analysis of derivatives designated as hedges The Santander UK group applies hedge accounting on both a fair value and cash flow basis depending on the nature of the underlying exposure. We establish the hedge ratio by matching the notional of the derivative with the underlying position being hedged. Only the designated risk is hedged and therefore other risks, such as credit risk are managed but not hedged. For interest rate hedges, the designated hedged risk is determined with reference to the underlying benchmark rate. Fair value hedges Portfolio hedges of interest rate risk Santander UK holds various portfolios of fixed rate assets and liabilities which expose it to changes in fair value due to movements in market interest rates. We manage these exposures by entering into interest rate swaps. Each portfolio contains assets or liabilities that are similar in nature and share the risk exposure that is designated as being hedged. The interest rate risk component is the change in fair value of fixed rate instruments for changes in the designated benchmark rate. Such changes are usually the largest component of the overall change in fair value. Separate hedges are maintained for each underlying currency. Effectiveness is assessed by comparing changes in fair value of the hedged item attributable to changes in the designated benchmark interest rate, with changes in the fair value of the interest rate swaps. Micro hedges of interest rate risk and foreign currency risk Santander UK accesses international markets to obtain funding, issuing fixed rate debt in its functional currency and other currencies. We are therefore exposed to changes in fair value due to changes in market interest rates and/or foreign exchange rates, principally in USD and EUR, which we mitigate through the use of receive fixed/pay floating rate interest rate swaps and/or receive fixed/pay floating rate cross currency swaps. The interest rate risk component is the change in fair value of the fixed rate debt due to changes in the benchmark LIBOR rate or risk free rate. The foreign exchange component is the change in the fair value of the fixed rate debt issuance due to changes in foreign exchange rates prevailing from the time of execution. Effectiveness is assessed by using linear regression techniques to compare changes in the fair value of the debt caused by changes in the benchmark interest rate and foreign exchange rates, with changes in the fair value of the interest rate swaps and/or cross currency swaps. Cashflow hedges Hedges of interest rate risk Santander UK manages its exposure to the variability in cash flows of floating rate assets and liabilities attributable to movements in market interest rates by entering into interest rate swaps. The interest rate risk component is determined with reference to the underlying benchmark rate attributable to the floating rates asset or liability. Designated benchmark rates referenced are currently SONIA or LIBOR. Effectiveness is assessed by comparing changes in the fair value of the interest rate swap with changes in the fair value of the hedged item attributable to the hedged risk, applying a hypothetical derivative method using linear regression techniques. Hedges of foreign currency risk As Santander UK obtains funding in international markets, we assume significant foreign currency risk exposure, mainly in USD and EUR. In addition, the Santander UK group also holds debt securities for liquidity purposes which assumes foreign currency exposure, principally in JPY. Santander UK manages the exposures to the variability in cash flows of foreign currency denominated assets and liabilities to movements in foreign exchange rates by entering into either foreign exchange contracts (spot, forward and swaps) or cross currency swaps. These instruments are entered into to match the cash flow profile and maturity of the estimated interest and principal repayments of the hedged item. The foreign currency risk component is the change in cash flows of the foreign currency debt arising from changes in the relevant foreign currency forward exchange rate. Such changes constitute a significant component of the overall changes in cash flows of the instrument. Effectiveness is assessed by comparing changes in the fair value of the cross currency or foreign exchange swaps with changes in the fair value of the hedged debt attributable to the hedged risk applying a hypothetical derivative method using linear regression techniques. LIBOR Reform Details of the notional value of hedging instruments by benchmark interest rate impacted by IBOR reform are disclosed in Note 43. Hedge effectiveness measurement Hedge effectiveness is assessed by using either dollar offset or linear regression techniques to compare changes in the fair value of the hedged item attributable to changes in the designated hedged risk and the hedging instrument. For cash flow hedges, a hypothetical derivative method is used to model the cash flows of the hedged item. Possible sources of hedge ineffectiveness For both fair value and cash flow hedges, hedge ineffectiveness can arise from hedging derivatives with a non-zero fair value at the date of initial designation. In addition, for: Fair value hedges Hedge ineffectiveness can also arise due to differences in discounting between the hedged item and the hedging instrument as cash collateralised swaps discount using Overnight Indexed Swaps discount curves not applied to the hedged item; and where counterparty credit risk impacts the fair value of the derivative but not the hedged item. For portfolio hedges of interest rate risk, it can also arise due to differences in the expected and actual volume of prepayments. Cash flow hedges Hedge ineffectiveness can also arise due to differences in the timing of cash flows between the hedged item and the hedging instrument. For micro hedges of interest rate risk, it can also arise due to differences in the basis of cash flows between the hedged item and the hedging instrument. Maturity profile and average price/rate of hedging instruments The following table sets out the maturity profile and average price/rate of the hedging instruments used in the Santander UK group’s hedging strategies: Group 2020 Hedging Instruments ≤1 month >1 and ≤3 months >3 and ≤12 months >1 and ≤5 years >5 years Total Fair value hedges: Interest rate risk Interest rate contracts- Nominal Amount (£m) 2,429 7,617 27,791 47,749 7,889 93,475 Average fixed interest rate - GBP 0.69 % 0.65 % 0.82 % 0.73 % 3.61 % Average fixed interest rate - EUR 1.18 % 0.23 % 3.02 % 0.98 % 2.34 % Average fixed interest rate - USD 1.87 % 1.72 % 2.89 % 2.49 % 4.16 % Interest rate/FX risk Exchange rate contracts - Nominal Amount (£m) — — 132 461 196 789 Interest rate contracts - Nominal Amount (£m) — — — 236 37 273 Average GBP - EUR exchange rate — — 1.1405 1.1700 1.1669 Average GBP - USD exchange rate — — — — — Average fixed interest rate - EUR — — 4.64 % 1.78 % 3.56 % Average fixed interest rate - USD — — — — — Cash flow hedges: Interest rate risk Interest rate contracts – Notional amount (£m) — 897 2,528 7,964 1,061 12,450 Average fixed interest rate - GBP — 0.46 % 0.57 % 1.45 % 1.33 % FX risk Exchange rate contracts- Nominal Amount (£m) 1,439 2,015 3,877 7,113 1,119 15,563 Interest rate contracts- Nominal Amount (£m) — — — 366 — 366 Average GBP - JPY exchange rate — 137.9769 135.6073 132.2714 — Average GBP - EUR exchange rate — — — 1.1629 1.1787 Average GBP - USD exchange rate 1.2931 1.3164 1.3231 1.3039 — Interest rate/FX risk Exchange rate contracts - Nominal Amount (£m) 732 — 2,583 6,550 1,592 11,457 Interest rate contracts - Nominal Amount (£m) 732 — 882 4,062 915 6,591 Average GBP - EUR exchange rate — — 1.3543 1.2525 1.1965 Average GBP - USD exchange rate 1.4649 — — 1.6085 1.3811 Average fixed interest rate – GBP 2.01 % — 3.18 % 2.48 % 3.39 % 2019 Fair value hedges: Interest rate risk Interest rate contracts- Nominal Amount (£m) 4,354 5,804 27,405 43,652 13,099 94,314 Average fixed interest rate - GBP 0.77 % 0.90 % 0.88 % 1.33 % 3.00 % Average fixed interest rate - EUR (0.41) % 0.29 % 2.21 % 1.36 % 2.36 % Average fixed interest rate - USD — 1.54 % 1.99 % 2.69 % 4.56 % Interest rate/FX risk Exchange rate contracts - Nominal Amount (£m) — 755 — 317 410 1,482 Interest rate contracts - Nominal Amount (£m) — — — 18 218 236 Average GBP - EUR exchange rate — — — 1.1781 1.1603 Average GBP - USD exchange rate — 1.5110 — — — Average fixed interest rate - EUR — — — 3.52 % 2.12 % Average fixed interest rate - USD — 2.38 % — — — Cash flow hedges: Interest rate risk Interest rate contracts - Nominal Amount (£m) — 339 1,066 4,671 500 6,576 Average fixed interest rate - GBP (%) — 0.76 % 0.82 % 1.46 % 0.40 % — FX risk Exchange rate contracts- Nominal Amount (£m) 1,187 2,119 3,758 5,217 — 12,281 Interest rate contracts- Nominal Amount (£m) — — — 755 — 755 Average GBP - JPY exchange rate — 145.9275 143.0857 140.8152 — Average GBP - EUR exchange rate — 1.1444 1.1167 1.1526 — Average GBP - USD exchange rate 1.2856 1.2624 1.2925 1.2991 — Interest rate/FX risk Exchange rate contracts - Nominal Amount (£m) 812 — 3,367 8,009 4,033 16,221 Interest rate contracts - Nominal Amount (£m) — — 3,121 4,829 2,170 10,120 Average GBP - EUR exchange rate 1.2742 — 1.1689 1.3114 1.2090 Average GBP - USD exchange rate — — 1.5357 1.5811 1.4499 Average fixed interest rate - GBP (%) 2.49 % — 2.16 % 2.87 % 2.96 % Net gains or losses arising from fair value and cash flow hedges included in other operating income Group 2020 2019 2018 £m £m £m Fair value hedging: (Losses)/gains on hedging instruments (299) (360) 4 Gains/(losses) on hedged items attributable to hedged risks 365 414 75 Fair value hedging ineffectiveness 66 54 79 Cash flow hedging ineffectiveness (46) (46) (45) 20 8 34 Hedge ineffectiveness can be analysed by risk category as follows: Group 2020 2019 Change in FV of hedging instruments Change in FV of hedged items Recognised in income statement Change in FV of hedging instruments Change in FV of hedged items Recognised in income statement £m £m £m £m £m £m Fair value hedges: Interest rate risk (358) 385 27 (264) 284 20 Interest rate/FX risk 59 (20) 39 (96) 130 34 (299) 365 66 (360) 414 54 Group 2020 2019 Hedging Instruments Hedging Instruments Income statement line item affected by reclassification Change in FV Recognised in OCI Recognised in Income Statement Reclassified from reserves to income Change in FV Recognised in OCI Recognised in Income Statement Reclassified from reserves to income £m £m £m £m £m £m £m £m Cash flow hedges: Interest rate risk Net interest income 185 (179) 6 33 34 (33) 1 13 FX risk Net interest income/other operating income (42) 38 (4) 2 (333) 329 (4) (316) Equity risk Operating expenses — — — — — — — — Interest rate/FX risk Net interest income/other operating income 782 (830) (48) 773 (604) 561 (43) (709) 925 (971) (46) 808 (903) 857 (46) (1,012) In 2020, cash flow hedge accounting of £4m (2019: £4m) had to cease due to the hedged cash flows no longer being expected to occur. The following table provides a reconciliation by risk category of components of equity and analysis of OCI items (before tax) resulting from hedge accounting. Group 2020 2019 £m £m Balance at 1 January 481 326 Effective portion of changes in fair value: – Interest rate risk 179 33 – Foreign currency risk (38) (329) – Equity risk — — – Interest rate/foreign currency risk 830 (561) 971 (857) Income statement transfers – Interest rate risk (33) (13) – Foreign currency risk (2) 316 – Equity risk — — – Interest rate/foreign currency risk (773) 709 (808) 1,012 Balances at 31 December 644 481 Hedged exposures Santander UK hedges its exposures to various risks, including interest rate risk and foreign currency risk, as set out in the following table. Group 2020 2019 Accumulated amount of FV hedge adjustments Change in value to calculate hedge ineffectiveness Accumulated amount of FV hedge adjustments Change in value to calculate hedge ineffectiveness Carrying value Hedged item Portfolio hedge of interest rate risks Of which Discontinued hedges Carrying value Hedged item Portfolio hedge of interest rate risks Of which Discontinued hedges £m £m £m £m £m £m £m £m £m £m Fair value hedges Interest rate risk: Loans and advances to customers 54,118 — 1,189 892 334 43,098 — 870 630 258 Other financial assets at amortised cost 772 — 36 13 121 6,627 — 142 121 83 Reverse repos – non trading 12,149 — 1 — 3 17,121 — (2) — (2) Other financial assets at FVOCI 5,129 155 — 74 88 5,944 102 — 82 125 Deposits by customers (7,309) (158) (10) (77) (73) (9,944) (85) 4 (11) (110) Deposits by banks — — — — — (517) (16) — (17) (1) Debt securities in issue (5,885) (375) (137) (239) (61) (8,099) (303) (166) (300) (50) Subordinated liabilities (636) (185) (41) (166) (27) (707) (181) (48) (204) (19) Interest rate/FX risk: Other financial assets at FVOCI 299 5 — — 15 241 3 — — (4) Debt securities in issue (621) (94) — (76) (34) (1,396) (135) — (122) 136 Subordinated liabilities 3 3 — 3 (1) 7 7 — 7 (2) 58,019 (649) 1,038 424 365 52,375 (608) 800 186 414 Group 2020 2019 Change in value to calculate hedge ineffectiveness Cash flow hedge reserve Balances on cash flow hedge reserve for discontinued hedges Change in value to calculate hedge ineffectiveness Cash flow hedge reserve Balances on cash flow hedge reserve for discontinued hedges Hedged item balance sheet line item £m £m £m £m £m £m Cash flow hedges: Interest rate risk: Loans and advances to customers (183) 165 1 (34) 21 (11) Loans and advances to banks (2) 1 — (2) — — Reverse repurchase agreements – non trading (2) 1 — — — — Deposits by banks 7 (2) — 3 (2) — Debt securities in issue — — — — — — Repurchase agreements – non trading 1 (1) — — — — FX risk: Other financial assets at FVOCI 40 6 — (122) 3 — Not applicable – highly probable forecast transactions 33 3 — 267 2 — Deposits by customers (5) 14 — 3 — — Deposits by banks — — — 4 — — Debt securities in issue (15) (60) — 177 (3) — Repurchase agreements – non trading (15) — — — — — Equity risk: Other liabilities — — — — — (1) Interest rate/FX risk: Debt securities in issue/loans and advances to customers (569) 236 (2) 630 280 20 Deposits by customers (132) 87 — — — — Subordinated liabilities/loans and advances to customers (130) 194 — (69) 180 — (972) 644 (1) 857 481 8

Other Financial Assets At Fair

Other Financial Assets At Fair Value Through Profit Or Loss	12 Months Ended
	Dec. 31, 2020
Financial assets at fair value through profit or loss [abstract]
Other Financial Assets At Fair Value Through Profit Or Loss	12. OTHER FINANCIAL ASSETS AT FAIR VALUE THROUGH PROFIT OR LOSS Group 2020 2019 £m £m Loans and advances to customers: Loans to housing associations 13 12 Other loans 86 80 99 92 Debt securities 109 294 Equity securities — — Reverse repurchase agreements - non trading — — 208 386 For the Santander UK group, other financial assets at FVTPL comprised £13m (2019: £12m) of financial assets designated at FVTPL and £195m (2019: £374m) of financial assets mandatorily held at FVTPL. Loans and advances to customers principally represented other loans, being a portfolio of roll-up mortgages. These are managed, and have their performance evaluated, on a fair value basis in accordance with a documented investment strategy, and information about them is provided on that basis to management. Since 2009, the Santander UK group’s policy has been not to designate similar new loans at fair value through profit or loss. In 2019 £2.1bn of senior tranches of credit linked notes related to an SRT securitisation, which were previously classified as debt securities in the table above, were presented on a net basis. This followed a deed of amendment, including a legal right of set-off between the principal amounts of the senior tranches of credit linked notes and the related cash deposits included as collateral in Note 36. At 31 December 2020, the amount of this netting was £825m (2019: £1.5bn). The net (loss)/gain in the year attributable to changes in credit risk for loans and advances at fair value through profit or loss was £nil (2019: £nil, 2018: £(1)m). The cumulative net loss attributable to changes in credit risk for loans and advances at fair value through profit or loss at 31 December 2020 was £2m (2019: £2m).

Loans and Advances to Customers

Loans and Advances to Customers	12 Months Ended
	Dec. 31, 2020
Net Loans and Advances to Customers [abstract]
Loans and Advances to Customers	13. LOANS AND ADVANCES TO CUSTOMERS Group 2020 2019 £m £m Loans secured on residential properties 166,714 165,356 Corporate loans 24,474 27,043 Finance leases 6,554 6,264 Secured advances — — Other unsecured loans 9,933 7,096 Amounts due from fellow Banco Santander subsidiaries and joint ventures 2,425 2,366 Amounts due from Santander UK Group Holdings plc 7 8 Amounts due from subsidiaries — — Loans and advances to customers 210,107 208,133 Credit impairment loss allowances on loans and advances to customers (1,303) (785) RV and voluntary termination provisions on finance leases (54) (61) Net loans and advances to customers 208,750 207,287 For movements in expected credit losses, see the Credit risk section of the Risk review. Finance lease and hire purchase contract receivables may be analysed as follows: Group 2020 2019 Gross investment Unearned finance income Net investment Gross investment Unearned finance income Net investment £m £m £m £m £m £m No later than one year 3,468 (297) 3,171 3,060 (303) 2,757 Later than one year and not later than two years 1,829 (173) 1,656 2,046 (207) 1,839 Later than two years and not later than three years 1,099 (106) 993 1,157 (119) 1,038 Later than three years and not later than four years 575 (55) 520 541 (54) 487 Later than four years and not later than five years 231 (25) 206 41 (4) 37 Later than five years 8 — 8 147 (41) 106 7,210 (656) 6,554 6,992 (728) 6,264 The prior year analysis by maturity in the table above has been restated to conform with the current year presentation. The Santander UK group enters into finance leasing arrangements primarily for the financing of motor vehicles and a range of assets for its corporate customers. Included in the carrying value of net investment in finance leases and hire purchase contracts is £3,552m (2019: £3,512m) of unguaranteed RV at the end of the current lease terms, which is expected to be recovered through re-payment, re-financing or sale. Contingent rent income of £nil (2019: £nil, 2018: £nil) was earned during the year, which was classified in ‘Interest and similar income’. Finance income on the net investment in finance leases was £308m (2019: £299m, 2018: £346m). Finance lease receivable balances are secured over the asset leased. The Santander UK group is not permitted to sell or repledge the asset in the absence of default by the lessee. The Directors consider that the carrying amount of the finance lease receivables approximates to their fair value.

Securitisations and Covered Bon

Securitisations and Covered Bonds	12 Months Ended
	Dec. 31, 2020
Disclosure of securitisations and covered bonds [abstract]
Securitisations and Covered Bonds	14. SECURITISATIONS AND COVERED BONDS The information in this Note relates to securitisations and covered bonds for consolidated structured entities, used to obtain funding or collateral. It excludes structured entities relating to credit protection vehicles. The Santander UK group uses structured entities to securitise some of the mortgage and other loans to customers that it originates. The Santander UK group also issues covered bonds, which are guaranteed by, and secured against, a pool of the Santander UK group’s mortgage loans transferred to Abbey Covered Bonds LLP. The Santander UK group issues mortgage-backed securities, other asset-backed securities and covered bonds mainly in order to obtain diverse, low cost funding, but also to use as collateral for raising funds via third party bilateral secured funding transactions or for liquidity purposes in the future. The Santander UK group has successfully used bilateral secured transactions as an additional form of medium-term funding; this has allowed the Santander UK group to further diversify its medium-term funding investor base. Loans and advances to customers include portfolios of residential mortgage loans, and receivables derived from credit agreements with retail customers for the purchases of financed vehicles, which are subject to non-recourse finance arrangements. These loans and receivables have been purchased by, or assigned to, structured entities or Abbey Covered Bonds LLP, and have been funded primarily through the issue of mortgage-backed securities, other asset-backed securities or covered bonds. No gain or loss has been recognised as a result of these sales. The structured entities and Abbey Covered Bonds LLP are consolidated as subsidiaries. The Company and its subsidiaries do not own directly, or indirectly, any of the share capital of any of the structured entities. a) Securitisations i) Master trust structures The Santander UK group makes use of master trust structures, whereby a pool of residential mortgage loans is assigned to a trust company by the asset originator. A funding entity acquires a beneficial interest in the pool of assets held by the trust company with funds borrowed from qualifying structured entities, which at the same time issue asset-backed securities to third-party investors or the Santander UK group. Santander UK plc and its subsidiaries receive payments from the securitisation companies in respect of fees for administering the loans, and payment of deferred consideration for the sale of the loans. Santander UK plc and its subsidiaries have no right or obligation to repurchase any securitised loan, except if certain representations and warranties given by Santander UK plc or its subsidiaries at the time of transfer are breached and, in certain cases, if there is a product switch or further advance, if a securitised loan is in arrears for over two months or if a securitised loan does not comply with regulatory requirements. In April 2020, Santander UK plc followed FCA guidance on how they expect mortgage lenders and administrators to treat customers fairly during the Covid-19 pandemic and restructured all its securitisations to accommodate its obligations as servicer under the principles set out in the FCA Handbook and Mortgage Conduct of Business rules. The granting of payment holidays to any securitised loans results in a corresponding decrease in revenue receipts available to the trust company to distribute to the funding entity on each distribution date. To mitigate the potential impact to the securitisations, the qualifying structured entities were amended to direct a cash payment to the funding entity in an amount equal to the funding entity's share of the aggregate amount of the interest that would have been due on any loans which are the subject of a payment holiday. To effect such cash payment, Santander UK plc’s share of revenue receipts is reduced by such amount and the funding entity's share of revenue receipts increased accordingly, making the impact neutral to the securitisation. ii) Other securitisation structures The Santander UK group issues notes through pass-through stand-alone vehicles for the securitisation of receivables derived from credit agreements with retail customers for the purchase of financed vehicles. Santander UK plc and its subsidiaries are under no obligation to support any losses that may be incurred by the master trust or other structures, securitisation companies or holders of the securities, and do not intend to provide such further support. b) Covered bonds Santander UK plc also issues covered bonds, which are its direct, unsecured and unconditional obligation. The covered bonds benefit from a guarantee from Abbey Covered Bonds LLP. Santander UK plc makes a term advance to Abbey Covered Bonds LLP equal to the sterling proceeds of each issue of covered bonds. Abbey Covered Bonds LLP uses the proceeds of the term advance to purchase portfolios of residential mortgage loans and their security from Santander UK plc. Under the terms of the guarantee, Abbey Covered Bonds LLP has agreed to pay an amount equal to the guaranteed amounts when the same shall become due for payment but which would otherwise be unpaid by Santander UK plc. c) Analysis of securitisations and covered bonds The Santander UK group’s principal securitisation programmes and covered bond programme, together with the balances of the advances subject to securitisation (or for the covered bond programme assigned) and the carrying value of the notes in issue at 31 December 2020 and 2019 are listed below. Gross assets External notes in issue Notes issued to Santander UK plc/subsidiaries as collateral 2020 2019 2020 2019 2020 2019 £m £m £m £m £m £m Mortgage-backed master trust structures: – Holmes 3,073 4,262 829 1,931 334 463 – Fosse 2,258 3,708 290 295 1,402 1,404 – Langton 2,782 3,076 — — 2,355 2,354 8,113 11,046 1,119 2,226 4,091 4,221 Other asset-backed securitisation structures: – Motor 189 490 104 324 97 197 – Auto ABS UK Loans 1,460 1,532 1,107 1,229 361 368 1,649 2,022 1,211 1,553 458 565 Total securitisation programmes 9,762 13,068 2,330 3,779 4,549 4,786 Covered bond programmes – Euro 35bn Global Covered Bond Programme 23,670 23,323 19,285 19,004 — — Total securitisation and covered bond programmes 33,432 36,391 21,615 22,783 4,549 4,786 Less: held by Santander UK group: – Euro 35bn Global Covered Bond Programme — — Total securitisation and covered bond programmes (See Note 26) 21,615 22,783 The following table sets out the internal and external issuances and redemptions in 2020 and 2019 for each securitisation and covered bond programme. Internal issuances External issuances Internal redemptions External redemptions 2020 2019 2020 2019 2020 2019 2020 2019 £bn £bn £bn £bn £bn £bn £bn £bn Mortgage-backed master trust structures: – Holmes — — — — 0.3 — 0.9 1.1 – Fosse — 1.4 — 0.1 — — — — – Langton — — — — — — — — Other asset-backed securitisation structures: – Motor — — — — 0.1 0.2 0.2 0.4 – Auto ABS UK Loans — 0.1 0.3 0.2 — 0.1 0.1 0.2 Covered bond programme — — 3.0 2.9 — 0.5 2.7 1.5 — 1.5 3.3 3.2 0.4 0.8 3.9 3.2 Holmes Funding Ltd has a beneficial interest of £1.0bn (2019: £2.1bn) in the residential mortgage loans held by Holmes Trustees Ltd. The remaining share of the beneficial interest in residential mortgage loans held by Holmes Trustees Ltd belongs to Santander UK plc. Fosse Funding (No.1) Ltd has a beneficial interest of £1.7bn (2019: £1.7bn) in the residential mortgage loans held by Fosse Trustee (UK) Ltd. The remaining share of the beneficial interest in residential mortgage loans held by Fosse Trustee (UK) Ltd belongs to Santander UK plc. Langton Funding (No.1) Ltd has a beneficial interest of £2.4bn (2019: £2.4bn) in the residential mortgage loans held by Langton Mortgage Trustee (UK) Ltd. The remaining share of the beneficial interest in residential mortgage loans held by Langton Mortgage Trustee (UK) Ltd belongs to Santander UK plc. The Holmes securitisation companies have cash deposits of £186m (2019: £283m), which have been accumulated to finance the redemption of a number of securities issued by the Holmes securitisation companies. The share of Holmes Funding Ltd in the trust assets is therefore reduced by this amount. Fosse Master Issuer plc has cash deposits of £nil (2019: £nil), which have been accumulated to finance the redemption of a number of securities issued by Fosse Master Issuer plc. Fosse Funding (No.1) Ltd’s beneficial interest in the assets held by Fosse Trustee (UK) Ltd is therefore reduced by this amount.

Transfers of Financial Assets N

Transfers of Financial Assets Not Qualifying for Derecognition	12 Months Ended
	Dec. 31, 2020
Disclosure of transferred financial assets that are not derecognised in their entirety [abstract]
Transfers of Financial Assets Not Qualifying for Derecognition	15. TRANSFERS OF FINANCIAL ASSETS NOT QUALIFYING FOR DERECOGNITION The Santander UK group enters into transactions in the normal course of business by which it transfers recognised financial assets directly to third parties or to structured entities. These transfers may give rise to the full or partial derecognition of those financial assets. Transferred financial assets that do not qualify for derecognition consist of (i) securities held by counterparties as collateral under repurchase agreements, (ii) securities lent under securities lending agreements, and (iii) loans that have been securitised under arrangements by which the Santander UK group retains a continuing involvement in such transferred assets. As the substance of the sale and repurchase and securities lending transactions is secured borrowings, the asset collateral continues to be recognised in full and the related liability reflecting the Santander UK group’s obligation to repurchase the transferred assets for a fixed price at a future date is recognised in deposits from banks or customers, as appropriate. As a result of these transactions, the Santander UK group is unable to use, sell or pledge the transferred assets for the duration of the transaction. The Santander UK group remains exposed to interest rate risk and credit risk on these pledged instruments. The counterparty’s recourse is not limited to the transferred assets. The Santander UK group securitisation transfers do not qualify for derecognition. The Santander UK group remains exposed to credit risks arising from the mortgage loans or credit agreements and has retained control of the transferred assets. Circumstances in which the Santander UK group has continuing involvement in the transferred assets may include retention of servicing rights over the transferred assets, entering into a derivative transaction with the securitisation vehicle, retaining an interest in the securitisation vehicle or providing a cash reserve fund. Where the Santander UK group has continuing involvement it continues to recognise the transferred assets to the extent of its continuing involvement and recognises an associated liability. The net carrying amount of the transferred assets and associated liabilities reflects the rights and obligations that the Santander UK group has retained. The following table analyses the carrying amount of financial assets that did not qualify for derecognition and their associated financial liabilities: Group 2020 2019 Assets Liabilities Assets Liabilities Nature of transaction £m £m £m £m Sale and repurchase agreements 1,597 (1,340) 7,592 (6,739) Securities lending agreements 918 (752) 195 (143) Securitisations (See Notes 14 and 26) 6,980 (2,330) 9,992 (3,779) 9,495 (4,422) 17,779 (10,661)

Reverse Repurchase Agreements -

Reverse Repurchase Agreements - Non Trading	12 Months Ended
	Dec. 31, 2020
Disclosure of reserve repurchase agreements non-trading [abstract]
Reverse Repurchase Agreements - Non Trading	16. REVERSE REPURCHASE AGREEMENTS – NON TRADING Group 2020 2019 £m £m Agreements with banks 1,258 2,161 Agreements with customers 18,341 21,475 19,599 23,636

Other Financial Assets At Amort

Other Financial Assets At Amortised Cost	12 Months Ended
	Dec. 31, 2020
Financial assets at fair value through profit or loss [abstract]
Other Financial Assets At Amortised Cost	17. OTHER FINANCIAL ASSETS AT AMORTISED COST Group 2020 2019 £m £m Asset backed securities 491 532 Debt securities 672 6,524 1,163 7,056

Financial Assets At Fair Value

Financial Assets At Fair Value Through Other Comprehensive Income	12 Months Ended
	Dec. 31, 2020
Financial assets at fair value through other comprehensive income [abstract]
Financial Assets At Fair Value Through Other Comprehensive Income	18. FINANCIAL ASSETS AT FAIR VALUE THROUGH OTHER COMPREHENSIVE INCOME Group 2020 2019 £m £m Debt securities 8,929 9,691 Loans and advances to customers 21 56 8,950 9,747

Interests in Other Entities

Interests in Other Entities	12 Months Ended
	Dec. 31, 2020
Investments accounted for using equity method [abstract]
Interests in Other Entities	19. INTERESTS IN OTHER ENTITIES Group 2020 2019 £m £m Subsidiaries — — Joint ventures 172 117 0 172 117 The Santander UK group consists of a parent company, Santander UK plc, incorporated and domiciled in the UK and a number of subsidiaries and joint ventures held directly and indirectly by the Company. The Company has no individually significant associates. a) Interests in subsidiaries The Company holds directly or indirectly 100% of the issued ordinary share capital of its principal subsidiaries. All companies operate principally in their country of incorporation or registration. Subsidiaries with significant non-controlling interests The only subsidiary with significant non-controlling interests is PSA Finance UK Limited, which operates in the UK. In 2020 and 2019, the proportion of ownership interests and voting rights held by non-controlling interests was 50%. 2020 2019 £m £m Profit attributable to non-controlling interests 19 19 Accumulated non-controlling interests of the subsidiary 162 160 Dividends paid to non-controlling interests 15 12 Summarised financial information: Total assets 3,451 3,228 Total liabilities 3,127 2,905 Profit for the year 38 40 Total comprehensive income for the year 33 40 Interests in consolidated structured entities Structured entities are formed by Santander UK to accomplish specific and well-defined objectives. Santander UK consolidates these structured entities when the substance of the relationship indicates control, as described in Note 1. In addition to the structured entities disclosed in Note 14 which are used for securitisation and covered bond programmes, the only other structured entities consolidated by Santander UK are described below. All the external assets and liabilities in these entities are included in the financial statements and in relevant Notes. Other than as set out below, no significant judgements were required with respect to control or significant influence. i) Guaranteed Investment Products 1 PCC Limited (GIP) All protected cells in GIP, a Guernsey-incorporated, closed-ended, protected cell company, were redeemed in 2020 with the final redemption taking place on 31 December 2020. Santander Guarantee Company, a Santander UK group company, guaranteed the shareholders of cells a fixed return on their investment and/or the investment amount on redemption. As Santander UK has no remaining exposure to the variable risks and returns through Santander Guarantee Company’s guarantee, it no longer consolidates GIP. ii) Motor Securities 2018-1 Designated Activity Company (Motor 2018) Motor 2018 is a credit protection entity, and a Designated Activity Company limited by shares, incorporated in Ireland. It has issued a series of credit linked notes varying in seniority which reference portfolios of Santander UK group loans. Concurrently, these entities sell credit protection to Santander UK in respect of the referenced loans and, in return for a fee, are liable to make protection payments to Santander UK upon the occurrence of a credit event in relation to any of the referenced loans. Motor 2018 is consolidated as Santander UK holds a variable interest by retaining the junior tranche of notes issued by the entity. b) Interests in joint ventures Santander UK does not have any individually material interests in joint ventures. As set out in the accounting policies in Note 1, interests in joint ventures are accounted for using the equity method. In 2020, Santander UK’s share in the profit after tax of its joint ventures was £20m (2019: £30m) before elimination of transactions between Santander UK and the joint ventures. At 31 December 2020, the carrying amount of Santander UK’s interest was £172m (2019: £117m). At 31 December 2020 and 2019, the joint ventures had no commitments and contingent liabilities. c) Interests in unconsolidated structured entities Structured entities sponsored by the Santander UK group Santander UK has interests in structured entities which it sponsors but does not control. Santander UK considers itself a sponsor of a structured entity when it facilitates the establishment of the structured entity. Other than as set out below, no significant judgements were required with respect to control or significant influence. The structured entities sponsored but not consolidated by Santander UK are as follows. i) Santander (UK) Common Investment Fund The Santander (UK) Common Investment Fund (the Fund) is a common investment fund that was established to hold the assets of the Santander (UK) Group Pension Scheme. The Fund is not consolidated by Santander UK, but its assets of £13,553m (2019: £12,446m) are accounted for as part of the defined benefit assets and obligations recognised on Santander UK’s balance sheet. For more on the Fund, see Note 30. As the Fund holds the assets of the pension scheme, it is outside the scope of IFRS 10. Santander UK’s maximum exposure to loss is the carrying amount of the assets held. ii) Credit protection entities Santander UK has established three (2019: four) unconsolidated credit protection entities, which are Designated Activity Companies limited by shares, incorporated in Ireland. Each entity has issued a series of credit linked notes varying in seniority which reference portfolios of Santander UK group loans. Concurrently, these entities sell credit protection to Santander UK in respect of the referenced loans and, in return for a fee, are liable to make protection payments to Santander UK upon the occurrence of a credit event in relation to any of the referenced loans. Senior credit linked notes, which amounted to £2,160m (2019: £3,766m), are issued to, and held by, Santander UK. Junior credit linked notes, which amounted to £678m (2019: £825m), are all held by third party investors and suffer the first losses incurred in the referenced portfolios. Funds raised by the sale of the credit linked notes are deposited with Santander UK as collateral for the credit protection. The senior credit linked notes, along with the deposits and associated guarantees, are presented on a net basis, to reflect a legal right of set-off between the principal amounts of senior notes and the cash deposits. Deposits and associated guarantees in respect of the junior credit linked notes are included in ‘Deposits by customers’ (see Note 23). The entities are not consolidated by Santander UK because the third party investors have the exposure, or rights, to all of the variability of returns from the performance of the entities. No assets are transferred to, or income received from, these entities. Since the credit linked notes (including those held by Santander UK) are fully cash collateralised, Santander UK’s maximum exposure to loss is equal to any unamortised fees paid to the entities in connection with the credit protection outlined above. Structured entities not sponsored by the Santander UK group Santander UK also has interests in structured entities which it does not sponsor or control. These consist of holdings of mortgage and other asset backed securities issued by entities that were established and/or sponsored by other unrelated financial institutions. These securities comprise the asset backed securities included in Note 17. Management has concluded that the Santander UK group has no control or significant influence over these entities and that the carrying value of the interests held in these entities represents the maximum exposure to loss.

Intangible Assets

Intangible Assets	12 Months Ended
	Dec. 31, 2020
Disclosure of detailed information about intangible assets [abstract]
Intangible Assets	20. INTANGIBLE ASSETS a) Goodwill Group Cost Accumulated impairment Net book value £m £m £m At 31 December 2019, 1 January 2020 and 31 December 2020 1,269 (66) 1,203 Impairment of goodwill In 2020 and 2019, no impairment of goodwill was recognised. Goodwill is tested for impairment annually at 30 November, with a review for indicators of impairment at 30 June and 31 December. Goodwill is tested for impairment if reviews identify an indicator of impairment or when events or changes in circumstances dictate. At 30 November 2020, the annual review identified the continuing uncertainty created by the Covid-19 pandemic and its potential impact on the carrying value of goodwill as indicators of impairment for all cash-generating units (CGUs). As a result, management performed an updated impairment test at 31 December 2020 for all CGUs. Basis of the recoverable amount The recoverable amount of all CGUs was determined based on its value in use (VIU) at each testing date for 2020 and 2019. For each CGU, the VIU is calculated by discounting management’s cash flow projections for the CGU. The cash flow projections also take account of increased internal capital allocations needed to achieve internal and regulatory capital targets including the leverage ratio. The key assumptions used in the VIU calculation for each CGU are set out below. Key assumptions in the VIU calculation. Goodwill Discount rate Growth rate beyond initial cash flow projections 2020 2019 2020 2019 2020 2019 CGU £m £m % % % % Personal financial services 1,169 1,169 13.6 9.8 1.6 1.6 Private banking 30 30 8.9 9.8 1.6 1.6 Other 4 4 13.6 9.8 1.6 1.6 1,203 1,203 The CGUs do not carry on their balance sheets any other intangible assets with indefinite useful lives. Management’s judgement in estimating the cash flows of a CGU The cash flow projections for the purpose of impairment testing for each CGU are derived from the latest 3-year plan presented to the Board. The Board challenges and endorses management’s planning assumptions in light of internal capital allocations needed to support Santander UK’s strategy, current market conditions and the macro-economic outlook. For the goodwill impairment tests conducted at 31 December 2020, the determination of the carrying value of the Personal Financial Services CGU was based on an allocation of regulatory capital and management’s cash flow projections until the end of 2023. The assumptions included in the cash flow projections reflect an allocation to the cost of capital to support future growth, as well as the expected impact of Covid-19 on the UK economic environment and the financial outlook within which the CGUs operate. The cash flow projections are supported by Santander UK’s base case economic scenario. For more on the base case economic scenario, including our forecasting approach and the assumptions in place at 31 December 2020, see the Credit risk – Santander UK group level section of the Risk review. Cash flow projections for the purpose of impairment testing do not take account of any adverse outcomes arising from contingent liabilities (see Note 31), whose existence will be confirmed by uncertain future events or where any obligation is not probable or otherwise cannot be measured reliably, nor do they take account of the benefits arising from Santander UK’s transformation plans that had not yet been implemented or committed at 31 December 2020. Discount rate The rate used to discount the cash flows is based on the cost of equity assigned to each CGU, which is derived using a capital asset pricing model (CAPM). The CAPM depends on a number of inputs reflecting financial and economic variables, including the risk-free rate and a premium to reflect the inherent risk of the business being evaluated. These variables are based on the market’s assessment of the economic variables and management’s judgement. Growth rate beyond initial cash flow projections The growth rate for periods beyond the initial cash flow projections is used to extrapolate the cash flows in perpetuity because of the long-term perspective of CGUs. In line with the accounting requirements, management uses the UK Government’s official estimate of UK long-term average GDP growth rate, as this is lower than management's estimate of the long-term average growth rate of the business. Sensitivities of key assumptions in calculating VIU The VIU of each CGU remains higher than the carrying value of the related goodwill. However, at 31 December 2020, the Personal Financial Services CGU was sensitive to reasonably possible changes in the key assumptions supporting the recoverable amount, although no reasonably possible changes in any of the key assumptions would have resulted in an impairment. The table below presents a summary of the key assumptions underlying the most sensitive inputs to the model for the Personal Financial Services CGU, the key risks associated with each and details of a reasonably possible change in assumptions that could result in an impairment. The VIU calculation is not sensitive to the UK long-term average GDP growth rate assumption given the amount of headroom as the increased profit after tax generated by growth of the business is mostly offset by the need to retain more profit to meet increased regulatory capital requirements driven by the growth in assets. Reasonably possible changes in key assumptions CGU Input Key assumptions Associated risks Reasonably possible change Personal Financial Services – Cash flow projections – BoE Bank Rate – UK house price growth – UK mortgage loan market growth – UK unemployment rate – Position in the market – Regulatory capital levels. – Uncertain market outlook – Persistent low interest rate environment – Customer remediation and regulatory action outcomes – Uncertain regulatory capital requirements. – Cash flow projections decrease by 5%. – Discount rate – Discount rate used is a reasonable estimate of a suitable market rate for the profile of the business. – Market rates of interest rise. – Discount rate increases by 100 basis points. Sensitivity of VIU changes to current assumptions to achieve nil headroom Although there was no impairment of goodwill at 31 December 2020, the amount by which the Personal Financial Services CGU’s recoverable amount exceeds the carrying value (the headroom) reduced and the test is now sensitive to some of the assumptions used. The changes in assumptions detailed below for the discount rate and cash flow projections would eliminate the current headroom. As a result, there is a risk of impairment in the future should business performance or economic factors diverge from forecasts. The sensitivity analysis presented below has been prepared on the basis that a change in each key assumption would not have a consequential impact on other assumptions used in the impairment review. However, due to the interrelationships between some of the assumptions, a change in one of the assumptions might impact one or more of the other assumptions and could result in a larger or smaller overall impact. Carrying value Value in use Headroom Increase in post tax discount rate Decrease in cash flows CGU £m £m £m bps % Personal Financial Services 6,758 8,602 1,844 239 22 At 31 December 2019, there were no reasonably possible changes in any of the key assumptions that would have resulted in an impairment. b) Other intangibles Group Accumulated Net book value Cost £m £m £m At 1 January 2020 1,249 (686) 563 Additions 102 — 102 Disposals (47) 47 — Charge — (197) (197) Impairment — (25) (25) At 31 December 2020 1,304 (861) 443 At 1 January 2019 1,090 (485) 605 Additions 173 — 173 Disposals (14) — (14) Charge — (191) (191) Impairment — (10) (10) At 31 December 2019 1,249 (686) 563

Property, Plant and Equipment

Property, Plant and Equipment	12 Months Ended
	Dec. 31, 2020
Property, plant and equipment [abstract]
Property, Plant and Equipment	21. PROPERTY, PLANT AND EQUIPMENT Group Property Office fixtures and equipment Computer software Operating lease assets Right-of-use assets Total (3) £m £m £m £m £m £m Cost: At 1 January 2020 1,270 1,436 439 738 212 4,095 Additions 61 43 2 185 8 299 Disposals (59) (104) (5) (203) (2) (373) At 31 December 2020 1,272 1,375 436 720 218 4,021 Accumulated depreciation: At 1 January 2020 454 1,016 434 164 60 2,128 Charge for the year (2) 79 111 — 92 58 340 Disposals (44) (59) — (78) — (181) At 31 December 2020 489 1,068 434 178 118 2,287 Net book value 783 307 2 542 100 1,734 Cost: At 1 January 2019 (1) 1,291 1,398 438 604 209 3,940 Additions 5 70 1 251 4 331 Disposals (26) (32) — (117) (1) (176) At 31 December 2019 1,270 1,436 439 738 212 4,095 Accumulated depreciation: At 1 January 2019 429 903 434 134 — 1,900 Charge for the year 39 161 — 81 60 341 Disposals (14) (48) — (51) — (113) At 31 December 2019 454 1,016 434 164 60 2,128 Net book value 816 420 5 574 152 1,967 (1) Represents the value of the right-of-use assets, principally premises, recognised on 1 January 2019 upon adoption of IFRS 16. (2) Following a review of the estimated useful lives of property as part of Santander UK's transformation program, the charge for the year includes accelerated property depreciation of £9m (2019: £2m). (3) Property, plant and equipment includes assets under construction of £55m (2019: £16m).

Other Financial Liabilities at

Other Financial Liabilities at Fair Value Through Profit or Loss	12 Months Ended
	Dec. 31, 2020
Disclosure of financial liabilities [abstract]
Other Financial Liabilities at Fair Value Through Profit or Loss	22. OTHER FINANCIAL LIABILITIES AT FAIR VALUE THROUGH PROFIT OR LOSS Group 2020 2019 £m £m US$30bn Euro Medium Term Note Programme 102 159 Structured Notes Programmes 805 809 Eurobonds 150 137 Structured deposits 375 435 Collateral and associated financial guarantees 2 173 Repurchase agreements - non trading — — 1,434 1,713 For the Santander UK group all (2019: all) of the other financial liabilities at fair value through profit or loss were designated as such. Collateral and associated financial guarantees in the table above represent collateral received, together with associated credit protection guarantees, in respect of the proceeds of the retained senior tranches of credit linked notes described in Note 12. The financial guarantees are valued using the same parameters as the related credit linked notes, such that changes in the respective valuations are offset exactly, and there is no charge or credit to the income statement. In 2019 £2.1bn of cash deposits, which were previously included within collateral and associated financial guarantees in the table above, were presented on a net basis. This followed a deed of amendment, including a legal right of set-off between the principal amounts of senior tranches of credit linked notes, classified as debt securities in Note 12, and the cash deposits. At 31 December 2020 the amount of this netting was £825m (2019: £1.5bn). For more, see ‘Credit protection entities’ in Note 19. Gains and losses arising from changes in the credit spread of securities issued by the Santander UK group reverse over the contractual life of the debt, provided that the debt is not repaid at a premium or a discount. The net loss during the year attributable to changes in the Santander UK group’s own credit risk on the above securities was £3m (2019: £77m loss, 2018: £84m gain). The cumulative net loss attributable to changes in the Santander UK group’s own credit risk on the above securities at 31 December 2020 was £3m (2019: £nil). At 31 December 2020, the amount that would be required to be contractually paid at maturity of the securities above was £11m higher ( 2019: £4m lower) than the carrying value.

Deposits by Customers

Deposits by Customers	12 Months Ended
	Dec. 31, 2020
Deposits from customers [abstract]
Deposits by Customers	23. DEPOSITS BY CUSTOMERS Group 2020 2019 £m £m Demand and time deposits (1) 185,879 171,736 Amounts due to other Santander UK Group Holdings plc subsidiaries 59 44 Amounts due to Santander UK Group Holdings plc (2) 7,883 8,869 Amounts due to fellow Banco Santander subsidiaries and joint ventures 1,314 1,234 195,135 181,883 (1) Includes equity index-linked deposits of £577m (2019: £1,139m). The capital amount guaranteed/protected and the amount of return guaranteed in respect of the equity index-linked deposits were £577m and £2m (2019: £1,139m and £18m) respectively. (2) Includes downstreamed funding from our immediate parent company Santander UK Group Holdings plc.

Deposits by Banks

Deposits by Banks	12 Months Ended
	Dec. 31, 2020
Disclosure of deposits from banks [abstract]
Deposits by Banks	24. DEPOSITS BY BANKS Group 2020 2019 £m £m Items in the course of transmission 375 337 Deposits held as collateral 2,063 2,169 Other deposits (1) 18,519 11,847 Amounts due to Santander UK subsidiaries 1 — 20,958 14,353 (1) Includes drawdown from the TFS of £6.3bn (2019: £10.8bn) and drawdown from the TFSME of £11.7bn (2019:£nil).

Repurchase Agreements - Non Tra

Repurchase Agreements - Non Trading	12 Months Ended
	Dec. 31, 2020
Disclosure Of Repurchase Agreement [Abstract]
Repurchase Agreements - Non Trading	25. REPURCHASE AGREEMENTS – NON TRADING Group 2020 2019 £m £m Agreements with banks 6,358 10,227 Agreements with customers 9,490 8,059 15,848 18,286

Debt Securities in Issue

Debt Securities in Issue	12 Months Ended
	Dec. 31, 2020
Debt Securities In Issue [Abstract]
Debt Securities in Issue	26. DEBT SECURITIES IN ISSUE Group 2020 2019 £m £m Medium-term notes: – US$30bn Euro Medium Term Note Programme 1,694 4,679 – Euro 30bn Euro Medium Term Note Programme 388 1,896 – US SEC-registered Debt Programme – Santander UK plc 4,723 5,891 – Euro 750m Senior Unsecured Notes 673 — – US$1bn Senior Unsecured Notes 734 — 8,212 12,466 Euro 35bn Global Covered Bond Programme (See Note 14) 19,285 19,004 US$20bn Commercial Paper Programmes 2,824 3,014 Certificates of deposit 2,858 2,806 Credit linked notes 57 60 Securitisation programmes (See Note 14) 2,330 3,779 35,566 41,129

Subordinated Liabilities

Subordinated Liabilities	12 Months Ended
	Dec. 31, 2020
Disclosure of Subordinated Liabilities [Abstract]
Subordinated Liabilities	27. SUBORDINATED LIABILITIES Group 2020 2019 £m £m £325m Sterling preference shares 344 344 Undated subordinated liabilities 557 581 Dated subordinated liabilities 1,655 2,603 2,556 3,528 In 2020, the Santander UK group repurchased certain debt securities and subordinated liabilities as part of ongoing liability management exercises, resulting in a loss of £24m. In 2019, the Santander UK group did not repurchase any of its debt securities or subordinated liabilities. The above securities will, in the event of the winding up of the issuer, be subordinated to the claims of depositors and all other creditors of the issuer, other than creditors whose claims rank equally with, or are junior to, the claims of the holders of the subordinated liabilities. The subordination amongst each of the subordinated liabilities upon a winding up of the issuer is specified in their respective terms and conditions. In 2020 and 2019, the Santander UK group had no defaults of principal, interest or other breaches with respect to its subordinated liabilities. No repayment or purchase by the issuer of the subordinated liabilities may be made prior to their stated maturity without the consent of the PRA. Undated subordinated liabilities Group 2020 2019 First call date £m £m 10.0625% Exchangeable capital securities n/a 205 205 7.375% 20 Year Step-up perpetual callable subordinated notes 2020 — 15 7.125% 30 Year Step-up perpetual callable subordinated notes 2030 352 361 557 581 In common with other debt securities issued by Santander UK group companies and notwithstanding the issuer’s first call dates in the table above, in the event of certain tax changes affecting the treatment of payments of interest on subordinated liabilities in the UK, the 7.125% 30 Year Step-up perpetual callable subordinated notes are redeemable at any time, and the 10.0625% Exchangeable capital securities are redeemable on any interest payment date – each in whole at the option of Santander UK plc, at their principal amount together with any accrued interest. The 7.375% 20 Year Step-up perpetual callable subordinated notes were redeemed in full on 28 September 2020. The 10.0625% Exchangeable capital securities are exchangeable into fully paid 10.375% non-cumulative non-redeemable sterling preference shares of £1 each, at the option of Santander UK plc, on the business day immediately following any interest payment date. Dated subordinated liabilities Group 2020 2019 Maturity £m £m 5% Subordinated notes (US$1,500m) 2023 542 1,132 4.75% Subordinated notes (US$1,000m) 2025 536 763 7.95% Subordinated notes (US$1,000m) 2029 242 280 6.50% Subordinated notes 2030 31 40 5.875% Subordinated notes 2031 10 9 5.625% Subordinated notes (US$500m) 2045 294 379 1,655 2,603 The dated subordinated liabilities are redeemable in whole at the option of Santander UK plc in the event of certain tax changes affecting the treatment of payments of interest on the subordinated liabilities in the UK, at their principal amount together with any accrued interest.

Other Liabilities

Other Liabilities	12 Months Ended
	Dec. 31, 2020
Subclassifications of assets, liabilities and equities [abstract]
Other Liabilities	28. OTHER LIABILITIES Group 2020 2019 £m £m Lease liabilities 97 137 Other 2,240 2,207 2,337 2,344

Provisions

Provisions	12 Months Ended
	Dec. 31, 2020
Disclosure of other provisions [abstract]
Provisions	29. PROVISIONS Group Conduct remediation PPI Other products Bank Levy Property Off balance sheet ECL Regulatory and other Total £m £m £m £m £m £m £m At 1 January 2020 189 25 46 59 78 175 572 Additional provisions (See Note 8) — — 72 9 — 208 289 Provisions released (See Note 8) — (15) — (6) (3) (1) (25) Utilisation and other (1) (113) (2) (94) (16) — (157) (382) Recharge (2) — — 10 — — — 10 At 31 December 2020 76 8 34 46 75 225 464 To be settled: – Within 12 months 76 2 34 25 75 111 323 – In more than 12 months — 6 — 21 — 114 141 76 8 34 46 75 225 464 a) Conduct remediation i) Payment Protection Insurance (PPI) At 31 December 2020, the remaining provision for PPI redress and related costs was £76m (2019: £189m). There was no additional provision in 2020. Cumulative complaints from the inception of the PPI complaints process to 31 December 2020, regardless of the likelihood of Santander UK incurring a liability, were 4.6m. At 31 December 2020, there were an estimated 3,500 complaints still requiring assessment and we had also entered into a commercial negotiation with the Official Receiver. Although the deadline for bringing complaints has passed, customers can still commence litigation concerning the historical sale of PPI. Provision has been made for the best estimate of any obligation to pay compensation in respect of current stock and estimated future claims. However, there are ongoing factual issues to be resolved regarding such litigation which may have legal consequences including the volume and quality of future litigation claims. As a result, the extent of the potential liability and amount of any compensation to be paid remains uncertain. The provision for conduct remediation recognised represents management’s best estimate of Santander UK’s liability in respect of mis-selling of PPI policies. (ii) Other products A provision for conduct remediation has also been recognised in respect of sales of other products. A number of uncertainties remain as to the eventual costs with respect to conduct remediation in respect of these products given the inherent difficulties in determining the number of customers involved and the amount of any redress to be provided to them. The remaining provision for other conduct was £8m (2019: £25m), which primarily related to the sale of mortgage endowments. b) Bank Levy In addition to changes in UK corporation tax rates, Finance (No.2) Act 2015 reduced the UK Bank Levy rate from 0.21% via subsequent annual reductions to 0.10% from 1 January 2021. As a result, a rate of 0.14% applies for 2020 (2019: 0.15%). The UK Bank Levy cost for 2020 was £72m (2019: £86m, 2018: £69m). The Santander UK group paid £88m in 2020 (2019: £90m) and provided for a liability of £34m at 31 December 2020 (2019: £46m). c) Property Property provisions include vacant property provisions and property dilapidation provisions for leased properties within the scope of IFRS 16. Vacant property provisions are made by reference to an estimate of any expected sub-let income, compared to the head rent, and the possibility of disposing of Santander UK’s interest in the lease, taking into account conditions in the property market. These provisions are reassessed on a semi-annual basis and will normally run off over the period of the leases concerned. Where a property is disposed of earlier than anticipated, any remaining provision relating to that property is released. Property provisions were impacted by £1m of transformation charges in 2020. These relate to a multi-year project to deliver on our strategic priorities and enhance efficiency in order for us to better serve our customers and meet our medium-term targets. These charges largely related to restructuring of our branch network, and in particular the closure of university branches in 2020. d) Off-balance sheet ECL Provisions include expected credit losses relating to guarantees given to third parties and undrawn loan commitments. e) Regulatory and other Regulatory and other provisions principally comprised amounts in respect of regulatory charges (including fines), operational loss and operational risk provisions, restructuring charges and litigation and related expenses. A number of uncertainties exist with respect to these provisions given the uncertainties inherent in operational, restructuring and litigation matters that affect the amount and timing of any potential outflows with respect to which provisions have been established. These provisions are reviewed at least quarterly. At 31 December 2020 Regulatory and other provisions included an amount of £47m (2019: £68m) that arose from a systems-related historical issue identified by Santander UK, relating to compliance with certain requirements of the Consumer Credit Act (CCA). This provision is based on detailed reviews of relevant systems related to consumer credit business operations, supported by external legal and regulatory advice, and reflects our best estimate at 31 December 2020 of potential costs in respect of the identified issue. As detailed in Note 31, there are aspects of the issue which remain under review. The balance also included an amount in respect of our best estimate of liability relating to a legal dispute regarding allocation of responsibility for a specific PPI portfolio of complaints, further described in Note 31. No further information on the best estimate is provided on the basis that it would be seriously prejudicial. In 2020 there was a charge of £17m included in Regulatory and other provisions, relating to breaches of certain requirements to provide SMS warning alerts to customers regarding overdraft charges in our Retail Banking Business. It also included a charge of £65m as part of our multi-year transformation programme to improve future returns, focused on simplifying, digitising and automating the bank, a charge for operational risk provisions of £91m, and smaller charges for legal and redundancy provisions.

Retirement Benefit Plans

Retirement Benefit Plans	12 Months Ended
	Dec. 31, 2020
Disclosure of net defined benefit liability (asset) [abstract]
Retirement Benefit Plans	30. RETIREMENT BENEFIT PLANS The amounts recognised in the balance sheet were as follows: Group 2020 2019 £m £m Assets/(liabilities) Funded defined benefit pension scheme - surplus 495 669 Funded defined benefit pension scheme - deficit (361) (239) Unfunded pension and post retirement medical benefits (42) (41) Total net assets 92 389 Remeasurement losses/(gains) recognised in other comprehensive income during the year were as follows: Group 2020 2019 2018 £m £m £m Pension remeasurement 505 522 (470) An expense of £66m (2019: £66m, 2018: £67m) was recognised for defined contribution plans in the period and is included in staff costs classified within operating expenses (see Note 6). b) Defined benefit pension schemes The Santander UK group operates a number of defined benefit pension schemes. The main scheme is the Santander (UK) Group Pension Scheme (the Scheme). It comprises seven legally segregated sections. The Scheme covers 11% (2019: 11%) of the Santander UK group’s current employees and is a funded defined benefit scheme which is closed to new members. The corporate trustee of the Scheme is Santander (UK) Group Pension Scheme Trustees Limited (the Trustee), a private limited company incorporated in 1996 and a wholly-owned subsidiary of Santander UK Group Holdings plc. The principal duty of the Trustee is to act in the best interests of the members of the Scheme. The Trustee board comprises five (2019: five) Directors selected by Santander UK Group Holdings plc, plus five (2019: five) member-nominated Directors selected from eligible members who apply for the role. The assets of the funded schemes including the Scheme are held independently of the Santander UK group’s assets in separate trustee administered funds. Investment strategy across the sections of the Scheme remains under regular review. Investment decisions are delegated by the Trustee to a common investment fund, managed by Santander (CF Trustee) Limited, a private limited company owned by five Trustee directors, three appointed by Santander UK plc and two by the Trustee. The Santander (CF Trustee) Limited directors’ principal duty, within the investment powers delegated to them, is to act in the best interest of the members of the Scheme. Ultimate responsibility for investment policy and strategy rests with the Trustee of the Scheme who is required under the Pensions Act 2004 to prepare a statement of investment principles. The defined benefit pension schemes expose the Santander UK group to risks such as investment risk, interest rate risk, longevity risk and inflation risk. The Santander UK group does not hold material insurance policies over the defined benefit pension schemes and has not entered into any significant transactions with them. Formal actuarial valuations of the assets and liabilities of the defined benefit schemes are carried out on at least a triennial basis by independent professionally-qualified actuaries and valued for accounting purposes at each balance sheet date. The Scheme Trustee is responsible for the actuarial valuations and in doing so considers, or relies in part on, a report of a third-party expert. The latest formal actuarial valuation for the Scheme at 31 March 2019 was finalised in August 2019, with a deficit to be funded of £1,136m. The next scheduled triennial funding valuation will be at 31 March 2022. Any funding surpluses can be recovered by Santander UK plc from the Scheme through refunds as the Scheme is run off over time or could be used to pay for the cost of benefits which are accruing. The main differences between the assumptions used for assessing the defined benefit liabilities for the funding valuation and those used for IAS19 is that the financial and demographic assumptions used for the funding valuation are generally more prudent than those used for the IAS 19 valuation. The total amount charged to the income statement was as follows: Group 2020 2019 2018 £m £m £m Net interest income (10) (23) (7) Current service cost 36 34 41 Past service and GMP costs 1 1 41 Administration costs 8 8 8 35 20 83 On 26 October 2018, the High Court handed down a judgement concluding that defined benefit schemes should equalise pension benefits for men and women in relation to GMP and concluded on the methods that were appropriate. The estimated increase in liabilities at the date of the judgement was £40m and was based on a number of assumptions and the actual impact may be different. This was reflected in the income statement and in the closing net accounting surplus of the Scheme in 2018. The allowance included in the Scheme liabilities at 31 December 2020 increased by £5m (2019: £5m) to £50m (2019: £45m) to reflect the latest assumptions. This change was recognised in other comprehensive income. Work is being undertaken to consider and agree how to implement GMP equalisation. On 20 November 2020, a further court ruling on the Lloyds GMP equalisation case took place on the issue of whether or not there is an obligation to equalise transfers that occurred prior to October 2018. It was concluded that historic transfers should be equalised. The potential additional liability was estimated and was not material. As a result, no additional liability has been accounted for. The amounts recognised in other comprehensive income were as follows: Group 2020 2019 2018 £m £m £m Return on plan assets (excluding amounts included in net interest expense) (1,328) (855) 246 Actuarial losses/(gains) arising from changes in demographic assumptions 34 42 (56) Actuarial (gains)/losses arising from experience adjustments (141) (42) 15 Actuarial losses/(gains) arising from changes in financial assumptions 1,940 1,377 (675) Pension remeasurement 505 522 (470) Movements in the present value of defined benefit scheme obligations were as follows: Group 2020 2019 £m £m At 1 January (12,158) (10,804) Current service cost paid by Santander UK plc (24) (22) Current service cost paid by subsidiaries (12) (12) Interest cost (253) (308) Employer salary sacrifice contributions (2) (9) Past service cost (1) (1) Remeasurement due to actuarial movements arising from: – Changes in demographic assumptions (34) (42) – Experience adjustments 141 42 – Changes in financial assumptions (1,940) (1,377) Benefits paid 396 375 At 31 December (13,887) (12,158) Movements in the fair value of the schemes’ assets were as follows: Group 2020 2019 £m £m At 1 January 12,547 11,532 Interest income 263 331 Contributions paid by employer and scheme members 245 212 Contributions paid by fellow Banco Santander subsidiaries — — Administration costs paid (8) (8) Return on plan assets (excluding amounts included in net interest expense) 1,328 855 Benefits paid (396) (375) At 31 December 13,979 12,547 The composition and fair value of the schemes’ assets by category was: Group Quoted prices in active markets Prices not quoted in active markets Total 2020 £m % £m % £m % UK equities 40 — — — 40 — Overseas equities 1,271 9 1,004 7 2,275 16 Corporate bonds 1,121 8 457 3 1,578 11 Government fixed interest bonds 1,618 12 — — 1,618 12 Government index-linked bonds 6,695 48 — — 6,695 48 Property — — 1,454 10 1,454 10 Derivatives — — 312 2 312 2 Cash — — 1,161 8 1,161 8 Repurchase agreements (1) — — (2,198) (15) (2,198) (15) Other — — 1,044 8 1,044 8 10,745 77 3,234 23 13,979 100 2019 UK equities 128 1 — — 128 1 Overseas equities 1,742 14 933 7 2,675 21 Corporate bonds 1,333 11 444 4 1,777 15 Government fixed interest bonds 2,710 22 — — 2,710 22 Government index-linked bonds 4,543 35 — — 4,543 35 Property — — 1,332 11 1,332 11 Derivatives — — 94 1 94 1 Cash — — 984 8 984 8 Repurchase agreements (1) — — (3,263) (26) (3,263) (26) Other — — 1,567 12 1,567 12 10,456 83 2,091 17 12,547 100 (1) Sale and repurchase agreements net of purchase and resale agreements. Scheme assets are stated at fair value based upon quoted prices in active markets except for property, property funds, derivatives, private equity and those classified under ‘Other’. The ‘Other’ category includes annuities, infrastructure funds and hedge funds. The property funds and the infrastructure funds were valued using market valuations prepared by an independent expert. Investments in hedge funds that are included in the ‘Other’ category, and investments in foreign exchange, inflation, equity and interest rate derivatives that are included in the ‘Derivatives’ category, were valued by investment managers by reference to market observable data. Private equity funds were valued by reference to the latest valuation statements provided by the private equity managers. The insured annuities were valued by actuaries based on the liabilities insured. At 31 December 2020, the value of the insured annuities included in the ‘Other’ category included the value of a pensioner buy-in that was entered into on 27 May 2020 by the Trustee with an insurance company. This transaction insured 100% of the SMA section pensioner liabilities and 50% of the SPI section pensioner liabilities based on membership in the Scheme at 31 December 2018. At 31 December 2020, as highlighted above the Scheme was invested in certain assets whose values are not based on market observable data, such as investments in private equity funds and property. Due diligence has been conducted to ensure the values obtained in respect of these assets are appropriate and represent fair value. A strategy is in place to manage interest rate and inflation risk relating to the liabilities. In 2020, the level of interest rate and inflation rate hedging was increased. The Scheme also has in place an equity collar to manage equity risk and hedges a proportion of its foreign exchange exposure to manage currency risk. At 31 December 2020 the equity collar had a notional value of £1,076m (2019: £1,560m) and the currency forwards had a notional value of £2,378m (2019: £2,079m). Some asset de-risking took place in 2020, with disinvestments from equity funds and multi-asset funds. The proceeds from disinvestment were used in a liability driven investment portfolio. An ESG monitoring framework has also been implemented to ensure regulatory compliance and to support the consideration of ESG issues in the decision-making framework. The Santander UK group’s pension schemes did not directly hold any equity securities of the Company or any of its related parties at 31 December 2020 and 2019. The Santander UK group’s pension scheme assets do not include any property or other assets that are occupied or used by the Santander UK group. Funding In August 2019, in compliance with the Pensions Act 2004, the Trustee and the Santander UK group agreed to a new recovery plan in respect of the Scheme and schedule of contributions following the finalisation of the 31 March 2019 actuarial valuation. The funding target for this actuarial valuation is for the Scheme to have sufficient assets to make payments to members in respect of the accrued benefits as and when they fall due. In accordance with the terms of the Trustee agreement in place at the time, the Santander UK group contributed £236m in 2020 (2019: £203m) to the Scheme, of which £187m (2019: £153m) was in respect of agreed deficit repair contributions. The agreed schedule of the Santander UK group’s remaining contributions to the Scheme broadly comprises contributions of £187m each year from 30 September 2019 to 31 March 2026. In addition, the Santander UK group has agreed to pay further contingent contributions should the funding position have fallen behind plan. The Santander UK group also meets Scheme administration expenses. The funding valuation is used to judge the amount of cash contributions the Group needs to put into the pension scheme. It will always be different to the IAS 19 accounting deficit, which is an accounting rule concerning employee benefits and shown on the balance sheet of our financial statements. Actuarial assumptions The principal actuarial assumptions used for the defined benefit schemes were: Group 2020 2019 2018 % % % To determine benefit obligations: – Discount rate for scheme liabilities 1.3 2.1 2.9 – General price inflation 3.0 3.0 3.2 – General salary increase 1.0 1.0 1.0 – Expected rate of pension increase 2.9 2.9 2.9 Years Years Years Longevity at 60 for current pensioners, on the valuation date: – Males 27.5 27.3 27.3 – Females 30.0 29.8 30.1 Longevity at 60 for future pensioners currently aged 40, on the valuation date: – Males 29.0 28.9 28.7 – Females 31.5 31.3 31.6 Discount rate for scheme liabilities The rate used to discount the retirement benefit obligation for accounting purposes is based on the annual yield at the balance sheet date of high-quality corporate bonds on that date. There are only a limited number of higher quality Sterling-denominated corporate bonds, particularly those that are longer-dated. Therefore, in order to set a suitable discount rate, we need to construct a corporate bond yield curve. The model which we use for constructing the curve uses corporate bond data but excludes most convertible and asset-backed bonds. The curve is then constructed from this data by extrapolating the horizontal forward curve from 30 years, with the level of this forward rate being the average of the fitted forward rates over the 15 to 30 year range. When considering an appropriate assumption, we project forward the expected cash flows of the Scheme and adopt a single equivalent cash flow weighted discount rate, subject to management judgement. General price inflation Consistent with our discount rate methodology, we set the inflation assumption using the expected cash flows of the Scheme, fitting them to an inflation curve to give a weighted average inflation assumption. We then deduct an inflation risk premium to reflect the compensation holders of fixed rate instruments expect to receive for taking on the inflation risk. This premium is subject to a cap, to better reflect management’s view of inflation expectations. In 2020, management amended the general price inflation assumptions to reflect the expectation that the Retail Price Index would be brought in line with the Consumer Price Index from 2030. At 31 December 2020, this change increased the liabilities of the Scheme by £64m. General salary increase From 1 March 2015, a cap on pensionable pay increases of 1% each year was applied to staff in the Scheme. Expected rate of pension increase The pension increase assumption methodology uses a stochastic model, which is calibrated to consider both the observed historical volatility term structure and derivative pricing. The model allows for the likelihood that high or low inflation in one-year feeds into inflation remaining high or low in the next year. Mortality assumptions The mortality assumptions are based on an independent analysis of the Scheme’s actual mortality experience, carried out as part of the triennial actuarial valuation, together with recent evidence from the Continuous Mortality Investigation. An allowance is then made for expected future improvements to life expectancy based on the Continuous Mortality Investigation Tables. Following this review the S3 Medium all pensioner mortality table was adopted with appropriate adjustments to reflect the actual mortality experience. For future improvements, at 31 December 2020 the CMI 2019 projection model was adopted, with model parameters selected having had regard to the Scheme’s membership profile with an initial addition to improvements of 0.15% per annum, together with a long-term rate of future improvements to life expectancy of 1.25% for male and female members. Both of these are published by the Continuous Mortality Investigation. In 2019, the methodology for setting the demographic assumptions was changed to better represent current expectations, following a review carried out by the Trustee as part of the 2019 triennial valuation and a separate review conducted on early retirement experience. These reviews resulted in changes in the assumptions for commutation, family statistics and early retirement, which were retained at 31 December 2020. Actuarial assumption sensitivities The sensitivity analyses below have been determined based on reasonably possible changes of the respective assumptions occurring at the end of the reporting period, while holding all other assumptions constant. (Decrease)/increase 2020 2019 Assumption Change in pension obligation at period end from £m £m Discount rate 25 bps increase (662) (564) General price inflation 25 bps increase 365 407 Mortality Each additional year of longevity assumed 515 419 The 25 bps sensitivity to the inflation assumption includes the corresponding impact of changes in future pension increase assumptions before and after retirement. The sensitivity analysis presented above may not be representative of the actual change in the defined benefit obligation as it is unlikely that the changes in assumptions would occur in isolation of one another as some of the assumptions may be correlated. Furthermore, in presenting the sensitivity analysis, the present value of the defined benefit obligation has been calculated using the projected unit credit method at the end of the reporting period, which is the same method used to calculate the defined benefit obligation recognised in the balance sheet. There were no changes in the methods and assumptions used in preparing the sensitivity analyses from prior years. The benefits expected to be paid in each of the next five years, and in the aggregate for the five years thereafter are: Year ending 31 December £m 2021 359 2022 327 2023 336 2024 359 2025 382 Five years ending 2030 2,139 The average duration of the defined benefit obligation at 31 December 2020 was 19.4 years (2019: 18.8 years). Covid-19 The Covid-19 pandemic led to significant volatility in financial markets in 2020. This translated into a volatile pension surplus/deficit, with large movements going through Other Comprehensive Income as a result. Significant progress was made in 2020 to improve the Scheme’s resilience to market volatility, which includes the increase in the level of interest rate and inflation rate hedging, de-risking within the asset portfolio and investment in a buy-in. The Santander UK group has collaborated with the Trustee to ensure the delivery of key functions and services could be maintained throughout the Covid-19 pandemic, to include most vitally the payment of pensions to members. At the start of the Covid-19 pandemic, an enhanced risk monitoring framework was also established to identify and monitor such risks and ensure they were adequately managed.

Contingent Liabilities and Comm

Contingent Liabilities and Commitments	12 Months Ended
	Dec. 31, 2020
Disclosure of contingent liabilities [abstract]
Contingent Liabilities and Commitments	31. CONTINGENT LIABILITIES AND COMMITMENTS Group 2020 2019 £m £m Guarantees given to third parties 939 1,198 Formal standby facilities, credit lines and other commitments 42,221 40,397 43,160 41,595 1. In 2020, an administrative error was identified in which £5,053m of loan commitments had been omitted in 2019. This has been corrected. Capital support arrangements Santander UK plc, Cater Allen Limited and certain other non-regulated subsidiaries within the ring-fenced bank entered into a capital support deed dated 13 November 2018 (the RFB Sub-Group Capital Support Deed). The parties to the RFB Sub-Group Capital Support Deed are permitted by the PRA to form a core UK group, as defined in the PRA Rulebook, a permission which will expire on 31 December 2021. Exposures of each of the regulated entities to other members of the core UK group are exempt from large exposure limits that would otherwise apply. The purpose of the RFB Sub-Group Capital Support Deed is to facilitate the prompt transfer of available capital resources from, or repayment of liabilities by, the non-regulated parties to any of the regulated parties in the event that one of the regulated parties breaches or is at risk of breaching its capital resources requirements or risk concentrations requirements. Liquidity support arrangements We monitor and manage liquidity risk for the Santander UK plc group and SFS separately. Under this model, and the PRA’s liquidity rules, Santander UK plc and its subsidiary Cater Allen Limited form the RFB Domestic Liquidity Sub-group (the RFB DoLSub), which allows the entities to collectively meet regulatory requirements for the purpose of managing liquidity risk. Each member of the RFB DoLSub will support the other by transferring surplus liquidity in times of stress. Guarantees given to third parties Guarantees given to third parties consist primarily of letters of credit, bonds and guarantees granted as part of normal product facilities which are offered to customers. Formal standby facilities, credit lines and other commitments Standby facilities, credit lines and other commitments are also granted as part of normal product facilities which are offered to customers. Retail facilities comprise undrawn facilities granted on flexible mortgages, bank overdrafts and credit cards. On flexible mortgages, the credit limit is set at the point of granting the loan through property value and affordability assessments. Ongoing assessments are made to ensure that credit limits remain appropriate considering any change in the security value or the customer’s financial circumstances. For unsecured overdraft facilities and credit cards, the facilities are granted based on new business risk assessment and are reviewed more frequently based on internal, as well as external data. The delinquency status of the account would result in the withdrawal of the facility. Corporate facilities can comprise standby and revolving facilities which are subject to ongoing compliance with covenants and may require the provision of agreed security. Failure to comply with these terms can result in the withdrawal of the unutilised facility headroom. FSCS The FSCS is the UK’s independent statutory compensation fund for customers of authorised financial services firms and pays compensation if a firm is unable to pay claims against it. The FSCS is funded by levies on the industry (and recoveries and borrowings where appropriate). Following the default of a number of deposit takers since 2008, the FSCS borrowed funds from HM Treasury to meet the compensation costs for customers of those firms. The remaining debt due to the FSCS, that related to the failure of Bradford & Bingley plc, has now been repaid. This has enabled the FSCS to make a corresponding repayment of the balance of its loan to HM Treasury. The provision was £nil in 2020 (2019: £nil). Loan representations and warranties In connection with the securitisations and covered bond transactions described in Note 14, the Santander UK group entities selling the relevant loans into the applicable securitisation or covered bond portfolios make representations and warranties with respect to such loans, in each case as of the date of the sale of the loans into the applicable portfolio. These representations and warranties cover, among other things, the ownership of the loan by the relevant Santander UK group entity, absence of a material breach or default by the relevant borrower under the loan, the loan’s compliance with applicable laws and absence of material disputes with respect to the relevant borrower, asset and loan. The specific representations and warranties made by Santander UK group companies which act as sellers of loans in these securitisations and covered bond transactions depend in each case on the nature of the transaction and the requirements of the transaction structure. In addition, market conditions and credit rating agency requirements may affect the representations and warranties required of the relevant Santander UK group companies in these transactions. In the event that there is a material breach of the representations and warranties given by Santander UK plc as seller of loans under the residential mortgage-backed securitisations or the covered bond programmes included in Note 14, or if such representations and warranties prove to be materially untrue as at the date when they were given (being the sale date of the relevant mortgage loans), Santander UK plc may be required to repurchase the affected mortgage loans (generally at their outstanding principal balance plus accrued interest). These securitisations and covered bond programmes are collateralised by prime residential mortgage loans. Santander UK plc is principally a retail prime lender and has no appetite or product offering for any type of sub-prime business. In addition, Santander UK plc’s credit policy explicitly prohibits such lending. Similarly, under the auto loan securitisations in Note 14, in the event that there is a breach or inaccuracy in respect of a representation or warranty relating to the loans, the relevant Santander UK group entity who sold the auto loans into the securitisation portfolio will be required to repurchase such loans from the structure (also at their outstanding principal balance plus accrued interest). In addition to breaches of representation and warranties, under the auto loan securitisations, the seller may also have a repurchase obligation if certain portfolio limits are breached (which include, amongst other things, limits as to the size of a loan given to an individual customer, LTV ratio, average term to maturity and average seasoning). In the case of a repurchase of a loan from the relevant securitisation or covered bond programmes, the Santander UK group may bear any subsequent credit loss on such loan. The Santander UK group manages and monitors its securitisation and covered bond activities closely to minimise potential claims. Other legal actions and regulatory matters Santander UK engages in discussion, and co-operates, with the FCA, PRA, CMA and other regulators and government agencies in various jurisdictions in their supervision and review of Santander UK including reviews exercised under statutory powers, regarding its interaction with past and present customers, both as part of general thematic work and in relation to specific products, services and activities. During the ordinary course of business, Santander UK is also subject to complaints and threatened legal proceedings brought by or on behalf of current or former employees, customers, investors or other third parties, in addition to legal and regulatory reviews, challenges and tax or enforcement investigations or proceedings in various jurisdictions. All such matters are assessed periodically to determine the likelihood of Santander UK incurring a liability. In those instances where it is concluded that it is not yet probable that a quantifiable payment will be made, for example because the facts are unclear or further time is required to fully assess the merits of the case or to reasonably quantify the expected payment, no provision is made. In addition, where it is not currently practicable to estimate the possible financial effect of these matters, no provision is made. Payment Protection Insurance In relation to a specific PPI portfolio of complaints, a legal dispute regarding allocation of liability is in its early stages. The dispute relates to the liability for PPI mis-selling complaints relating to pre-2005 PPI policies underwritten by AXA France IARD and AXA France Vie (together, AXA France - previously Financial Insurance Company Ltd and Financial Assurance Company Ltd respectively) and involves Santander Cards UK Limited (a former GE Capital Corporation entity and distributor of pre-2005 PPI known as GE Capital Bank Limited which was acquired by Banco Santander S.A. in 2008 and subsequently transferred to Santander UK plc) and a Banco Santander S.A. subsidiary Santander Insurance Services UK Limited (together the Santander Entities). During the relevant period, AXA France were owned by Genworth Financial International Holdings, Inc. (Genworth). In September 2015 AXA S.A. acquired AXA France from Genworth. In July 2017, the Santander Entities notified AXA France that they did not accept liability for losses on PPI policies relating to this period. Santander UK plc entered into a Complaints Handling Agreement (CHA) with AXA France pursuant to which it agreed to handle complaints on their behalf, and AXA France agreed to pay redress assessed to be due to relevant policyholders on a without prejudice basis. A standstill agreement was entered into between the Santander Entities and AXA France as a condition of the CHA. In July 2020 Genworth announced that it had agreed to pay AXA circa £624m in respect of PPI mis-selling losses in settlement of the related dispute concerning obligations under the sale and purchase agreement pursuant to which Genworth sold AXA France to AXA. The CHA between Santander UK plc and AXA France terminated on 26 December 2020. On 30 December 2020, AXA France provided written notice to the Santander Entities to terminate the standstill agreement and claimed that the Santander Entities are liable to reimburse AXA France for pre-2005 PPI mis-selling losses, currently estimated at £631 million. This dispute is at an early stage and there are ongoing factual issues to be resolved which may have legal consequences including in relation to liability. These issues create uncertainties which mean that it is difficult to reliably predict the outcome or the timing of the resolution of the matter. The Regulatory and other provision in Note 29 includes our best estimate of the Santander Entities’ liability to the specific portfolio. Further information has not been provided on the basis that it would be seriously prejudicial to the Santander Entities’ interests in connection with the dispute. In addition, and in relation to PPI more generally the PPI provision includes an amount relating to legal claims challenging the FCA’s industry guidance on the treatment of Plevin / recurring non-disclosure assessments. This provision is based on current stock levels, future projected claims, and average redress. There remains a risk that volumes received in future may be higher than forecast. The provision in Note 29 includes our best estimate of Santander UK’s liability for the specific issue. The actual cost of customer compensation could differ from the amount provided. It is not currently practicable to provide an estimate of the risk and amount of any further financial impact. German dividend tax arbitrage transactions In June 2018 the Cologne Criminal Prosecution Office and the German Federal Tax Office commenced an investigation in relation to the historical involvement of Santander UK plc, Santander Financial Services plc and Cater Allen International Limited (all subsidiaries of Santander UK Group Holdings plc) in German dividend tax arbitrage transactions (known as cum/ex transactions). These transactions allegedly exploited a loophole of a specific German settlement mechanism through short-selling and complex derivative structuring which resulted in the German government either refunding withholding tax where such tax had not been paid or refunding it more than once. The German authorities are investigating numerous institutions and individuals in connection with alleged transactions and practices which may be found to be illegal under German law. During 2020 we have continued to cooperate with the German authorities and, with the assistance of external experts, to progress an internal investigation into the matters in question. From Santander UK plc’s perspective the investigation is focused principally on the period 2009-2011 and remains on-going. There remain factual issues to be resolved which may have legal consequences including potentially material financial penalties. These issues create uncertainties which mean that it is difficult to predict the resolution of the matter including timing or the significance of the possible impact. Any potential losses, claims or expenses suffered or incurred by Santander Financial Services plc in respect of these matters have been fully indemnified by Santander UK plc, as part of the ring-fencing transfer scheme between Santander UK plc, Santander Financial Services plc and Banco Santander SA. Consumer credit The Santander UK group’s unsecured lending and other consumer credit business is governed by consumer credit law and related regulations, including the CCA. Claims brought by customers in relation to these requirements, including potential breaches, could result in costs to the Santander UK group where such potential breaches are not found to be de minimis. The CCA includes very detailed and prescriptive requirements for lenders, including in relation to post contractual information. As described in Note 29, other provisions include an amount of £47m arising from a systems-related historical issue identified by Santander UK, relating to compliance with certain requirements of the CCA. This provision has been based on detailed reviews of relevant systems related to consumer credit business operations, supported by external legal and regulatory advice. The Regulatory and other provision in Note 29 includes our best estimate of Santander UK’s liability for the specific issue. The actual cost of customer compensation could differ from the amount provided. It is not practicable to provide an estimate of the risk and amount of any further financial impact. FCA civil regulatory investigation into Santander UK plc financial crime systems, processes and controls and compliance with the Money Laundering Regulations 2007 Santander UK plc is cooperating with an FCA civil regulatory investigation which commenced in July 2017 into our compliance with the Money Laundering Regulations 2007 and potential breaches of FCA principles and rules relating to anti-money laundering and financial crime systems and controls. The FCA’s investigation focuses primarily on the period 2012 to 2017 and includes consideration of high risk customers including Money Service Businesses. It is not currently possible to make a reliable assessment of any liability resulting from the investigation including any financial penalty. Taxation The Santander UK group engages in discussion, and co-operates, with HM Revenue & Customs (HMRC) in their oversight of the Santander UK group’s tax matters. The Santander UK group adopted the UK’s Code of Practice on Taxation for Banks in 2010. Certain leases in which the Santander UK group is or was the lessor are currently under review by HMRC in connection with claims for tax allowances. Under the terms of the lease agreements, the Santander UK group is fully indemnified in all material respects by the respective lessees for any liability arising from the disallowance of tax allowances plus accrued interest, which could be up to £152m. Whilst legal opinions have been obtained to support the Santander UK group’s position, the matter remains uncertain pending formal resolution with HMRC and any subsequent litigation. In 2020, as required under the terms of the leases these matters have moved to formal litigation and it is currently anticipated that hearings will be held at the First Tier Tax Tribunal in 2021 or 2022. On 2 November 2015, Visa Europe Ltd agreed to sell 100% of its share capital to Visa Inc. The deal closed on 21 June 2016. As a member and shareholder of Visa Europe Ltd, Santander UK received upfront consideration made up of cash and convertible preferred stock. The convertible preferred stock is now held by Santander Equity Investments Limited (SEIL), outside the ring fenced bank. Conversion of the preferred stock into Class A Common Stock of Visa Inc. depends on the outcome of litigation against Visa involving UK & Ireland (UK&I) multilateral interchange fees (MIFs). In June 2020 the Supreme Court issued a judgment finding that MIFs restricted competition. In addition, Santander UK and certain other UK&I banks have agreed to indemnify Visa Inc. in the event that the preferred stock is insufficient to meet the costs of this litigation. Visa Inc. has recourse to this indemnity once more than €1bn of losses relating to UK&I MIFs have arisen or once the total value of the preferred stock issued to UK&I banks on closing has been reduced to nil. Whilst Santander UK's liability under this indemnity is capped at €39.85m, Visa Inc. may have recourse to a general indemnity in place under Visa Europe Operating Regulations for damages not satisfied through the above mechanism. At this stage, it is unclear whether the litigation will give rise to more than €1bn of losses relating to UK&I MIFs which means it is difficult to predict the resolution of the matter including the timing or the significance of the possible impact. As part of the sale of subsidiaries, businesses and other entities, and as is normal in such circumstances, Santander UK has given warranties and indemnities to the purchasers. Obligations under stock borrowing and lending agreements Obligations under stock borrowing and lending agreements represent contractual commitments to return stock borrowed. These obligations are offset by a contractual right to receive stock under other contractual agreements. See Note 36. Other off-balance sheet commitments The Santander UK group has commitments to lend at fixed interest rates which expose us to interest rate risk. For more, see the Risk review. Lease commitments as lessee The majority of leases are subject to a third party outsourcing contract which expired on 24 December 2020 and the remainder are held directly by the Santander UK group with third party landlords. The leasehold interests subject to the outsourcing contract are held directly by the Santander UK group from 25 December 2020. Where Santander UK group leases have expired, negotiations are in progress/will be progressed with the landlords of these properties, to agree renewal terms, where occupation is still required. Negotiations will be in accordance with a conventional landlord and tenant negotiation on lease expiry, subject to a lease renewal being available from the external landlords. Where the freehold interest in a property is held by the outsourcing company (91 properties in total), new lease terms have been negotiated and all these leases are due to complete in January 2021, save for two, where the outsourcing company own a long leasehold interest and the leases will be completed upon receipt of landlords consent.

Share Capital

Share Capital	12 Months Ended
	Dec. 31, 2020
Disclosure of classes of share capital [abstract]
Share Capital	32. SHARE CAPITAL Group Ordinary shares £300m Preference shares of £1,000 each Total Issued and fully paid share capital No. £m No. £m £m At 1 January 2020 31,051,768,866 3,105 — — 3,105 Redemption of preference shares — — — — — At 31 December 2020 31,051,768,866 3,105 — — 3,105 At 1 January 2018, 31 December 2018 and 1 January 2019 31,051,768,866 3,105 13,780 14 3,119 Redemption of preference shares — — (13,780) (14) (14) At 31 December 2019 31,051,768,866 3,105 — — 3,105 Group 2020 2019 Share premium £m £m At 1 January and 31 December 5,620 5,620 The Company has one class of ordinary shares which carries no right to fixed income. The Company’s £325m sterling preference shares are classified as Subordinated Liabilities as described in Note 27. Fixed/Floating Rate Non-Cumulative Callable Preference Shares

Other Equity Instruments

Other Equity Instruments	12 Months Ended
	Dec. 31, 2020
Disclosure of classes of share capital [abstract]
Other Equity Instruments	33. OTHER EQUITY INSTRUMENTS Group Interest rate 2020 2019 % Next call date £m £m £300m Step-up Callable Perpetual Reserve Capital Instruments 7.037 February 2026 235 235 AT1 securities: - £500m Fixed Rate Reset Perpetual AT1 Capital Securities 6.75 June 2024 496 496 - £750m Fixed Rate Reset Perpetual AT1 Capital Securities 7.375 June 2022 750 750 - £500m Fixed Rate Reset Perpetual AT1 Capital Securities 5.18 March 2021 210 210 - £500m Fixed Rate Reset Perpetual AT1 Capital Securities 6.30 March 2025 500 500 2,191 2,191 Step-up Callable Perpetual Reserve Capital Instruments These instruments are redeemable by Santander UK plc on 14 February 2026 or on any coupon payment date thereafter, subject to the prior approval of the PRA. They are perpetual and pay interest annually. The coupon rate resets every five years, based on the UK five AT1 securities The AT1 securities issued by the Company were subscribed by its immediate parent company, Santander UK Group Holdings plc. The AT1 securities are perpetual and pay a distribution on 24 March, June, September and December. At each distribution payment date, the Company can decide whether to pay the distribution, which is non-cumulative, in whole or in part. The distribution rate resets every five years. The securities will be automatically written down and the investors will lose their entire investment in the securities should the CET1 capital ratio of the Santander UK prudential consolidation group fall below 7%. They are redeemable at the option of the Company on their first call date or on any reset date thereafter in the cases of the 6.75% and 7.375% Fixed Rate Reset Perpetual AT1 Capital Securities, and on any distribution payment date thereafter in the cases of the 5.18% and 6.30% Fixed Rate Reset Perpetual AT1 Capital Securities. No such redemption may be made without the consent of the PRA.

Non-Controlling Interests

Non-Controlling Interests	12 Months Ended
	Dec. 31, 2020
Disclosure Of Non Controlling Interests [Abstract]
Non-Controlling Interests	34. NON-CONTROLLING INTERESTS 2020 2019 £m £m PSA Finance UK Limited 162 160 162 160

Changes in Liabilities Arising

Changes in Liabilities Arising from Financing Activities	12 Months Ended
	Dec. 31, 2020
Disclosure of reconciliation of liabilities arising from financing activities [abstract]
Changes in Liabilities Arising from Financing Activities	35. CHANGES IN LIABILITIES ARISING FROM FINANCING ACTIVITIES The table below shows the changes in liabilities arising from financing activities. Group 2020 2019 Balance sheet line item Balance sheet line item Debt securities in issue Subordinated liabilities Other equity instruments Dividends paid Total Debt securities in issue Subordinated liabilities Other equity instruments Dividends paid Total £m £m £m £m £m £m £m £m £m £m At 1 January 41,129 3,528 2,191 — 46,848 46,692 3,601 1,991 — 52,284 Cash flows from financing activities (7,201) (659) — (292) (8,152) (3,839) — 196 (469) (4,112) Cash flows from operating activities 1,201 — — — 1,201 (529) — — — (529) Non-cash changes: – Unrealised foreign exchange 376 22 — — 398 (1,320) (50) — — (1,370) – Other changes 61 (335) — 292 18 125 (23) 4 469 575 At 31 December 35,566 2,556 2,191 — 40,313 41,129 3,528 2,191 — 46,848

Assets Charged As Security For

Assets Charged As Security For Liabilities And Collateral Accepted As Security For Assets	12 Months Ended
	Dec. 31, 2020
Disclosure of assets pledged as security [abstract]
Assets Charged As Security For Liabilities And Collateral Accepted As Security For Assets	36. ASSETS CHARGED AS SECURITY FOR LIABILITIES AND COLLATERAL ACCEPTED AS SECURITY FOR ASSETS The following transactions are conducted under terms that are usual and customary to collateralised transactions including, where relevant, standard securities lending and repurchase agreements. a) Assets charged as security for liabilities The financial assets below are analysed between those assets accounted for on-balance sheet and off-balance sheet. Group 2020 2019 £m £m On-balance sheet: Cash and balances at central banks 985 1,080 Loans and advances to banks 550 403 Loans and advances to customers – securitisations and covered bonds (See Note 14) 31,138 36,225 Loans and advances to customers – other 23,655 16,282 Other financial assets at amortised cost 648 3,026 Financial assets at fair value through other comprehensive income 5,581 6,009 Total on-balance sheet 62,557 63,025 Total off-balance sheet 24,701 15,098 The Santander UK group provides assets as collateral in the following areas of the business. Sale and repurchase agreements The Santander UK group enters into sale and repurchase agreements and similar transactions of debt securities, which are accounted for as secured borrowings. Upon entering into such transactions, the Santander UK group provides collateral in excess of the borrowed amount. The carrying amount of assets that were so provided at 31 December 2020 was £25,874m (2019: £20,686m), of which £1,392m (2019: £2,067m) was classified within ‘Loans and advances to customers – securitisations and covered bonds’ in the table above. Securitisations and covered bonds As described in Note 14, Santander UK plc and certain of its subsidiaries issue securitisations and covered bonds. At 31 December 2020, there were £33,432m (2019: £36,391m) of gross assets in these secured programmes and £2,294m (2019: £166m) of these related to internally retained issuances that were available for use as collateral for liquidity purposes in the future. At 31 December 2020, a total of £4,530m (2019: £4,728m) of notes issued under securitisation and covered bond programmes had been retained internally, a proportion of which had been used as collateral via third party bilateral secured funding transactions, which totalled £1,114m at 31 December 2020 (2019: £1,581m), or for use as collateral for liquidity purposes in the future. Stock borrowing and lending agreements Asset balances under stock borrowing and lending agreements represent stock lent by the Santander UK group. These balances amounted to £30,040m at 31 December 2020 (2019: £21,563m) and are offset by contractual commitments to return stock borrowed or cash received. Derivatives business In addition to the arrangements described, collateral is also provided in the normal course of derivative business to counterparties. At 31 December 2020 £1,598m (2019: £1,716m) of such collateral in the form of cash had been provided by the Santander UK group and is included in the table. b) Collateral accepted as security for assets The collateral held as security for assets, analysed between those liabilities accounted for on balance sheet and off-balance sheet, was: Group 2020 2019 £m £m On-balance sheet: Deposits by banks 2,063 2,169 Total on-balance sheet 2,063 2,169 Total off-balance sheet 30,510 25,120 Purchase and resale agreements The Santander UK group also enters into purchase and resale agreements and similar transactions of debt securities, which are accounted for as collateralised loans. Upon entering into such transactions, the Santander UK group receives collateral in excess of the loan amount. The level of collateral held is monitored daily and if required, further calls are made to ensure the market values of collateral remains at least equal to the loan balance. The subsidiaries are permitted to sell or repledge the collateral held in the absence of default. At 31 December 2020, the fair value of such collateral received was £25,972m (2019: £20,444m). Of the collateral received, almost all was sold or repledged. The subsidiaries have an obligation to return collateral that they have sold or pledged. Stock borrowing and lending agreements Obligations representing contractual commitments to return stock borrowed by the Santander UK group amounted to £4,538m at 31 December 2020 (2019: £4,676m) and are offset by a contractual right to receive stock lent. Derivatives business In addition to the arrangements described, collateral is also received from counterparties in the normal course of derivative business. At 31 December 2020, £2,063m (2019: £2,169m) of such collateral in the form of cash had been received by the Santander UK group and is included in the table. Lending activities In addition to the collateral held as security for assets, the Santander UK group may obtain a charge over a customer’s property in connection with its lending activities. Details of these arrangements are set out in the ‘Credit risk’ section of the Risk review.

Share-Based Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2020
Disclosure of terms and conditions of share-based payment arrangement [abstract]
Share-Based Compensation	37. SHARE-BASED COMPENSATION The Santander UK group operates share schemes and arrangements for eligible employees. The main current schemes are the Sharesave Schemes, the Long-Term Incentive Plan (the LTIP), the Deferred Shares Bonus Plan and the Partnership Shares scheme. All the share options and awards relate to shares in Banco Santander SA. The amount charged to the income statement in respect of share-based payment transactions is set out in Note 6. a) Sharesave Schemes The Santander UK group launched its thirteenth HM Revenue & Customs approved Sharesave invitation under Banco Santander SA ownership in September 2020. Sharesave invitations have been offered since 2008 under broadly similar terms. Under the Sharesave Scheme’s HMRC-approved savings limits, eligible employees may enter into contracts to save between £5 and £500 per month. For all schemes, at the end of a fixed term of three or five years after the grant date, the employees can use these savings to buy shares in Banco Santander SA at a discount, calculated in accordance with the rules of the scheme. This year no discount was applied. The option price is calculated as the average middle market quoted price of Banco Santander SA shares over the first three dealing days prior to invitation. The vesting of awards under the scheme depends on continued employment with the Banco Santander SA group. Participants in the scheme have six months from the date of vest to exercise the option. In 2018, as part of the implementation of our ring-fencing plans, the Sharesave Schemes were transferred to SEIL, which was subsequently transferred outside of the Santander UK group, but remained within the Santander UK Group Holdings plc group. Information relating to the Sharesave Schemes administered by SEIL is presented below as employees of the Santander UK group are participants. The table below summarises movements in the number of options, and changes in weighted average exercise price over the same period. 2020 2019 Number of options Weighted average exercise price Number of options Weighted average exercise price ‘000 £ ‘000 £ Outstanding at 1 January 23,373 3.03 26,838 3.12 Granted 11,642 1.65 9,594 2.83 Exercised (860) 2.75 (7,978) 2.83 Forfeited/expired (12,993) 2.96 (5,081) 3.42 Outstanding at 31 December 21,162 2.32 23,373 3.03 Exercisable at 31 December 1,805 3.59 2,519 3.62 The weighted average share price at the date the options were exercised was £2.92 (2019: £3.18). The following table summarises the range of exercise prices and weighted average remaining contractual life of the options at 31 December 2020 and 2019. 2020 2019 Range of exercise prices Weighted average remaining contractual life Weighted average exercise price Weighted average remaining contractual life Weighted average exercise price Years £ Years £ £1 to £2 4 1.65 — — £2 to £3 2 2.81 3 2.80 £3 to £4 1 3.38 2 3.38 £4 to £5 1 4.02 2 4.13 The fair value of each option at the date of grant is estimated using a partial differentiation equation model. This model uses assumptions on the share price, the EUR/GBP FX rate, the risk-free interest rate, dividend yields, the expected volatility of the underlying shares and the expected lives of options granted. The weighted average grant-date fair value of options granted during the year was £0.21 (2019: £0.49). At 31 December 2020, the carrying amount of liabilities arising from share-based payment transactions in the Santander UK Group Holdings plc group was £0.5m (2019: £2.4m), of which £nil had vested at 31 December 2020 (2019: £1.4m). b) LTIP In 2015, a conditional cash award was made to certain Executive Directors, Key Management Personnel (as defined in Note 38) and other nominated individuals which are converted into shares in Banco Santander SA at the time of vesting and deferred for three years. There have been no LTIP awards granted since 2015 due to the introduction of a single variable remuneration framework across the Banco Santander group in 2016. The LTIP plan granted in 2015 involved a one-year performance cycle for vesting, deferred for a further three-year period dependent upon performance conditions applied. Beneficiaries were granted an initial award determined in GBP which was converted into shares in Banco Santander SA in January 2016 respectively based on performance over the performance cycle. The 2015 LTIP vested in January 2016, was deferred over three years and was subject to performance conditions based on Banco Santander SA’s Earnings Per Share (EPS) and Return on Tangible Equity (RoTE) performance against budget. The conditions of the 2015 LTIP were met and payment was made to the remaining eligible population in March 2019 at 65.78% of the original award. The following table summarises the movement in the value of conditional awards in the LTIP in 2020 and 2019: 2015 LTIP 2020 2019 £000 £000 Outstanding at 1 January — 6,374 Payments made — (4,578) Forfeited/cancelled — (1,796) Outstanding at 31 December — — c) Deferred shares bonus plan Deferred bonus awards are designed to align employee performance with shareholder value and encourage increased retention of senior employees. During 2019 and 2020, conditional share awards were made to employees (designated as Material Risk Takers). Such employees receive part of their annual bonus as a deferred award comprising 50% in shares, and 50%in cash. Any deferred awards are dependent on continued employment or subject to Santander UK's discretion for leavers. For 2019 and 2020 bonus awards, deferral of the award is over a three, five or seven-year period from the anniversary of the initial award. Deferred bonus awards in shares are subject to an additional one-year retention period from the point of delivery. Material Risk Takers are required to defer either 40% or 60% of any annual bonus (40% for variable pay of less than £500,000, 60% for variable pay at or above this amount). Vesting of both deferred bonus awards and long-term bonus awards is subject to risk and performance adjustment in the event of deficient performance and prudent financial control provisions. d) Partnership Shares scheme A Partnership Shares scheme is operated for eligible employees under the Share Incentive Plan (SIP) umbrella. Participants can choose to invest up to £1,800 per tax year (or no more than 10% of an employee’s salary for the tax year) from pre-tax salary to buy Banco Santander SA shares. Shares are held in trust for the participants. There are no vesting conditions attached to these shares, and no restrictions as to when the shares can be removed from the trust. However, if a participant chooses to sell the shares before the end of five years, they will be liable for the taxable benefit received when the shares are taken out of the trust. The shares can be released from trust after five years free of income tax and national insurance contributions. 3,254,900 shares were outstanding at 31 December 2020 (2019: 2,396,909 shares).

Transactions With Directors and

Transactions With Directors and Other Key Management Personnel	12 Months Ended
	Dec. 31, 2020
Disclosure Of Remuneration Of Directors And Other Key Management Personnel [Abstract]
Transactions With Directors and Other Key Management Personnel	38. TRANSACTIONS WITH DIRECTORS AND OTHER KEY MANAGEMENT PERSONNEL a) Remuneration of Directors and Other Key Management Personnel The remuneration of the Directors and Other Key Management Personnel of the Santander UK group is set out in aggregate below. 2020 2019 2018 Directors’ remuneration £ £ £ Salaries and fees 5,353,980 5,025,665 5,028,434 Performance-related payments (1) 1,458,911 3,864,965 5,194,317 Other fixed remuneration (pension and other allowances & non-cash benefits) 1,107,348 1,367,069 1,467,011 Expenses 6,772 42,526 25,198 Total remuneration 7,927,011 10,300,225 11,714,960 2020 2019 2018 Directors' and Other Key Management Personnel compensation £ £ £ Short-term employee benefits (2) 18,432,698 21,925,975 24,445,189 Post-employment benefits (3) 2,084,645 3,590,466 2,399,261 Total Compensation 20,517,343 25,516,441 26,844,450 (1) In line with the Code, a proportion of the performance-related payment was deferred. Further details can be found in Note 37. (2) In addition to the remuneration in the table above, grants of shares in Banco Santander SA were made as buy-outs of deferred performance-related payments of shares in connection with previous employment for two individuals. One to a director of £1,293,678 of which £242,605 vested in the year and one to Key Management Personnel of £924,133 of which £60,500 vested in the year (2019: £nil). (3) No Termination payments were paid in 2020 to key management persons (2019: £1,076,435 for one individual; 2018: £847,388 for two individuals). In 2020, the remuneration, excluding pension contributions, of the highest paid Director, was £2,134,134 (2019: £3,725,993) of which £726,040 (2019: £1,989,900) was performance related. In 2020, there was no pension benefit accrued for the highest paid Director. b) Retirement benefits Defined benefit pension schemes are provided to certain employees. See Note 30 for details of the schemes and the related costs and obligations. One director has a deferred pension benefit accruing under a defined benefit scheme. Ex gratia pensions paid to former Directors of Santander UK plc in 2020, which have been provided for previously, amounted to £366,248 (2019: £335,202; 2018: £87,300). In 1992, the Board decided not to award any new such ex gratia pensions. c) Transactions with Directors, Other Key Management Personnel and each of their connected persons Directors, Other Key Management Personnel (defined as the Executive Committee of Santander UK plc who served during the year) and their connected persons have undertaken the following transactions with the Santander UK group in the ordinary course of business. 2020 2019 No. £000 No. £000 Secured loans, unsecured loans and overdrafts At 1 January 18 4,920 16 3,035 Net movements (6) (1,280) 2 1,885 At 31 December 12 3,640 18 4,920 Deposit, bank and instant access accounts and investments At 1 January 32 11,975 30 10,963 Net movements (9) (3,780) 2 1,012 At 31 December 23 8,195 32 11,975 In 2020 and 2019, no Director held any interest in the shares of any company in the Santander UK group and no Director exercised or was granted any rights to subscribe for shares in any company in the Santander UK group. In addition, in 2020 and 2019, no Directors exercised share options over shares in Banco Santander SA, the ultimate parent company of the Company. Secured loans, unsecured loans and overdrafts are made to Directors, Other Key Management Personnel and their connected persons, in the ordinary course of business, with terms prevailing for comparable transactions and on the same terms and conditions as applicable to other employees in the Santander UK group. Such loans do not involve more than the normal risk of collectability or present any unfavourable features. Amounts deposited by Directors, Other Key Management Personnel and their connected persons earn interest at the same rates as those offered to the market or on the same terms and conditions applicable to other employees in the Santander UK group. Deposits, bank and instant access accounts and investments are entered into by Directors, Other Key Management Personnel and their connected persons on normal market terms and conditions, or on the same terms and conditions as applicable to other employees in Santander UK group. In 2020, loans were made to seven Directors (2019: nine Directors), with a principal amount of £1,829,267 outstanding at 31 December 2020 (2019: £1,767,066). In 2020, loans were made to five Other Key Management Personnel (2019: nine), with a principal amount of £1,811,171 outstanding at 31 December 2020 (2019: £3,153,343). In 2020 and 2019, there were no other transactions, arrangements or agreements with Santander UK in which Directors, Other Key Management Personnel or their connected persons had a material interest. In addition, in 2020 and 2019, no Director had a material interest in any contract of significance with Santander UK other than a service contract.

Related Party Disclosures

Related Party Disclosures	12 Months Ended
	Dec. 31, 2020
Disclosure of transactions between related parties [abstract]
Related Party Disclosures	39. RELATED PARTY DISCLOSURES a) Parent undertaking and controlling party The Company’s immediate parent is Santander UK Group Holdings plc, a company incorporated in England and Wales. Its ultimate parent and controlling party is Banco Santander SA, a company incorporated in Spain. The smallest and largest groups into which the Santander UK group’s results are included are the group accounts of Santander UK Group Holdings plc and Banco Santander SA, respectively copies of which may be obtained from Shareholder Relations, 2 Triton Square, Regent’s Place, London NW1 3AN. b) Transactions with related parties Transactions with related parties during the year and balances outstanding at the year-end: Group Interest, fees and Interest, fees and Amounts owed by Amounts owed to 2020 2019 2018 2020 2019 2018 2020 2019 2020 2019 £m £m £m £m £m £m £m £m £m £m Ultimate parent (119) (130) (72) 105 266 217 1,557 1,560 (2,151) (2,143) Immediate parent (7) (7) (3) 316 317 275 8 8 (10,121) (10,012) Fellow subsidiaries (58) (66) (86) 157 173 178 223 204 (559) (544) Associates & joint ventures (29) (29) (28) — — — 2,234 2,194 (1,034) (930) (213) (232) (189) 578 756 670 4,022 3,966 (13,865) (13,629) For more on this, see ‘Balances with other Banco Santander companies’ in the Risk review. In addition, transactions with pension schemes operated by the Santander UK group are described in Note 30. The above transactions were made in the ordinary course of business, except those carried out with Banco Santander SA and subsidiaries of the Company as part of our ring-fencing implementation in 2018, on substantially the same terms as for comparable transactions with third party counterparties, and within limits acceptable to the PRA. Such transactions do not involve more than the normal risk of collectability or present any unfavourable features. In addition, in July 2018 we transferred £1.4bn of customer loans, £21.5bn of other assets and £20.7bn of liabilities from Santander UK to Banco Santander London Branch. Of these transfers £19.7bn of assets and £18.8bn of liabilities related to derivatives business. These transfers reduced RWAs by £5.5bn and we paid an associated dividend of £668m. Furthermore in 2018, and as described in more detail in Note 19, Santander UK plc sold 100% of the share capital of SFS plc to Santander UK Group Holdings plc, for a consideration of £337m , and the business of the Jersey and Isle of Man branches of Santander UK plc was subsequently acquired by SFS plc.

Financial Instruments

Financial Instruments	12 Months Ended
	Dec. 31, 2020
Disclosure of financial assets [abstract]
Financial Instruments	40. FINANCIAL INSTRUMENTS a) Measurement basis of financial assets and liabilities Financial assets and financial liabilities are measured on an ongoing basis either at fair value or at amortised cost. Note 1 describes how the classes of financial instruments are measured, and how income and expenses, including fair value gains and losses, are recognised. b) Fair value measurement and hierarchy (i) Fair value measurement The fair value of financial instruments is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date in the principal, or in its absence, the most advantageous market to which Santander UK has access at that date. The fair value of a liability reflects its non-performance risk. Financial instruments valued using observable market prices If a quoted market price in an active market is available for an instrument, the fair value is calculated as the current bid price multiplied by the number of units of the instrument held. Financial instruments valued using a valuation technique In the absence of a quoted market price in an active market, management uses internal models to make its best estimate of the price that the market would set for that financial instrument. In order to make these estimations, various techniques are employed, including extrapolation from observable market data and observation of similar financial instruments with similar characteristics. Wherever possible, valuation parameters for each product are based on prices directly observable in active markets or that can be derived from directly observable market prices. Chosen valuation techniques incorporate all the factors that market participants would take into account in pricing transactions. Santander UK manages certain groups of financial assets and liabilities on the basis of its net exposure to either market risks or credit risk. As a result it has elected to use the exception under IFRS 13 which permits the fair value measurement of a group of financial assets and financial liabilities on the basis of the price that would be received to sell a net long position for a particular risk exposure or paid to transfer a net short position for a particular risk exposure in an orderly transaction between market participants at the measurement date under current market conditions. (ii) Fair value hierarchy Santander UK applies the following fair value hierarchy that prioritises the inputs to valuation techniques used in measuring fair value. The hierarchy establishes three categories for valuing financial instruments, giving the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities and the lowest priority to unobservable inputs. The three categories are: quoted prices in active markets (Level 1), internal models based on observable market data (Level 2) and internal models based on other than observable market data (Level 3). If the inputs used to measure an asset or a liability fall to different levels within the hierarchy, the classification of the entire asset or liability will be based on the lowest level input that is significant to the overall fair value measurement of the asset or liability. Santander UK categorises assets and liabilities measured at fair value within the fair value hierarchy based on the inputs to the valuation techniques as follows: Level 1 Unadjusted quoted prices for identical assets or liabilities in an active market that Santander UK can access at the measurement date. Active markets are assessed by reference to average daily trading volumes in absolute terms and, where applicable, by reference to market capitalisation for the instrument. Level 2 Quoted prices in inactive markets, quoted prices for similar assets or liabilities, recent market transactions, inputs other than quoted market prices for the asset or liability that are observable either directly or indirectly for substantially the full term, and inputs to valuation techniques that are derived principally from or corroborated by observable market data through correlation or other statistical means for substantially the full term of the asset or liability. Level 3 Significant inputs to the pricing or valuation techniques are unobservable. These unobservable inputs reflect the assumptions that market participants would use when pricing assets or liabilities and are considered significant to the overall valuation. Changes in the observability of significant valuation inputs during the reporting period may result in a transfer of assets and liabilities within the fair value hierarchy. The Santander UK group recognises transfers between levels of the fair value hierarchy when there is a significant change in either its principal market or the level of observability of the inputs to the valuation techniques as at the end of the reporting period. c) Valuation techniques The main valuation techniques employed in internal models to measure the fair value of the financial instruments at 31 December 2020 and 2019 are set out below. In substantially all cases, the principal inputs into these models are derived from observable market data. Santander UK did not make any material changes to the valuation techniques and internal models it used in 2020, 2019 and 2018. A. In the valuation of financial instruments requiring static hedging (for example interest rate, currency derivatives and property derivatives) and in the valuation of loans and advances and deposits, the ‘present value’ method is used. Expected future cash flows are discounted using the interest rate curves of the applicable currencies or forward house price index levels, as well as credit spreads. The interest rate curves are generally observable market data and reference yield curves derived from quoted interest rates in appropriate time bandings, which match the timings of the cash flows and maturities of the instruments. The forward commodity house price index levels are generally observable market data. B. In the valuation of equity financial instruments requiring dynamic hedging (principally equity securities, options and other structured instruments), proprietary local volatility and stochastic volatility models are used. These types of models are widely accepted in the financial services industry. Observable market inputs used in these models include the bid-offer spread, foreign currency exchange rates, volatility and correlation between indices. In limited circumstances, other inputs may be used in these models that are based on unobservable market data, such as the Halifax’s UK HPI volatility, HPI forward growth, HPI spot rate, mortality and mean reversion. C. In the valuation of financial instruments exposed to interest rate risk that require either static or dynamic hedging (such as interest rate futures, caps and floors, and options), the present value method (futures), Black’s model (caps/floors) and the Hull/White and Markov functional models (Bermudan options) are used. These types of models are widely accepted in the financial services industry. The significant inputs used in these models are observable market data, including appropriate interest rate curves, volatilities, correlations and exchange rates. In limited circumstances, other inputs may be used in these models that are based on unobservable market data, such as HPI volatility, HPI forward growth, HPI spot rate and mortality. D. In the valuation of linear instruments such as credit risk and fixed-income derivatives, credit risk is measured using dynamic models similar to those used in the measurement of interest rate risk. In the case of non-linear instruments, if the portfolio is exposed to credit risk such as credit derivatives, the probability of default is determined using the credit default spread market. The main inputs used to determine the underlying cost of credit of credit derivatives are quoted credit risk premiums and the correlation between the quoted credit derivatives of various issuers. The fair values of the financial instruments arising from Santander UK’s internal models take into account, among other things, contract terms and observable market data, which include such factors as bid-offer spread, interest rates, credit risk, exchange rates, the quoted market price of equity securities, volatility and prepayments. In all cases, when it is not possible to derive a valuation for a particular feature of an instrument, management uses judgement to determine the fair value of the particular feature. In exercising this judgement, a variety of tools are used including proxy observable data, historical data and extrapolation techniques. Extrapolation techniques take into account behavioural characteristics of equity markets that have been observed over time, and for which there is a strong case to support an expectation of a continuing trend in the future. Estimates are calibrated to observable market prices when they become available. Santander UK believes its valuation methods are appropriate and consistent with other market participants. Nevertheless, the use of different valuation methods or assumptions, including imprecision in estimating unobservable market inputs, to determine the fair value of certain financial instruments could result in different estimates of fair value at the reporting date and the amount of gain or loss recorded for a particular instrument. Most of the valuation models are not significantly subjective, because they can be tested and, if necessary, recalibrated by the internal calculation of and subsequent comparison to market prices of actively traded securities, where available. d) Control framework Fair values are subject to a control framework designed to ensure that they are either determined or validated by a function independent of the risk-taker. To this end, ultimate responsibility for the determination of fair values lies with the Risk Department. For all financial instruments where fair values are determined by reference to externally quoted prices or observable pricing inputs to models, independent price determination or validation is utilised. In inactive markets, direct observation of a traded price may not be possible. In these circumstances, Santander UK will source alternative market information to validate the financial instrument’s fair value, with greater weight given to information that is considered to be more relevant and reliable. The factors that are considered in this regard include: – The extent to which prices may be expected to represent genuine traded or tradeable prices – The degree of similarity between financial instruments – The degree of consistency between different sources – The process followed by the pricing provider to derive the data – The elapsed time between the date to which the market data relates and the balance sheet date – The manner in which the data was sourced. The source of pricing data is considered as part of the process that determines the classification of the level of a financial instrument. Consideration is given to the quality of the information available that provides the current mark-to-model valuation and estimates of how different these valuations could be on an actual trade, taking into consideration how active the market is. For spot assets that cannot be sold due to illiquidity, forward estimates are discounted to estimate a realisable value over time. Adjustments for illiquid positions are regularly reviewed to reflect changing market conditions. For fair values determined using a valuation model, the control framework may include, as applicable, independent development and / or validation of: (i) the logic within the models; (ii) the inputs to those models; and (iii) any adjustments required outside the models. Internal valuation models are validated independently within the Risk Department. A validation report is produced for each model-derived valuation that assesses the mathematical assumptions behind the model, the implementation of the model and its integration within the trading system. Group 2020 2019 Fair value Carrying Fair value Carrying Level 1 Level 2 Level 3 Total value Level 1 Level 2 Level 3 Total value £m £m £m £m £m £m £m £m £m £m Assets Loans and advances to customers — — 211,279 211,279 208,750 — — 211,796 211,796 207,287 Loans and advances to banks — 1,682 — 1,682 1,682 — 1,739 116 1,855 1,855 Reverse repurchase agreements - non trading — 19,382 226 19,608 19,599 — 23,634 — 23,634 23,636 Other financial assets at amortised cost 799 393 — 1,192 1,163 6,575 535 — 7,110 7,056 799 21,457 211,505 233,761 231,194 6,575 25,908 211,912 244,395 239,834 Liabilities Deposits by customers — 108 195,134 195,242 195,135 — 95 181,918 182,013 181,883 Deposits by banks — 20,951 16 20,967 20,958 — 13,956 407 14,363 14,353 Repurchase agreements - non trading — 15,847 — 15,847 15,848 — 18,292 — 18,292 18,286 Debt securities in issue — 34,967 1,430 36,397 35,566 — 42,694 — 42,694 41,129 Subordinated liabilities — 2,830 239 3,069 2,556 — 4,220 — 4,220 3,528 — 74,703 196,819 271,522 270,063 — 79,257 182,325 261,582 259,179 Valuation methodology for financial instruments carried at amortised cost The valuation approach to specific categories of financial instruments is described below. Assets: Loans and advances to customers The approach to estimating the fair value of loans and advances to customers has been determined by discounting expected cash flows to reflect either current market rates or credit spreads relevant to the specific industry of the borrower. The determination of their fair values is an area of considerable estimation and uncertainty as there is no observable market and values are significantly affected by customer behaviour. i) Advances secured on residential property The fair value of the mortgage portfolio is calculated by discounting contractual cash flows by different spreads for each LTV Band, after taking account of expected customer prepayment rates. The spread is based on new business interest rates derived from publicly available competitor market information. ii) Corporate loans The determination of the fair values of performing loans takes account of the differential between existing margins and estimated new business rates for similar loans in terms of segment and maturity. Provisions are considered appropriate for the book that is not impaired. A discount has been applied to impaired loans. Although exits have generally been achieved at carrying value, this does not reflect the discount a purchaser would require. A discount has therefore been applied based on the target return sought by distressed bond funds, who are the typical purchaser of the assets. iii) Other loans These consist of unsecured personal loans, credit cards, overdrafts and consumer (auto) finance. The weighted average lives of these portfolios are short and relate to relatively new business. As a result, contractual interest rates approximate new business interest rates, and therefore no mark-to-market surplus or deficit has been recorded with respect to the performing book, with the exception of unsecured personal loans and consumer (auto) finance loans, where a small surplus or deficit has been recognised based on the differential between existing portfolio margins and the current contractual interest rates. Loans and advances to banks These comprise secured loans, short-term placements with banks including collateral and unsettled financial transactions. The secured loans have been valued based on a discounted spread for the term of the loans using valuation technique A as described above. The carrying amount of the other items is deemed a reasonable approximation of their fair value, as the transactions are very short-term in duration. Reverse repurchase agreements - non-trading The fair value of the reverse repurchase agreements - non trading has been estimated using valuation technique A as described above, using a spread appropriate to the underlying collateral. Other financial assets at amortised cost These consist of asset backed securities and debt securities. The asset backed securities can be complex products and in some instances are valued with the assistance of an independent, specialist valuation firm. These fair values are determined using industry-standard valuation techniques, including discounted cash flow models. The inputs to these models used in these valuation techniques include quotes from market makers, prices of similar assets, adjustments for differences in credit spreads, and additional quantitative and qualitative research. The debt security investments consist of a portfolio of government debt securities. The fair value of this portfolio has been determined using quoted market prices. Liabilities: Deposits by customers The majority of deposit liabilities are payable on demand and therefore can be deemed short-term in nature with the fair value equal to the carrying value. Certain of the deposit liabilities are at a fixed rate until maturity. The deficit/surplus of fair value over carrying value of these liabilities has been estimated by reference to the market rates available at the balance sheet date for similar deposit liabilities of similar maturities. The fair value of such deposit liabilities has been estimated using valuation technique A as described above. Deposits by banks The fair value of deposits by banks, including repos, has been estimated using valuation technique A as described above, discounted at the appropriate credit spread. Repurchase agreements - non trading The fair value of the repurchase agreements - non trading has been estimated using valuation technique A as described above, discounted at a spread appropriate to the underlying collateral. Debt securities in issue and subordinated liabilities Where reliable prices are available, the fair value of debt securities in issue and subordinated liabilities has been calculated using quoted market prices. f) Fair values of financial instruments measured at fair value The following tables summarise the fair values of the financial assets and liabilities accounted for at fair value at 31 December 2020 and 31 December 2019, analysed by their levels in the fair value hierarchy - Level 1, Level 2 and Level 3. Group 2020 2019 Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Total Valuation £m £m £m £m £m £m £m £m technique Assets Derivative financial instruments Exchange rate contracts — 2,455 2 2,457 — 2,317 6 2,323 A Interest rate contracts — 2,566 14 2,580 — 1,915 9 1,924 A & C Equity and credit contracts — 71 52 123 — 223 60 283 B & D Netting — (1,754) — (1,754) — (1,214) — (1,214) — 3,338 68 3,406 — 3,241 75 3,316 Other financial assets at FVTPL Loans and advances to customers — — 99 99 — — 92 92 A Debt securities — — 109 109 — — 294 294 A, B & D Equity securities B Reverse repurchase agreements – non trading A — — 208 208 — — 386 386 Financial assets at FVOCI Debt securities 8,501 428 — 8,929 9,209 482 — 9,691 D Loans and advances to customers — — 21 21 — — 56 56 D 8,501 428 21 8,950 9,209 482 56 9,747 Total assets at fair value 8,501 3,766 297 12,564 9,209 3,723 517 13,449 Liabilities Derivative financial instruments Exchange rate contracts — 833 — 833 — 660 4 664 A Interest rate contracts — 2,447 3 2,450 — 1,836 2 1,838 A & C Equity and credit contracts — 26 29 55 — 134 26 160 B & D Netting — (1,754) — (1,754) — (1,214) — (1,214) — 1,552 32 1,584 — 1,416 32 1,448 Other financial liabilities at FVTPL Debt securities in issue — 1,051 6 1,057 — 1,099 6 1,105 A Structured deposits — 375 — 375 — 406 29 435 A Repurchase agreements – non trading A Collateral and associated financial guarantees — — 2 2 — 147 26 173 D — 1,426 8 1,434 — 1,652 61 1,713 Total liabilities at fair value — 2,978 40 3,018 — 3,068 93 3,161 Transfers between levels of the fair value hierarchy In 2020 and 2019, there were no significant transfers of financial instruments between levels of the fair value hierarchy. g) Fair value adjustments The internal models incorporate assumptions that Santander UK believes would be made by a market participant to establish fair value. Fair value adjustments are adopted when Santander UK considers that there are additional factors that would be considered by a market participant that are not incorporated in the valuation model. Santander UK classifies fair value adjustments as either ‘risk-related’ or ‘model-related’. The fair value adjustments form part of the portfolio fair value and are included in the balance sheet values of the product types to which they have been applied. The magnitude and types of fair value adjustment are listed in the following table: 2020 2019 £m £m Risk-related: - Bid-offer and trade specific adjustments (8) (12) - Uncertainty 23 17 - Credit risk adjustment 11 6 - Funding fair value adjustment 3 6 29 17 Model-related — — Day One profit — — 29 17 Risk-related adjustments Risk-related adjustments are driven, in part, by the magnitude of Santander UK’s market or credit risk exposure, and by external market factors, such as the size of market spreads. (i) Bid-offer and trade specific adjustments Portfolios are marked at bid or offer, as appropriate. Valuation models will typically generate mid-market values. The bid-offer adjustment reflects the cost that would be incurred if substantially all residual net portfolio market risks were closed using available hedging instruments or by disposing of or unwinding the position. For debt securities, the bid-offer spread is based on a market price at an individual security level. For other products, the major risk types are identified. For each risk type, the net portfolio risks are first classified into buckets, and then a bid-offer spread is applied to each risk bucket based upon the market bid-offer spread for the relevant hedging instrument. (ii) Uncertainty Certain model inputs may be less readily determinable from market data, and/or the choice of model itself may be more subjective. In these circumstances, a range of possible values exists that the financial instrument or market parameter may assume, and an adjustment may be needed to reflect the likelihood that in estimating the fair value of the financial instrument, market participants would adopt more conservative values for uncertain parameters and/or model assumptions than those used in the valuation model. (iii) Credit risk adjustment Credit risk adjustments comprise credit and debit valuation adjustments. The credit valuation adjustment (CVA) is an adjustment to the valuation of OTC derivative contracts to reflect within fair value the possibility that the counterparty may default, and Santander UK may not receive the full market value of the transactions. The debit valuation adjustment (DVA) is an adjustment to the valuation of the OTC derivative contracts to reflect within the fair value the possibility that Santander UK may default, and that Santander UK may not pay full market value of the transactions. Santander UK calculates a separate CVA and DVA for each Santander UK legal entity, and within each entity for each counterparty to which the entity has exposure. Santander UK calculates the CVA by applying the probability of default of the counterparty to the expected positive exposure to the counterparty, and multiplying the result by the loss expected in the event of default i.e. LGD. Conversely, Santander UK calculates the DVA by applying the PD of the Santander UK group, conditional on the non-default of the counterparty, to the expected positive exposure of the counterparty to Santander UK and multiplying the result by the LGD. Both calculations are performed over the life of the potential exposure. For most products Santander UK uses a simulation methodology to calculate the expected positive exposure to a counterparty. This incorporates a range of potential exposures across the portfolio of transactions with the counterparty over the life of the portfolio. The simulation methodology includes credit mitigants such as counterparty netting agreements and collateral agreements with the counterparty. The methodologies do not, in general, account for wrong-way risk. Wrong-way risk arises where the underlying value of the derivative prior to any credit risk adjustment is positively correlated to the probability of default of the counterparty. When there is significant wrong-way risk, a trade-specific approach is applied to reflect the wrong-way risk within the valuation. Exposure to wrong-way risk is limited via internal governance processes and deal pricing. Santander UK considers that an appropriate adjustment to reflect wrong-way risk is £nil (2019: £nil). (iv) Funding fair value adjustment (FFVA) The FFVA is an adjustment to the valuation of OTC derivative positions to include the net cost of funding uncollateralised derivative positions. This is calculated by applying a suitable funding cost to the expected future funding exposure of any uncollateralised component of the OTC derivative portfolio. Model-related adjustments Models used for portfolio valuation purposes may be based upon a simplifying set of assumptions that do not capture all material market characteristics. Additionally, markets evolve, and models that were adequate in the past may require development to capture all material market characteristics in current market conditions. In these circumstances, model limitation adjustments are adopted. As model development progresses, model limitations are addressed within the core revaluation models and a model limitation adjustment is no longer needed. The table below provides an analysis of financial instruments valued using internal models based on information other than market data together with further details on the valuation techniques used for each type of instrument. Each instrument is initially valued at transaction price: Balance sheet value Fair value movements recognised in profit/(loss) 2020 2019 2020 2019 2018 Balance sheet line item Category Financial instrument product type £m £m £m £m £m 1. Derivative assets Equity and credit contracts Reversionary property interests 51 52 3 2 30 2. FVTPL assets Loans and advances to customers Roll-up mortgage portfolio 56 51 6 — 8 3. FVTPL assets Loans and advances to customers Other loans 43 41 3 1 2 4. FVTPL assets Debt securities Reversionary property securities 107 120 6 (17) (28) 5. FVTPL assets Debt securities Credit linked notes 2 174 (16) 7 13 6. FVOCI assets Loans and advances to customers Other loans 21 56 (4) (2) (5) 7. Derivative liabilities Equity contracts Property options and forwards (29) (26) (3) — — 8. FVTPL liabilities Financial guarantees Credit protection guarantee (2) (26) 16 (7) (13) 249 442 11 (16) 7 Other Level 3 assets 17 23 7 16 (2) Other Level 3 liabilities (9) (41) (1) (5) 1 Total net assets 257 424 — — — Total income/(expense) 17 (5) 6 Valuation techniques 1. Derivative assets – Equity and credit contracts These are valued using a probability weighted set of HPI forward prices, which are assumed to be a reasonable representation of the increase in value of the Santander UK group’s reversionary interest portfolio underlying the derivatives. The probability used reflects the likelihood of the home owner vacating the property and is calculated from mortality rates and acceleration rates which are a function of age and gender, obtained from the relevant mortality tables. Indexing is felt to be appropriate due to the size and geographical dispersion of the reversionary interest portfolio. These are determined using HPI Spot Rates adjusted to reflect estimated forward growth. Non-seasonally adjusted (NSA) national and regional HPI are used in the valuation model to avoid any subjective judgement in the adjustment process which is made by Markit, which publishes the Halifax House Price Index. The inputs used to determine the value of the reversionary property derivatives are HPI spot, HPI forward growth and mortality rates. The principal pricing parameter is HPI forward growth. 2. FVTPL assets – Loans and advances to customers – roll-up mortgage portfolio These represent roll-up mortgages (sometimes referred to as lifetime mortgages), which are an equity release scheme under which a property owner takes out a loan secured against their home. The owner may not make any interest payments during their lifetime in which case the fixed interest payments are rolled up into the mortgage. The loan or mortgage (capital and rolled-up interest) is repaid upon the owner’s vacation of the property and the value of the loan is only repaid from the value of the property. This is known as a ‘no negative equity guarantee’. Santander UK suffers a loss if the sale proceeds from the property are insufficient to repay the loan, as it is unable to pursue the homeowner’s estate or beneficiaries for the shortfall. The value of the mortgage ‘rolls up’ or accretes until the owner vacates the property. In order to value the roll-up mortgages, Santander UK uses a probability-weighted set of European option prices (puts) determined using the Black-Scholes model, in which the ‘no negative equity guarantee’ is valued as short put options. The probability weighting applied is calculated from mortality rates and acceleration rates as a function of age and gender, taken from mortality tables. The inputs used to determine the value of these instruments are HPI spot, HPI forward growth, HPI volatility, mortality rates and repayment rates. The principal pricing parameter is HPI forward growth. The HPI forward growth rate used is unobservable and is the same as used in the valuation of Instrument 1 above. The other parameters do not have a significant effect on the value of the instruments. 3. FVTPL assets – Loans and advances to customers – other loans These relate to loans to transport and education companies. The fair value of these loans is estimated using the ‘present value’ model based on a credit curve derived from current market spreads. Loan specific credit data is unobservable, so a proxy population is applied based on industry sector and credit rating. 4. FVTPL assets – Debt securities These consist of reversionary property securities and are an equity release scheme, where the property owner receives an upfront lump sum in return for paying a fixed percentage of the sales proceeds of the property when the owner vacates the property. These reversionary property securities are valued using a probability-weighted set of HPI forward prices which are assumed to be a reasonable representation of the increase in value of Santander UK’s reversionary interest portfolio underlying the derivatives. The probability weighting used reflects the probability of the home owner vacating the property through death or moving into care and is calculated from mortality rates and acceleration factors which are a function of age and gender, obtained from the relevant mortality table. The inputs used to determine the value of these instruments are HPI spot, HPI forward growth and mortality rates. The principal pricing parameter is HPI forward growth. Discussion of the HPI spot rate, HPI forward growth rate and mortality rates for this financial instrument is the same as Instrument 1 above. An adjustment is also made to reflect the specific property risk. Discussion of the specific property risk adjustment is the same as Instrument 1 above. 5 . FVTPL assets – Debt securities (Credit linked notes) These consist of the retained senior tranches of credit linked notes in respect of credit protection vehicles sponsored by Santander UK and are mandatorily held at fair value through profit or loss. These vehicles provide credit protection on reference portfolios of Santander UK group loans with junior notes sold to external investors. The notes retained by Santander UK are classified as level 3 financial instruments as their valuation depends upon unobservable parameters relating to the underlying reference portfolios of loans, including credit spreads, correlations and prepayment speed, which have a significant effect on the overall valuation. For more information, see ‘Credit protection entities’ in Note 19. 6 . FVOCI assets – Loans and advances to customers – other loans These relate to shipping and construction loans. The fair value of these loans is estimated using the ‘present value’ model based on a credit curve derived from current market spreads. Loan specific credit data is unobservable, so a proxy population is applied based on industry sector and credit rating. 7 . Derivative liabilities – Equity contracts There

Offsetting Financial Assets and

Offsetting Financial Assets and Liabilities	12 Months Ended
	Dec. 31, 2020
Disclosure of offsetting of financial liabilities [abstract]
Offsetting Financial Assets and Liabilities	41. OFFSETTING FINANCIAL ASSETS AND LIABILITIES Financial assets and financial liabilities are reported on a net basis on the balance sheet only if there is a legally enforceable right to set off the recognised amounts and there is an intention to settle on a net basis, or to realise the asset and settle the liability simultaneously. The following table shows the impact of netting arrangements on: – All financial assets and liabilities that are reported net on the balance sheet – All derivative financial instruments and repurchase agreements and other similar secured lending and borrowing agreements that are subject to enforceable master netting arrangements or similar agreements, but do not qualify for balance sheet netting. The table identifies the amounts that have been offset in the balance sheet and also those amounts that are covered by enforceable netting arrangements (offsetting arrangements and financial collateral) but do not qualify for netting under the requirements described above. For derivative contracts, the ‘Financial instruments’ column identifies financial assets and liabilities that are subject to set off under netting agreements, such as the ISDA Master Agreement or derivative exchange or clearing counterparty agreements, whereby all outstanding transactions with the same counterparty can be offset and close-out netting applied across all outstanding transactions covered by the agreements if an event of default or other predetermined events occur. Financial collateral refers to cash and non-cash collateral obtained, typically daily or weekly, to cover the net exposure between counterparties by enabling the collateral to be realised in an event of default or if other predetermined events occur. For repurchase and reverse repurchase agreements and other similar secured lending and borrowing, the ‘Financial instruments’ column identifies financial assets and liabilities that are subject to set off under netting agreements, such as global master repurchase agreements and global master securities lending agreements, whereby all outstanding transactions with the same counterparty can be offset and close-out netting applied across all outstanding transactions covered by the agreements if an event of default or other predetermined events occur. Financial collateral typically comprises highly liquid securities which are legally transferred and can be liquidated if a counterparty defaults. Santander UK engages in a variety of counterparty credit mitigation strategies in addition to netting and collateral arrangements. Therefore, the net amounts presented in the tables below do not purport to represent Santander UK’s actual credit exposure. Group Amounts subject to enforceable netting arrangements Assets not subject to enforceable netting arrangements (2) Effects of offsetting on balance sheet Related amounts not offset Gross Amounts Net amounts Financial Financial collateral (1) Net Balance sheet total (3) 2020 £m £m £m £m £m £m £m £m Assets Derivative financial assets 5,071 (1,754) 3,317 (782) (1,840) 695 89 3,406 Reverse repurchase, securities borrowing & similar agreements: – Amortised cost 26,084 (6,485) 19,599 (129) (19,470) — — 19,599 – Fair value — — — — — — — — Loans and advances to customers and banks⁽⁴⁾ 7,454 (1,073) 6,381 — — 6,381 204,051 210,432 38,609 (9,312) 29,297 (911) (21,310) 7,076 204,140 233,437 Liabilities Derivative financial liabilities 3,261 (1,754) 1,507 (782) (551) 174 77 1,584 Repurchase, securities lending & similar agreements: – Amortised cost 22,333 (6,485) 15,848 (129) (15,719) — — 15,848 – Fair value — — — — — — — — Deposits by customers and banks⁽⁴⁾ 11,159 (1,073) 10,086 — (502) 9,584 206,007 216,093 36,753 (9,312) 27,441 (911) (16,772) 9,758 206,084 233,525 2019 Assets Derivative financial assets 4,446 (1,214) 3,232 (768) (1,915) 549 84 3,316 Reverse repurchase, securities borrowing & similar agreements: – Amortised cost 25,312 (1,676) 23,636 (537) (23,099) — — 23,636 – Fair value — — — — — — — — Loans and advances to customers and banks⁽⁴⁾ 5,247 (1,248) 3,999 — — 3,999 205,143 209,142 35,005 (4,138) 30,867 (1,305) (25,014) 4,548 205,227 236,094 Liabilities Derivative financial liabilities 2,616 (1,214) 1,402 (768) (572) 62 46 1,448 Repurchase, securities lending & similar agreements: – Amortised cost 19,962 (1,676) 18,286 (537) (17,749) — — 18,286 – Fair value — — — — — — — — Deposits by customers and banks⁽⁴⁾ 11,395 (1,248) 10,147 — (502) 9,645 186,089 196,236 33,973 (4,138) 29,835 (1,305) (18,823) 9,707 186,135 215,970 (1) Financial collateral is reflected at its fair value, but has been limited to the net balance sheet exposure so as not to include any over-collateralisation. (2) This column includes contractual rights of set-off that are subject to uncertainty under the laws of the relevant jurisdiction. (3) The balance sheet total is the sum of ‘Net amounts reported on the balance sheet’ that are subject to enforceable netting arrangements and ‘Amounts not subject to enforceable netting arrangements’. ’.

Events After the Balance Sheet

Events After the Balance Sheet Date	12 Months Ended
	Dec. 31, 2020
Disclosure of non-adjusting events after reporting period [abstract]
Events After the Balance Sheet Date	42. EVENTS AFTER THE BALANCE SHEET DATE There have been no significant events between 31 December 2020 and the date of approval of these financial statements which would require a change to or additional disclosure in the financial statements, except as follows: Proposed transfer of the Corporate & Investment Banking (CIB) business to the London Branch of Banco Santander SA (SLB) As part of our drive for continuous improvement in customer experience and following a review of the way we operate the CIB business in the UK, we intend to conduct substantially all of this business from SLB beginning later in 2021. To undertake this change, and subject to court approval, we are proposing to transfer substantially all of the CIB business to SLB in H2 2021 by way of a banking business transfer scheme under Part VII of the Financial Services and Markets Act 2000. Although this decision was made prior to 31 December 2020, the transfer, which is subject to court approval, is proposed to take effect in H2 2021. We will continue to engage with regulators in relation to the proposed transfer.

Interest Rate Benchmark Reform

Interest Rate Benchmark Reform	12 Months Ended
	Dec. 31, 2020
Interest Rate Benchmark Reform [Abstract]
Interest Rate Benchmark Reform	43. INTEREST RATE BENCHMARK REFORM In September 2019, the IASB amended IFRS 9 'Financial Instruments', IAS 39 'Financial Instruments: Recognition and Measurement' and IFRS 7 'Financial Instruments: Disclosures' to address issues affecting financial reporting in the period before the reform of an interest rate benchmark, including the replacement of an interest rate benchmark with an alternative benchmark rate (the Phase 1 amendments). These Phase 1 amendments provided temporary exceptions to specific hedge accounting requirements because of the uncertainty arising from the reform. After issuing the Phase 1 amendments, in August 2020, the IASB issued further amendments to various IFRSs to address issues that might affect financial reporting during the reform of an interest rate benchmark, including the effects of changes to contractual cash flows or hedging relationships arising from the replacement of an interest rate benchmark with an alternative benchmark rate (the Phase 2 amendments). The Phase 2 amendments do not supersede the Phase 1 amendments. Phase 1 amendments The amendments provide temporary exceptions from applying specific hedge accounting requirements to hedging relationships that are directly affected by the IBOR reform. The exceptions have the effect that IBOR reform should not generally cause hedge accounting to terminate, however any hedge ineffectiveness continues to be recognised in the income statement. The exceptions end at the earlier of: – when the uncertainty regarding the timing and the amount of interest rate benchmark-based cash flows is no longer present, and – discontinuance of the hedge relationship (or reclassification of all amounts from the cash flow hedge reserve). The amendments apply to all hedging relationships directly affected by uncertainties related to IBOR reform and had no impact on hedge relationships for affected hedges. The main assumptions or judgements made by Santander UK in applying the amendments are outlined below. – For cash flow hedges affected by IBOR reform, Santander UK management has assumed that the interest rate benchmark on which hedged cash flows are based is not altered as a result of IBOR reform when assessing whether the future cash flows are highly probable. For discontinued hedging relationships, the same assumption has been applied for determining whether the hedged future cash flows are expected to occur. – In making its prospective hedge effectiveness assessments, Santander UK assumes that the interest rate benchmark on which the hedged item and the hedging instrument are based is not altered as a result of IBOR reform. – Santander UK will not discontinue hedge accounting during the period of IBOR-related uncertainty solely because the retrospective effectiveness falls outside the required 80-125% range. – For hedges of a non-contractually specified benchmark portion of an interest rate, Santander UK only considers at inception of such a hedging relationship whether the separately identifiable requirement is met. Phase 2 amendments These amendments apply only to changes required by IBOR reform to financial instruments and hedging relationships. Changes are directly required by IBOR reform if, and only if, the change is necessary as a direct consequent of interest rate benchmark reform, and the new basis for determining the contractual cash flow is economically equivalent to the previous basis. The exceptions given by the amendments mean that IBOR reform did not result in the discontinuation of hedge accounting and any hedge ineffectiveness continued to be recognised in profit or loss for affected hedges at and for the year ended 31 December 2020. The amendments address the accounting issues for financial instruments when IBOR reform is implemented as described below. Practical expedient for changes to contractual cash flows For instruments to which the amortised cost measurement applies, the amendments require entities, as a practical expedient, to account for a change in the basis for determining the contractual cash flows by updating the effective interest rate using the guidance in IFRS 9 resulting in no immediate gain or loss being recognised, provided that, the change is directly required by IBOR reform and takes place on an economically equivalent basis. Whereas some instruments were converted to alternative benchmark interest rates during 2020, the majority of instruments referencing LIBOR or other IBORs will transition to alternative benchmark interest rates during 2021. Santander UK has no lease contracts which are indexed to LIBOR or other IBORs. Consequently, the application of the practical expedient had no material impact for the Santander UK group for 2020. Relief from specific hedge accounting requirements The table below sets out the hedge accounting amendments, and their impact for Santander UK, which provide additional temporary reliefs from applying specific IAS 39 hedge accounting requirements to hedging relationships directly affected by IBOR reform. Hedge accounting amendment Impact for the Santander UK group Allow amendment of the designation of a hedging relationship to reflect changes that are required by the reform. The hedge designation must be amended by the end of the reporting period in which the changes are made. This amendment means any change to hedge documentation will not result in discontinuation of hedge accounting nor the designation of a new hedge relationship. The Santander UK group did not use this relief in 2020. It expects the majority of its hedge relationships will transition to alternative benchmark rates in 2021. When a hedged item in a cash flow hedge is amended to reflect the changes that are required by the reform, the amount accumulated in the cash flow hedge reserve will be deemed to be based on the alternative benchmark rate on which the hedged future cash flows are determined. This amendment would result in the release of the cash flow hedge reserve to profit or loss in the same period or periods in which the hedged cash flows that are now based on the alternative benchmark interest rate affect profit or loss. At 31 December 2020, the Santander UK group’s existing cash flow hedges have not yet transitioned to alternative benchmark interest rates which is expected to take place in 2021. An entity may, on an individual hedge basis, reset to zero the cumulative fair value changes of the hedged item and hedging instrument when ceasing to apply the retrospective effectiveness assessment relief provided by the Phase 1 amendments. At 31 December 2020, the amendment had no impact on the Santander UK group. The Santander UK group will assess on a hedge-by-hedge basis as the hedging instrument transitions away from LIBORs or other IBORs in 2021 and expects resetting the cumulative fair value changes to zero will have no effect on the amounts recorded in profit or loss. All hedge ineffectiveness including any outside the 80-125% range arising from IBOR reform will be recognised in profit or loss. When amending the hedge relationships for groups of items, hedged items are allocated to sub-groups based on the alternative benchmark interest rate being hedged, and the benchmark rate for each sub-group is designated as the hedged risk. At 31 December 2020, the amendment had no impact on the Santander UK group as the affected portfolio has not yet begun transitioning away from LIBOR. An alternative benchmark interest rate designated as a non-contractually specified risk component, that is not separately identifiable at the date when it is designated, is deemed to have met the requirements at that date if the entity reasonably expects that it will meet the requirements within a period of 24 months from the date of first designation. The 24-month period will apply to each alternative benchmark interest rate separately. The risk component will, however, be required to be reliably measurable. The amendment eases transition to alternative benchmark interest rates by allowing hedging relationships to be designated and to continue even before the new benchmark interest rates are fully established as market benchmarks. At 31 December 2020, for the Santander UK group, the majority of hedge relationships, where an alternative benchmark interest rate is to be designated as a non-contractually specified risk component, are GBP fair value hedge relationships. Santander UK group expects alternative benchmark interest rates to be separately identifiable and reliably measurable when they transition in 2021. For other changes that are not as a direct consequence of IBOR reform, Santander UK separately assesses those changes to determine if they result in derecognition or discontinuation of hedge accounting by applying the relevant accounting policies as set out in Note 1. The following table shows the notional amounts of assets, liabilities and off-balance sheet commitments at 31 December 2020 affected by IBOR reform that have yet to transition to an alternative benchmark interest rate as provided internally to key management personnel. Group GBP (2) LIBOR USD (2) LIBOR Other (2) Total £m £m £m £m Assets Derivatives (1) 33,486 4,514 2,149 40,149 Other financial assets at fair value through profit and loss 968 22 — 990 Financial assets at amortised cost 15,062 1,191 90 16,343 Financial assets at fair value through comprehensive income 428 — — 428 49,944 5,727 2,239 57,910 Liabilities Derivatives (1) 35,217 5,205 88 40,510 Other financial liabilities at fair value through profit and loss 1,129 69 — 1,198 Financial liabilities at amortised cost 2,354 1,319 — 3,673 38,700 6,593 88 45,381 Off-balance sheet commitments given 11,400 2,126 573 14,099 (1) Many of the Santander UK group’s derivatives subject to IBOR reform are governed by ISDA definitions. In October 2020 ISDA issued an IBOR fallbacks supplement setting out how the amendments to new alternative benchmark rates will be accomplished, the effect of which is to create fallback provisions in derivatives that describe what floating rates will apply on the permanent discontinuation of certain key IBORs or upon ISDA declaring a non-representative determination of an IBOR. The Santander UK group has adhered to the protocol to implement the fallbacks to derivative contracts that were entered into before the effective date of the supplement (25 January 2021). If derivative counterparties also adhere to the protocol, new fallbacks will automatically be implemented in existing derivative contracts when the supplement becomes effective. (2) Cessation dates are :- GBP, JPY, NOK LIBOR 31/12/2021, USD LIBOR 30/06/23, EONIA 03/01/2022 The following tables show the notional amount of derivatives in hedging relationships directly affected by uncertainties related to IBOR reform. Group GBP USD Other Total 2020 £m £m £m £m Total notional value of hedging instruments: – Cash flow hedges 15,198 5,119 — 20,317 – Fair value hedges 32,223 1,077 778 34,078 47,421 6,196 778 54,395 Maturing after cessation date (1) – Cash flow hedges 10,553 2,562 — 13,115 – Fair value hedges 12,477 162 720 13,359 23,030 2,724 720 26,474 (1) Cessation dates are :- GBP, JPY, NOK LIBOR 31/12/2021, USD LIBOR 30/06/23, EONIA 03/01/2022 The Santander UK group’s GBP LIBOR and USD LIBOR cash flow hedges extend beyond the anticipated cessation dates for both LIBORs. The Santander UK group expects that GBP LIBOR and USD LIBOR will be replaced by SONIA and SOFR respectively but there remains uncertainty over the timing and amount of the replacement rate cash flows. Hedging relationships impacted by uncertainty about IBOR reform may experience ineffectiveness due to market participants’ expectations of when the shift from the existing IBOR benchmark rate to an alternative benchmark interest rate will occur or because transition of the hedged item and the hedging instrument could occur at different times. The Santander UK group will cease to apply the assumptions that the hedged benchmark interest rate, the cash flows of the hedged item and/or hedging instrument will not be altered because of IBOR reform when the uncertainty arising from IBOR reform is no longer present. This will require amendment to hedge documentation by the end of the reporting period in which the changes occur. Cumulative changes in the hedged cash flows and the hedging instrument based on new alternative benchmark rates will also be remeasured when IBOR reform uncertainty is removed. Further details of the significant interest rate benchmarks to which hedging relationships are exposed, the extent of risk exposure that is affected by IBOR reform, the effect of IBOR reform on interest rate risk management and how Santander UK’s transition to alternative benchmark interest rates is being managed, are disclosed in the Banking market risk section of the Risk review.

Transfer of Mortgages to Santan

Transfer of Mortgages to Santander Financial Services PLC	12 Months Ended
	Dec. 31, 2020
Subclassifications of assets, liabilities and equities [abstract]
Transfer of Mortgages to Santander Financial Services PLC	44. TRANSFER OF MORTGAGES TO SANTANDER FINANCIAL SERVICES PLC In October 2020, Santander UK plc transferred a portfolio of mortgage assets with a book value of £3,163m to Santander Financial Services plc for a cash consideration of £3,174m, including a purchase price premium of £11m. The mortgages transferred consisted only of assets classified as Stage 1 for ECL purposes, and were therefore not credit-impaired. There were no significant costs to sell. Santander UK plc recognised an initial gain of £11m from the sale in other income and a £0.3m release of ECL provision previously recognised. As the mortgage portfolio sold contained fixed rate mortgages included in the mortgage fair value macro hedge, a £14m fair value hedge accounting adjustment was also released, resulting in an overall loss on sale of £3m.

Accounting Policies (Policies)

Accounting Policies (Policies) - For Year-end	12 Months Ended
	Dec. 31, 2020
Statement [Line Items]
Basis of preparation	Basis of preparation These financial statements incorporate the financial statements of the Company and entities controlled by the Company (its subsidiaries) made up to 31 December each year. The consolidated financial statements have been prepared on the going concern basis using the historical cost convention, except for financial assets and liabilities that have been measured at fair value. An assessment of the appropriateness of the adoption of the going concern basis of accounting is disclosed in the statement of going concern in the Directors’ Report. Compliance with International Financial Reporting Standards The consolidated financial statements of the Santander UK group and the separate financial statements of the Company comply with international accounting standards in conformity with the requirements of the Companies Act 2006 and have also applied international financial reporting standards adopted pursuant to Regulation (EC) No 1606/2002 as it applies in the European Union. The financial statements are also prepared in accordance with International Financial Reporting Standards (IFRSs) as issued by the International Accounting Standards Board (IASB), including interpretations issued by the IFRS Interpretations Committee, as there are no applicable differences from IFRSs as issued by the IASB for the periods presented. Disclosures required by IFRS 7 ‘Financial Instruments: Disclosure’ relating to the nature and extent of risks arising from financial instruments, and IAS 1 ‘Presentation of Financial Statements’ relating to objectives, policies and processes for managing capital, can be found in the Risk review. Those disclosures form an integral part of these financial statements. Recent accounting developments Interest Rate Benchmark Reform In September 2019, the IASB issued ‘Interest Rate Benchmark Reform: Amendments to IFRS 9, IAS 39 and IFRS 7’. The Santander UK group applies IAS 39 hedge accounting so the amendments to IFRS 9 do not apply. The IAS 39 amendments apply to all hedging relationships directly affected by uncertainties related to interbank offered rate (IBOR) reform and must be applied for annual periods beginning on or after 1 January 2020. Following their endorsement for use in the European Union, the Santander UK group adopted the IAS 39 and IFRS 7 amendments in the preparation of the financial statements for the year ended 31 December 2019. The exceptions given by the IAS 39 amendments mean that IBOR reform had no impact on hedge relationships for affected hedges. In August 2020, the IASB issued ‘Interest Rate Benchmark Reform – Phase 2 - Amendments to IFRS 9, IAS 39, IFRS 7, IFRS 4 and IFRS 16’. These amendments apply only to changes required by IBOR reform to financial instruments and hedging relationships. The amendments are effective from 1 January 2021 and must be applied retrospectively without restating comparative information. Following their endorsement for use in the European Union and the UK, the Santander UK group has elected to apply the amendments in the preparation of these financial statements. The amendments address the accounting issues for financial instruments when IBOR reform is implemented including providing a practical expedient for changes to contractual cash flows, giving relief from specific hedge accounting requirements, and specifying a number of additional disclosures to enable users of financial statements to understand the effect of IBOR reform on an entity’s financial instruments and risk management strategy. Further details of the impact of these amendments on the financial statements for the year ended 31 December 2020 and the additional disclosures required are provided in Note 43. Other changes The Santander UK group adopted IFRS 16 and amendments to IAS 12 in 2019 and adopted IFRS 9 in 2018, with the impact included in the statement of changes in equity for that year end. Future accounting developments At 31 December 2020, for the Santander UK group, there were no significant new or revised standards and interpretations, and amendments thereto, which have been issued but which are not yet effective or which have otherwise not been early adopted where permitted.
Consolidation	The Consolidated Financial Statements incorporate the financial statements of the Company and entities (including structured entities) controlled by it and its subsidiaries. Control is achieved where the Company (i) has power over the investee; (ii) is exposed, or has rights, to variable returns from its involvement with the investee; and (iii) has the ability to use its power to affect its returns. The Company reassesses whether or not it controls an investee if facts and circumstances indicate that there are changes to one or more of the three elements of control listed above. When the Company has less than a majority of the voting rights of an investee, it has power over the investee when the voting rights are sufficient to give it the practical ability to direct the relevant activities of the investee unilaterally. The Company considers all relevant facts and circumstances in assessing whether or not the Company's voting rights in an investee are sufficient to give it power, including: – The size of the Company's holding of voting rights relative to the size and dispersion of holdings of the other vote holders – Potential voting rights held by the Company, other vote holders or other parties – Rights arising from other contractual arrangements – Any additional facts and circumstances that indicate that the Company has, or does not have, the current ability to direct the relevant activities at the time that decisions need to be made, including voting patterns at previous shareholders' meetings. Consolidation of a subsidiary begins when the Company obtains control over the subsidiary and ceases when the Company loses control of the subsidiary. Specifically, the results of a subsidiary acquired or disposed of during the year are included in the consolidated income statement and the consolidated statement of comprehensive income from the date the Company gains control until the date when the Company ceases to control the subsidiary. Inter-company transactions, balances and unrealised gains on transactions between Santander UK group companies are eliminated; unrealised losses are also eliminated unless the cost cannot be recovered. The acquisition method of accounting is used to account for the acquisition of subsidiaries which meet the definition of a business. The cost of an acquisition is measured at the fair value of the assets given up, shares issued or liabilities undertaken at the date of acquisition. Acquisition-related costs are expensed as incurred. The excess of the cost of acquisition, as well as the fair value of any interest previously held, over the fair value of the Santander UK group’s share of the identifiable net assets of the subsidiary at the date of acquisition is recorded as goodwill. When the Santander UK group loses control of a subsidiary, the profit or loss on disposal is calculated as the difference between (i) the aggregate of the fair value of the consideration received and the fair value of any retained interest and (ii) the previous carrying amount of the assets (including goodwill), less liabilities of the subsidiary and any non-controlling interests. Amounts previously recognised in other comprehensive income in relation to the subsidiary are accounted for (i.e. reclassified to profit or loss or transferred directly to retained earnings) in the same manner as would be required if the relevant assets or liabilities are disposed of. The fair value of any investment retained in a former subsidiary at the date when control is lost is regarded as the fair value on initial recognition for subsequent accounting under IFRS 9 or, when applicable, the costs on initial recognition of an investment in an associate or joint venture. Business combinations between entities under common control (i.e. fellow subsidiaries of Banco Santander SA, the ultimate parent) are outside the scope of IFRS 3 – ‘Business Combinations’, and there is no other guidance for such transactions under IFRS. The Santander UK group elects to account for business combinations between entities under common control at their book values in the acquired entity by including the acquired entity’s results from the date of the business combination and not restating comparatives. Reorganisations of entities within the Santander UK group are also accounted for at their book values. Interests in subsidiaries are eliminated during the preparation of the Consolidated Financial Statements. Interests in subsidiaries in the Company unconsolidated financial statements are held at cost subject to impairment. Credit protection entities established as part of significant risk transfer (SRT) transactions are not consolidated by the Santander UK group in cases where third party investors have the exposure, or rights, to all of the variability of returns from the performance of the entities. b) Joint ventures Joint ventures are joint arrangements whereby the parties that have joint control of the arrangement have rights to its net assets. Joint control is the contractually agreed sharing of control of an arrangement, which exists only when decisions about the relevant activities require unanimous consent of the parties sharing control. Accounting policies of joint ventures have been aligned to the extent there are differences from the Santander UK group’s policies. Investments in joint ventures are accounted for by the equity method of accounting and are initially recorded at cost and adjusted each year to reflect the Santander UK group’s share of their post-acquisition results. When the Santander UK group's share of losses of a joint venture exceed its interest in that joint venture, the Santander UK group discontinues recognising its share of further losses. Further losses are recognised only to the extent that the Santander UK group has incurred legal or constructive obligations or made payments on behalf of the joint venture. .
Foreign currency translation	Foreign currency translation Items included in the financial statements of each entity in the Santander UK group are measured using the currency that best reflects the economic substance of the underlying events and circumstances relevant to that entity (the functional currency). The Consolidated Financial Statements are presented in sterling, which is the functional currency of the Company. Income statements and cash flows of foreign entities are translated into the Santander UK group’s presentation currency at average exchange rates for the year and their balance sheets are translated at the exchange rates ruling on 31 December. Exchange differences on the translation of the net investment in foreign entities are recognised in other comprehensive income. When a foreign entity is sold, such exchange differences are recognised in the income statement as part of the gain or loss on sale. Foreign currency transactions are translated into the functional currency of the entity involved at the exchange rates prevailing at the dates of the transactions. Foreign exchange gains and losses resulting from the settlement of such transactions and from the translation of monetary assets and liabilities denominated in foreign currencies are recognised in the income statement unless recognised in other comprehensive income in connection with a cash flow hedge. Non-monetary items denominated in a foreign currency measured at historical cost are not retranslated. Exchange rate differences arising on non-monetary items measured at fair value are recognised in the consolidated income statement except for differences arising on equity securities measured at fair value through other comprehensive income (FVOCI), which are recognised in other comprehensive income.
Revenue recognition	Revenue recognition a) Interest income and expense Interest and similar income comprises interest income on financial assets measured at amortised cost, investments in debt instruments measured at FVOCI and interest income on hedging derivatives. Interest expense and similar charges comprises interest expense on financial liabilities measured at amortised cost, and interest expense on hedging derivatives. Interest income on financial assets measured at amortised cost, investments in debt instruments measured at FVOCI and interest expense on financial liabilities other than those at fair value through profit or loss (FVTPL) is determined using the effective interest rate method. The effective interest rate is the rate that discounts the estimated future cash payments or receipts over the expected life of the instrument or, when appropriate, a shorter period, to the gross carrying amount of the financial asset (i.e. its amortised cost before any impairment allowance) or to the amortised cost of a financial liability. When calculating the effective interest rate, the future cash flows are estimated after considering all the contractual terms of the instrument excluding expected credit losses. The calculation includes all amounts paid or received by the Santander UK group that are an integral part of the overall return, direct incremental transaction costs related to the acquisition, issue or disposal of the financial instrument and all other premiums or discounts. Interest income is calculated by applying the effective interest rate to the gross carrying amount of financial assets, except for financial assets that have subsequently become credit-impaired (or ‘Stage 3’), for which interest revenue is calculated by applying the effective interest rate to their amortised cost (i.e. net of the ECL provision). For more information on stage allocations of credit risk exposures, see ‘Significant increase in credit risk’ in the ‘Santander UK group level – credit risk management’ section of the Risk Review. During Q4 2020, we revised the accounting treatment for certain items of mortgage income. Mortgage account fees, which are normally paid at the end of the mortgage and were previously recognised as received in fee income, are now recognised in interest income as part of the effective interest rate method throughout the life of the mortgage to better reflect the requirements of IFRS. In addition, we no longer accrue interest income relating to the period after mortgages revert to the standard variable rate (or equivalent) beyond the incentive period. This better aligns our policy to current practice. These changes resulted in an increase in net interest income of £44m for 2020. The net impact of these changes is not material and comparatives have not been restated. b) Fee and commission income and expense Fees and commissions that are not an integral part of the effective interest rate are recognised when the service is performed. Most fee and commission income is recognised at a point in time. Certain commitment, upfront and management fees are recognised over time but are not material. For retail and corporate products, fee and commission income consists principally of collection services fees, commission on foreign currencies, commission and other fees received from retailers for processing credit card transactions, fees received from other credit card issuers for providing cash advances for their customers through the Santander UK group’s branch and ATM networks, annual fees payable by credit card holders and fees for non-banking financial products. For insurance products, fee and commission income consists principally of commissions and profit share arising from the sale of building and contents insurance and life protection insurance. Commissions arising from the sale of buildings and contents insurance are recognised over the period of insurance cover, adjusted to take account of cancelled policies. Profit share income from the sale of buildings and contents insurance which is not subject to any adjustment is recognised when the profit share income is earned. Commissions and profit share arising from the sale of life protection insurance is subject to adjustment for cancellations of policies within 3 years from inception. Fee and commission income which forms an integral part of the effective interest rate of a financial instrument (for example certain loan commitment fees) is recognised as an adjustment to the effective interest rate and recorded in ‘Interest income’. c) Dividend income Except for equity securities classified as trading assets or financial assets held at fair value through profit or loss, described below, dividend income is recognised when the right to receive payment is established. This is the ex-dividend date for equity securities. d) Other operating income Other operating income includes all gains and losses from changes in the fair value of financial assets and liabilities held at fair value through profit or loss (comprising financial assets and liabilities held for trading, trading derivatives and other financial assets and liabilities at fair value through profit or loss), together with related interest income, expense, dividends and changes in fair value of any derivatives managed in conjunction with these assets and liabilities. Changes in fair value of derivatives in a fair value hedging relationship are also recognised in other operating income. Other operating income also includes income from operating lease assets, and profits and losses arising on the sales of property, plant and equipment and subsidiary undertakings.
Borrowing costs	Borrowing costs Borrowing costs directly attributable to the acquisition, construction or production of qualifying assets, including computer software, which are assets that necessarily take a substantial period of time to develop for their intended use, are added to the cost of those assets, until the assets are substantially ready for their intended use. All other borrowing costs are recognised in profit or loss in the period in which they occur.
Pensions and other post-retirement benefits	Pensions and other post-retirement benefits a) Defined benefit schemes A defined benefit scheme is a pension scheme that guarantees an amount of pension benefit to be provided, usually as a function of one or more factors such as age, years of service or compensation. Pension costs are charged to ‘Administration expenses’, within the line item ‘Operating expenses before impairment losses, provisions and charges’ with the net interest on the defined benefit asset or liability included within ‘Net interest income’ in the income statement. The asset or liability recognised in respect of defined benefit pension schemes is the present value of the defined benefit obligation at the balance sheet date, less the fair value of scheme assets. The defined benefit obligation is calculated annually by independent actuaries using the projected unit credit method. The assets of the schemes are measured at their fair values at the balance sheet date. The present value of the defined benefit obligation is estimated by projecting forward the growth in current accrued pension benefits to reflect inflation and salary growth to the date of pension payment, then discounted to present value using the yield applicable to high-quality AA rated corporate bonds of the same currency and which have terms to maturity closest to the terms of the scheme liabilities, adjusted where necessary to match those terms. In determining the value of scheme liabilities, demographic and financial assumptions are made by management about life expectancy, inflation, discount rates, pension increases and earnings growth, based on past experience and future expectations. Financial assumptions are based on market conditions at the balance sheet date and can generally be derived objectively. Demographic assumptions require a greater degree of estimation and judgement to be applied to externally derived data. Any surplus or deficit of scheme assets over liabilities is recognised in the balance sheet as an asset (surplus) or liability (deficit). An asset is only recognised to the extent that the surplus can be recovered through reduced contributions in the future or through refunds from the scheme. The income statement includes the net interest income/expense on the net defined benefit liability/asset, current service cost and any past service cost and gain or loss on settlement. Remeasurement of defined benefit pension schemes, including return on scheme assets (excludes amounts included in net interest), actuarial gains and losses (arising from changes in demographic assumptions, the impact of scheme experience and changes in financial assumptions) and the effect of the changes to the asset ceiling (if applicable), are recognised in other comprehensive income. Remeasurement recognised in other comprehensive income will not be reclassified to the income statement. Past-service costs are recognised as an expense in the income statement at the earlier of when the scheme amendment or curtailment occurs and when the related restructuring costs or termination benefits are recognised. Curtailments include the impact of significant reductions in the number of employees covered by a scheme, or amendments to the terms of the scheme so that a significant element of future service will no longer qualify for benefits or will qualify only for reduced benefits. Curtailment gains and losses on businesses that meet the definition of discontinued operations are included in profit or loss for the year from discontinued operations. Gains and losses on settlements are recognised when the settlement occurs. b) Defined contribution plans A defined contribution plan is a pension scheme under which the Santander UK group pays fixed contributions as they fall due into a separate entity (a fund). The pension paid to the member at retirement is based on the amount in the separate fund for each member. The Santander UK group has no legal or constructive obligations to pay further contributions into the fund to ‘top up’ benefits to a certain guaranteed level. The regular contributions constitute net periodic costs for the year in which they are due and are included in staff costs within Operating expenses in the income statement. c) Post-retirement medical benefit plans Post-retirement medical benefit liabilities are determined using the projected unit credit method, with actuarial valuations updated at each year-end. The expected benefit costs are accrued over the period of employment using an accounting methodology similar to that for the defined benefit pension scheme.
Share-based payments	Share-based payments The Santander UK group engages in cash-settled and equity-settled share-based payment transactions in respect of services received from certain of its employees. Shares of the Santander UK group’s parent, Banco Santander SA are purchased in the open market by the Santander UK group (for the Employee Sharesave scheme) or are purchased by Banco Santander SA or another Banco Santander subsidiary (including awards granted under the Long-Term Incentive Plan and the Deferred Shares Bonus Plan) to satisfy share options or awards as they vest. Options granted under the Employee Sharesave scheme are accounted for as cash-settled share-based payment transactions. Awards granted under the Long-Term Incentive Plan and Deferred Shares Bonus Plan are accounted for as equity-settled share-based payment transactions. The fair value of the services received is measured by reference to the fair value of the shares or share options initially on the date of the grant for both the cash and equity settled share-based payments and then subsequently at each reporting date for the cash-settled share-based payments. The cost of the employee services received in respect of the shares or share options granted is recognised in the income statement in administration expenses over the period that the services are received i.e. the vesting period. A liability equal to the portion of the services received is recognised at the fair value determined at each balance sheet date for cash-settled share-based payments. A liability equal to the amount to be reimbursed to Banco Santander SA is recognised at the fair value determined at the grant date for equity-settled share-based payments. The fair value of the options granted under the Employee Sharesave scheme is determined using an option pricing model, which takes into account the exercise price of the option, the current share price, the risk free interest rate, the expected volatility of the Banco Santander SA share price over the life of the option and the dividend growth rate. The fair value of the awards granted for the Long-Term Incentive Plan was determined at the grant date using an option pricing model, which takes into account the share price at grant date, the risk free interest rate, the expected volatility of the Banco Santander SA share price over the life of the award and the dividend growth rate. Vesting conditions included in the terms of the grant are not taken into account in estimating fair value, except for those that include terms related to market conditions. Non-market vesting conditions are taken into account by adjusting the number of shares or share options included in the measurement of the cost of employee service so that, ultimately, the amount recognised in the income statement reflects the number of vested shares or share options. Where vesting conditions are related to market conditions, the charges for the services received are recognised regardless of whether or not the market–related vesting conditions are met, provided that the non-market vesting conditions are met. Where an award has been modified, as a minimum, the expense of the original award continues to be recognised as if it had not been modified. Where the effect of a modification is to increase the fair value of an award or increase the number of equity instruments, the incremental fair value of the award or incremental fair value of the modification of the award is recognised in addition to the expense of the original grant, measured at the date of modification, over the modified vesting period. Cancellations in the vesting period are treated as an acceleration of vesting, and recognised immediately for the amount that would otherwise have been recognised for services over the vesting period.
Goodwill and other intangible assets	Goodwill and other intangible assets Goodwill represents the excess of the cost of an acquisition, as well as the fair value of any interest previously held, over the fair value of the share of the identifiable net assets of the acquired subsidiary, associate, or business at the date of acquisition. Goodwill on the acquisition of subsidiaries and businesses is included in intangible assets. Goodwill on acquisitions of associates is included as part of investment in associates. Goodwill is tested for impairment annually, or more frequently when events or changes in circumstances dictate, and carried at cost less accumulated impairment losses. Gains and losses on the disposal of an entity or business include the carrying amount of goodwill relating to the entity or business sold. Other intangible assets are recognised if they arise from contractual or other legal rights or if they are capable of being separated or divided from Santander UK and sold, transferred, licensed, rented or exchanged. The value of such intangible assets is amortised on a straight-line basis over their useful economic life of three to seven years. Other intangible assets are reviewed annually for impairment indicators and tested for impairment where indicators are present. Software development costs are capitalised when they are direct costs associated with identifiable and unique software products that are expected to provide future economic benefits and the cost of those products can be measured reliably. These costs include payroll, materials, services and directly attributable overheads. Internally developed software meeting these criteria and externally purchased software are classified in intangible assets on the balance sheet and amortised on a straight-line basis over their useful life of three to seven years, unless the software is an integral part of the related computer hardware, in which case it is treated as property, plant and equipment as described below. Capitalisation of costs ceases when the software is capable of operating as intended. Costs of maintaining software are expensed as incurred.
Property, plant and equipment	Property, plant and equipment Property, plant and equipment include owner-occupied properties (including leasehold properties), office fixtures and equipment and computer software. Property, plant and equipment also includes operating leases where the Santander UK group is the lessor and right-of-use assets where the Santander UK group is the lessee, as described further in ‘Leases’ below. Property, plant and equipment are carried at cost less accumulated depreciation and accumulated impairment losses. A review for indications of impairment is carried out at each reporting date. Gains and losses on disposal are determined by reference to the carrying amount and are reported in other operating income. Repairs and renewals are charged to the income statement when the expenditure is incurred. Internally developed software meeting the criteria set out in ‘Goodwill and other intangible assets’ above and externally purchased software are classified in property, plant and equipment where the software is an integral part of the related computer hardware (for example operating system of a computer). Classes of property, plant and equipment are depreciated on a straight-line basis over their useful life, as follows: Owner-occupied properties Not exceeding 50 years Office fixtures and equipment 3 to 15 years Computer software 3 to 7 years Right-of-use assets (see ‘Leases – The Santander UK group as lessee’ below) Shorter of the lease term or the useful life of the underlying asset Depreciation is not charged on freehold land and assets under construction. Depreciation on operating lease assets where the Santander UK group is the lessor is described in 'Leases' below.
Financial instruments	Financial instruments a) Initial recognition and measurement Financial assets and liabilities are initially recognised when the Santander UK group becomes a party to the contractual terms of the instrument. The Santander UK group determines the classification of its financial assets and liabilities at initial recognition and measures a financial asset or financial liability at its fair value plus or minus, in the case of a financial asset or financial liability not at FVTPL, transaction costs that are incremental and directly attributable to the acquisition or issue of the financial asset or financial liability. Transaction costs of financial assets and financial liabilities carried at FVTPL are expensed in profit or loss. Immediately after initial recognition, an expected credit loss (ECL) allowance is recognised for financial assets measured at amortised cost and investments in debt instruments measured at FVOCI. A regular way purchase is a purchase of a financial asset under a contract whose terms require delivery of the asset within the timeframe established generally by regulation or convention in the market place concerned. Regular way purchases of financial assets classified as loans and receivables, issues of equity or financial liabilities measured at amortised cost are recognised on settlement date; all other regular way purchases and issues are recognised on trade date. b) Financial assets and liabilities i) Classification and subsequent measurement The Santander UK group classifies its financial assets in the measurement categories of amortised cost, FVOCI and FVTPL. Financial assets and financial liabilities are classified as FVTPL where there is a requirement to do so or where they are otherwise designated at FVTPL on initial recognition. Financial assets and financial liabilities which are required to be held at FVTPL include: – Financial assets and financial liabilities held for trading – Debt instruments that do not have solely payments of principal and interest (SPPI) characteristics. Otherwise, such instruments are measured at amortised cost or FVOCI, and – Equity instruments that have not been designated as held at FVOCI. Financial assets and financial liabilities are classified as held for trading if they are derivatives or if they are acquired or incurred principally for the purpose of selling or repurchasing in the near-term, or form part of a portfolio of financial instruments that are managed together and for which there is evidence of short-term profit taking. In certain circumstances, other financial assets and financial liabilities are designated at FVTPL where this results in more relevant information. This may arise because it significantly reduces a measurement inconsistency that would otherwise arise from measuring assets or liabilities or recognising the gains or losses on them on a different basis, where the assets and liabilities are managed and their performance evaluated on a fair value basis or, in the case of financial liabilities, where it contains one or more embedded derivatives which are not closely related to the host contract. The classification and measurement requirements for financial asset debt and equity instruments and financial liabilities are set out below. a) Financial assets: debt instruments Debt instruments are those instruments that meet the definition of a financial liability from the issuer’s perspective, such as loans and government and corporate bonds. Classification and subsequent measurement of debt instruments depend on the Santander UK group’s business model for managing the asset, and the cash flow characteristics of the asset. Business model The business model reflects how the Santander UK group manages the assets in order to generate cash flows and, specifically, whether the Santander UK group’s objective is solely to collect the contractual cash flows from the assets or is to collect both the contractual cash flows and cash flows arising from the sale of the assets. If neither of these is applicable, such as where the financial assets are held for trading purposes, then the financial assets are classified as part of an ‘other’ business model and measured at FVTPL. Factors considered in determining the business model for a group of assets include past experience on how the cash flows for these assets were collected, how the assets’ performance is evaluated and reported to key management personnel, and how risks are assessed and managed. SPPI Where the business model is to hold assets to collect contractual cash flows or to collect contractual cash flows and sell, the Santander UK group assesses whether the assets’ cash flows represent SPPI. In making this assessment, the Santander UK group considers whether the contractual cash flows are consistent with a basic lending arrangement (i.e. interest includes only consideration for the time value of money, credit risk, other basic lending risks and a profit margin that is consistent with a basic lending arrangement). Where the contractual terms introduce exposure to risk or volatility that is inconsistent with a basic lending arrangement, the related asset is classified and measured at FVTPL. Financial assets with embedded derivatives are considered in their entirety when determining whether their cash flows are SPPI. Based on these factors, the Santander UK group classifies its debt instruments into one of the following measurement categories: – Amortised cost – Financial assets that are held for collection of contractual cash flows where those cash flows represent SPPI, and that are not designated at FVTPL, are measured at amortised cost. The carrying amount of these assets is adjusted by any ECL recognised and measured as presented in Note 13. Interest income from these financial assets is included in ‘Interest and similar income’ using the effective interest rate method. When estimates of future cash flows are revised, the carrying amount of the respective financial assets or financial liabilities is adjusted to reflect the new estimate discounted using the original effective interest rate. Any changes are recognised in the income statement. – FVOCI – Financial assets that are held for collection of contractual cash flows and for selling the assets, where the assets’ cash flows represent SPPI, and that are not designated at FVTPL, are measured at FVOCI. Movements in the carrying amount are recognised in OCI, except for the recognition of impairment gains or losses, interest revenue and foreign exchange gains and losses on the instrument’s amortised cost which are recognised in profit or loss. When the financial asset is derecognised, the cumulative gain or loss previously recognised in OCI is reclassified from equity to profit or loss and recognised in ‘Other operating income’. Interest income from these financial assets is included in ‘Interest and similar income’ using the effective interest rate method. – FVTPL – Financial assets that do not meet the criteria for amortised cost or FVOCI are measured at FVTPL. A gain or loss on a debt instrument that is subsequently measured at FVTPL, including any debt instruments designated at fair value, is recognised in profit or loss and presented in the income statement in ‘Other operating income’ in the period in which it arises. The Santander UK group reclassifies financial assets when and only when its business model for managing those assets changes. The reclassification takes place from the start of the first reporting period following the change. Such changes are expected to be very infrequent. b) Financial assets: equity instruments Equity instruments are instruments that meet the definition of equity from the issuer’s perspective, being instruments that do not contain a contractual obligation to pay cash and that evidence a residual interest in the issuer’s net assets. All equity investments are subsequently measured at FVTPL, except where management has elected, at initial recognition, to irrevocably designate an equity investment at FVOCI. When this election is used, fair value gains and losses are recognised in OCI and are not subsequently reclassified to profit or loss, including on disposal. ECLs (and reversal of ECLs) are not reported separately from other changes in fair value. Dividends, when representing a return on such investments, continue to be recognised in profit or loss as other income when the right to receive payments is established. Gains and losses on equity investments at FVTPL are included in the ‘Other operating income’ line in the income statement. c) Financial liabilities Financial liabilities are classified as subsequently measured at amortised cost, except for: – Financial liabilities at fair value through profit or loss: this classification is applied to derivatives and other financial liabilities designated as such at initial recognition. Gains or losses on financial liabilities designated at fair value through profit or loss are presented partially in other comprehensive income (the amount of change in the fair value of the financial liability that is attributable to changes in the credit risk of that liability) and partially in profit or loss (the remaining amount of change in the fair value of the liability) – Financial liabilities arising from the transfer of financial assets which did not qualify for derecognition, whereby a financial liability is recognised for the consideration received for the transfer. In subsequent periods, the Santander UK group recognises any expense incurred on the financial liability, and – Financial guarantee contracts and loan commitments. Contracts involving the receipt of cash on which customers receive an index-linked return are accounted for as equity index-linked deposits. The principal products are Capital Guaranteed/Protected Products which give the customers a limited participation in the upside growth of an equity index. In the event the index falls in price, a cash principal element is guaranteed/protected. The equity index-linked deposits contain embedded derivatives. These embedded derivatives, in combination with the principal cash deposit element, are designed to replicate the investment performance profile tailored to the return agreed in the contracts with customers. The cash principal element is accounted for as deposits by customers at amortised cost. The embedded derivatives are separated from the host instrument and are separately accounted for as derivatives. d) Sale and repurchase agreements (including stock borrowing and lending) Securities sold subject to a commitment to repurchase them at a predetermined price (repos) under which substantially all the risks and rewards of ownership are retained by the Santander UK group remain on the balance sheet and a liability is recorded in respect of the consideration received. Securities purchased under commitments to resell (reverse repos) are not recognised on the balance sheet and the consideration paid is recorded as an asset. The difference between the sale and repurchase price is treated as trading income in the income statement, except where the repo is not treated as part of the trading book, in which case the difference is recorded in interest income or expense. Securities lending and borrowing transactions are generally secured, with collateral in the form of securities or cash advanced or received. Securities lent or borrowed are not reflected on the balance sheet. Collateral in the form of cash received or advanced is recorded as a deposit or a loan. Collateral in the form of securities is not recognised. e) Day One profit adjustments The fair value of a financial instrument on initial recognition is generally its transaction price (that is, the fair value of the consideration given or received). However, sometimes the fair value will be based on other observable current market transactions in the same instrument, without modification or repackaging, or on a valuation technique whose variables include only data from observable markets, such as interest rate yield curves, option volatilities and currency rates. When such evidence exists, the Santander UK group recognises a trading gain or loss at inception (Day One gain or loss), being the difference between the transaction price and the fair value. When significant unobservable parameters are used, the entire Day One gain or loss is deferred and is recognised in the income statement over the life of the transaction until the transaction matures, is closed out, the valuation inputs become observable or an offsetting transaction is entered into. ii) Impairment of debt instrument financial assets The Santander UK group assesses on a forward-looking basis the ECL associated with its debt instrument assets carried at amortised cost and FVOCI and with the exposure arising from financial guarantee contracts and loan commitments. The Santander UK group recognises a loss allowance for such losses at each reporting date. The measurement of ECL reflects: – An unbiased and probability-weighted amount that is determined by evaluating a range of possible outcomes – The time value of money, and – Reasonable and supportable information that is available without undue cost or effort at the reporting date about past events, current conditions and forecasts of future economic conditions. Grouping of instruments for losses measured on a collective basis We typically group instruments and assess them for impairment collectively where they share risk characteristics (as described in Retail Banking – credit risk management in the Risk review) using one or more statistical models. Where we have used internal capital or similar models as the basis for our ECL models, this typically results in a large number of relatively small homogenous groups which are determined by the permutations of the underlying characteristics in the statistical models. We calculate separate collective provisions for instruments in Stages 1, 2 and 3 where the instrument is not individually assessed, as described below. Individually assessed impairments (IAIs) We assess significant Stage 3 cases individually. We do this for CIB and Corporate & Commercial Banking cases, but not for Business Banking cases in Retail Banking which we assess collectively. To calculate the estimated loss, we estimate the future cash flows under several scenarios each of which uses case-specific factors and circumstances. We then probability-weight the net present value of the cash flows under each scenario to arrive at a weighted average provision requirement. We update our assessment process every quarter and more frequently if there are changes in circumstances that might affect the scenarios, cash flows or probabilities we apply. For more on how ECL is calculated, see the Credit risk section of the Risk review. a) Write-off For secured loans, a write-off is only made when all collection procedures have been exhausted and the security has been sold and/or a claim made on any mortgage indemnity guarantee or other insurance. In the corporate loan portfolio, there may be occasions where a write-off occurs for other reasons, such as following a consensual restructure or refinancing of the debt or where the debt is sold for strategic reasons into the secondary market at a value lower than its face value. There is no threshold based on past due status beyond which all secured loans are written off as there can be significant variations in the time needed to enforce possession and sale of the security, especially due to the different legal frameworks that apply in different regions of the UK. For unsecured loans, a write-off is only made when all internal avenues of collecting the debt have been exhausted. Where appropriate the debt is passed over to external collection agencies. A past due threshold is applied to unsecured debt where accounts that are 180 days past due are written off unless there is a dispute awaiting resolution. Contact is made with customers with the aim to achieve a realistic and sustainable repayment arrangement. Litigation and/or enforcement of security is usually carried out only when the steps described above have been undertaken without success. All write-offs are assessed / made on a case-by-case basis, taking account of the exposure at the date of write-off, after accounting for the value from any collateral or insurance held against the loan. The exception to this is in cases where fraud has occurred, where the exposure is written off once investigations have been completed and the probability of recovery is minimal. The time span between discovery and write-off will be short and may not result in an impairment loss allowance being raised. The write-off policy is regularly reviewed. Write-offs are charged against previously established loss allowances. b) Recoveries Recoveries of credit impairment losses are not included in the impairment loss allowance, but are taken to income and offset against credit impairment losses. Recoveries of credit impairment losses are classified in the income statement as ‘Credit impairment losses’. iii) Modifications of financial assets The treatment of a renegotiation or modification of the contractual cash flows of a financial asset normally depends upon whether the renegotiation or modification is due to financial difficulties of the borrower or for other commercial reasons. – Contractual modifications due to financial difficulties of the borrower: where the Santander UK group modifies the contractual conditions to enable the borrower to fulfil their payment obligations, the asset is not derecognised. The gross carrying amount of the financial asset is recalculated as the present value of the renegotiated/modified contractual cash flows that are discounted at the financial asset’s original EIR and any gain or loss arising from the modification is recognised in the income statement. – Contractual modifications for other commercial reasons: an assessment is performed to determine whether the terms of the new agreement are substantially different from the terms of the existing agreement, after considering changes in the cash flows arising from the modified terms and the overall instrument risk profile. Where terms are substantially different, such modifications are treated as a new transaction resulting in derecognition of the original financial asset, and the recognition of a ‘new’ financial asset with any difference between the carrying amount of the derecognised asset and the fair value of the new asset is recognised in the income statement as a gain or loss on derecognition. Where terms are not substantially different, the carrying value of the financial asset is adjusted to reflect the present value of modified cash flows discounted at the original EIR with any gain or loss arising from modification recognised immediately in the income statement. Any other contractual modifications, such as where a regulatory authority imposes a change in certain contractual terms or due to legal reasons, are assessed on a case-by-case basis to establish whether or not the financial asset should be derecognised. iv) Derecognition other than on a modification Financial assets are derecognised when the rights to receive cash flows have expired or the Santander UK group has transferred its contractual right to receive the cash flows from the assets and either: (1) substantially all the risks and rewards of ownership have been transferred; or (2) the Santander UK group has neither retained nor transferred substantially all of the risks and rewards, but has transferred control. Financial liabilities are derecognised when extinguished, cancelled or expired. c) Financial guarantee contracts and loan commitments Financial guarantee contracts are contracts that require the issuer to make specified payments to reimburse the holder for a loss it incurs because a specified debtor fails to make payments when due, in accordance with the terms of a debt instrument. Such financial guarantees are given to banks, financial institutions and others on behalf of customers to secure loans, overdrafts and other banking facilities. Financial guarantee contracts are initially measured at fair value and subsequently measured at the higher of the amount of the loss allowance, and the premium received on initial recognition less income recognised in accordance with the principles of IFRS 15. Loan commitments are measured as the amount of the loss allowance (determined in accordance with IFRS 9 as described in Credit risk section of the Risk review). The Santander UK group has not provided any commitment to provide loans at a below-market interest rate, or that can be settled net in cash or by delivering or issuing another financial instrument.
Derivative financial instruments	Derivative financial instruments (derivatives) Derivatives are contracts or agreements whose value is derived from one or more underlying indices or asset values inherent in the contract or agreement, which require no or little initial net investment and are settled at a future date. Transactions are undertaken in interest rate, cross currency, equity, residential property and other index-related swaps, forwards, caps, floors, swaptions, as well as credit default and total return swaps, equity index contracts and exchange traded interest rate futures, and equity index options. Derivatives are held for risk management purposes. Derivatives are classified as held for trading unless they are designated as being in a hedge accounting relationship. The Santander UK group chooses to designate certain derivatives as in a hedging relationship if they meet specific criteria, as further described in ‘Hedge accounting’ below. Derivatives are recognised initially (on the date on which a derivative contract is entered into), and are subsequently remeasured, at their fair value. Fair values of exchange-traded derivatives are obtained from quoted market prices. Fair values of over-the-counter derivatives are estimated using valuation techniques, including discounted cash flow and option pricing models. Certain derivatives may be embedded in hybrid contracts, such as the conversion option in a convertible bond. If the hybrid contract contains a host that is a financial asset, then the Santander UK group assesses the entire contract as described in the financial asset section above for classification and measurement purposes. Otherwise, embedded derivatives are treated as separate derivatives when their economic characteristics and risks are not closely related to those of the host contract; the terms of the embedded derivative would meet the definition of a stand-alone derivative if they were contained in a separate contract; and the combined contract is not held for trading or designated at fair value. These embedded derivatives are measured at fair value with changes in fair value recognised in the income statement. Contracts containing embedded derivatives are not subsequently reassessed for separation unless either there has been a change in the terms of the contract which significantly modifies the cash flows (in which case the contract is reassessed at the time of modification) or the contract has been reclassified (in which case the contract is reassessed at the time of reclassification). All derivatives are carried as assets when their fair value is positive and as liabilities when their fair value is negative, except where netting is permitted. The method of recognising fair value gains and losses depends on whether derivatives are held for trading or are designated as hedging instruments and, if the latter, the nature of the risks being hedged. Gains and losses from changes in the fair value of derivatives held for trading are recognised in the income statement, and included within Other operating income.
Offsetting financial assets and liabilities	Offsetting financial assets and liabilitiesFinancial assets and liabilities including derivatives are offset and the net amount reported in the balance sheet when there is a legally enforceable right to set off the recognised amounts and there is an intention to settle on a net basis, or realise the asset and settle the liability simultaneously. The Santander UK group is party to a number of arrangements, including master netting arrangements under industry standard agreements which facilitate netting of transactions in jurisdictions where netting agreements are recognised and have legal force. The netting arrangements do not generally result in an offset of balance sheet assets and liabilities for accounting purposes, as transactions are usually settled on a gross basis.
Hedge accounting	Hedge accounting The Santander UK group applies hedge accounting to represent, to the maximum possible extent permitted under accounting standards, the economic effects of its risk management strategies. Derivatives are used to hedge exposures to interest rates, exchange rates and certain indices such as retail price indices. At the time a financial instrument is designated as a hedge (i.e. at the inception of the hedge), the Santander UK group formally documents the relationship between the hedging instrument(s) and hedged item(s), its risk management objective and strategy for undertaking the hedge. The documentation includes the identification of each hedging instrument and respective hedged item, the nature of the risk being hedged (including the benchmark interest rate being hedged in a hedge of interest rate risk) and how the hedging instrument’s effectiveness in offsetting the exposure to changes in the hedged item’s fair value attributable to the hedged risk is to be assessed. Accordingly, the Santander UK group formally assesses, both at the inception of the hedge and on an ongoing basis, whether the hedging derivatives have been and will be highly effective in offsetting changes in the fair value attributable to the hedged risk during the period that the hedge is designated. A hedge is normally regarded as highly effective if, at inception and throughout its life, the Santander UK group can expect, and actual results indicate, that changes in the fair value or cash flow of the hedged items are effectively offset by changes in the fair value or cash flow of the hedging instrument. If at any point it is concluded that it is no longer highly effective in achieving its documented objective, hedge accounting is discontinued. Where derivatives are held for risk management purposes, and when transactions meet the required criteria for documentation and hedge effectiveness, the derivatives may be designated as either: (i) hedges of the change in fair value of recognised assets or liabilities or firm commitments (fair value hedges); (ii) hedges of the variability in highly probable future cash flows attributable to a recognised asset or liability, or a forecast transaction (cash flow hedges); or (iii) a hedge of a net investment in a foreign operation (net investment hedges). The Santander UK group applies fair value and cash flow hedge accounting, but not hedging of a net investment in a foreign operation. a) Fair value hedge accounting Changes in the fair value of derivatives that are designated and qualify as fair value hedges are recorded in the income statement, together with the changes in the fair value of the hedged asset or liability that are attributable to the hedged risk. Where the hedged item is measured at amortised cost, the fair value changes due to the hedged risk adjust the carrying amount of the hedged asset or liability. Changes in the fair value of portfolio hedged items are presented separately in the consolidated balance sheet in macro hedge of interest rate risk and recognised in the income statement within other operating income. If the hedge no longer meets the criteria for hedge accounting, changes in the fair value of the hedged item attributable to the hedged risk are no longer recognised in the income statement. For fair value hedges of interest rate risk, the cumulative adjustment that has been made to the carrying amount of the hedged item is amortised to the income statement using the effective interest method over the period to maturity. For portfolio hedged items, the cumulative adjustment is amortised to the income statement using the straight line method over the period to maturity. b) Cash flow hedge accounting The effective portion of changes in the fair value of qualifying cash flow hedges is recognised in other comprehensive income in the cash flow hedging reserve. The gain or loss relating to the ineffective portion is recognised immediately in the income statement. Amounts accumulated in equity are reclassified to the income statement in the periods in which the hedged item affects profit or loss. When a hedging instrument expires or is sold, or when a hedge no longer meets the criteria for hedge accounting, any cumulative gain or loss existing in equity at that time remains in equity and is recognised in the income statement when the forecast transaction is ultimately recognised in the income statement. When a forecast transaction is no longer expected to occur, the cumulative gain or loss that was reported in equity is immediately transferred to the income statement. The Santander UK group is exposed to cash flow interest rate risk on its floating rate assets, foreign currency risk on its fixed rate debt issuances denominated in foreign currency and equity price risk arises from the Santander UK group operating the Employee Sharesave scheme. Cash flow hedging is used to hedge the variability in cash flows arising from these risks.
Securitisation transactions	Securitisation transactions The Santander UK group has entered into arrangements where undertakings have issued mortgage-backed and other asset-backed securities or have entered into funding arrangements with lenders in order to finance specific loans and advances to customers. The Santander UK group has also entered into synthetic securitisation arrangements, as part of significant risk transfer (SRT) transactions to reduce its risk-weighted assets, where undertakings have issued credit-linked notes and deposited the funds raised as collateral for credit protection in respect of specific loans and advances to customers. As the Santander UK group has retained substantially all the risks and rewards of the underlying assets, such financial instruments continue to be recognised on the balance sheet, and a liability recognised for the proceeds of the funding transaction, or in the case of SRT transactions, collateral deposited.
Impairment of non-financial assets	Impairment of non-financial assets At each balance sheet date, or more frequently when events or changes in circumstances dictate, property plant and equipment (including operating lease assets) and intangible assets (including goodwill) are assessed for indicators of impairment. If indications are present, these assets are subject to an impairment review. The impairment review comprises a comparison of the carrying amount of the asset or cash generating unit with its recoverable amount: the higher of the asset’s or cash-generating unit’s fair value less costs to sell and its value in use. The cash-generating unit represents the lowest level at which non-financial assets, including goodwill, are monitored for internal management purposes and is not larger than an operating segment. The fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. Value in use is calculated by discounting management’s expected future cash flows obtainable as a result of the asset’s continued use (after making allowance for increases in regulatory capital requirements), including those resulting from its ultimate disposal, at a market-based discount rate on a pre-tax basis. The recoverable amounts of goodwill have been based on value in use calculations. The carrying values of property, plant and equipment, goodwill and other intangible assets are written down by the amount of any impairment and the loss is recognised in the income statement in the period in which it occurs. A previously recognised impairment loss relating to property, plant and equipment may be reversed in part or in full when a change in circumstances leads to a change in the estimates used to determine the property, plant and equipment’s recoverable amount. The carrying amount of the property, plant and equipment will only be increased up to the amount that would have been had the original impairment not been recognised. Impairment losses on goodwill are not reversed. For conducting goodwill impairment reviews, cash generating units are the lowest level at which management monitors the return on investment on assets.
Leases	Leases a) The Santander UK group as lessor Operating lease assets are recorded at cost and depreciated over the life of the asset after taking into account anticipated residual value (RV). Operating lease rental income and depreciation is recognised on a straight-line basis over the life of the asset. After initial recognition, residual values are reviewed regularly, and any changes are recognised prospectively through remaining depreciation charges. Amounts due from lessees under finance leases and hire purchase contracts are recorded as receivables at the amount of the Santander UK group’s net investment in the leases. Finance lease income is allocated to accounting periods to reflect a constant periodic rate of return on the Santander UK group’s net investment outstanding in respect of the leases and hire purchase contracts. A provision is recognised to reflect a reduction in any anticipated unguaranteed RV. A provision is also recognised for voluntary termination of the contract by the customer, where appropriate. b) The Santander UK group as lessee The Santander UK group assesses whether a contract is or contains a lease at the inception of the contract and recognises a right-of-use (ROU) asset representing its right to use the underlying leased asset and a lease liability representing its obligation to make lease payments for all leases, except for leases with a term of 12 months or less which are expensed in the income statement on a straight-line basis over the lease terms. Lease payments exclude irrecoverable VAT which is expensed in the income statement as lease payments are made. The lease liability, which is included in Other liabilities on the balance sheet, is initially measured at the present value of the lease payments that are not paid at the commencement date, discounted using the incremental borrowing rate appropriate to the lease term. The lease liability is subsequently measured at amortised cost using the effective interest rate method. Remeasurement of the lease liability occurs if there is a change in the lease payments (when a corresponding adjustment is made to the ROU asset), the lease term or in the assessment of an option to purchase the underlying asset. At inception, the ROU asset, which is included in Property, plant and equipment on the balance sheet, comprises the lease liability, initial direct costs and the obligations to restore the asset, less any incentives granted by the lessor. The ROU asset is depreciated over the shorter of the lease term or the useful life of the underlying asset and is reviewed for impairment as for owned assets. The obligation to restore the asset is included in Provisions on the balance sheet.
Income taxes, including deferred taxes	Income taxes, including deferred taxes The tax expense represents the sum of the income tax currently payable and deferred income tax. Income tax payable on profits, based on the applicable tax law in each jurisdiction, is recognised as an expense in the period in which profits arise. Taxable profit differs from net profit as reported in the income statement because it excludes items of income or expense that are taxable or deductible in other years and it further excludes items that are never taxable or deductible. The liability for current tax is calculated using tax rates that have been enacted or substantively enacted by the balance sheet date. Current taxes associated with the repurchase of equity instruments are reported directly in equity. A current tax liability for the current or prior period is measured at the amount expected to be paid to the tax authorities. Where the amount of the final tax liability is uncertain or where a position is challenged by a taxation authority, the liability recognised is the most likely outcome. Where a most likely outcome cannot be determined, a weighted average basis is applied. Deferred income tax is the tax expected to be payable or recoverable on income tax losses available to carry forward and on temporary differences arising between the tax bases of assets and liabilities and their carrying amounts in the Consolidated Financial Statements and is accounted for using the balance sheet liability method. Deferred tax liabilities are generally recognised for all taxable temporary differences and deferred tax assets are recognised to the extent that it is probable that taxable profits will be available against which the assets may be utilised as they reverse. Such deferred tax liabilities are not recognised if the temporary difference arises from the initial recognition of goodwill. Deferred tax assets and liabilities are not recognised from the initial recognition of other assets (other than in a business combination) and liabilities in a transaction that affects neither the taxable profit nor the accounting profit. Deferred tax is calculated at the tax rates that are expected to apply in the period when the liability is settled or the asset is realised based on rates enacted or substantively enacted at the balance sheet date. Deferred tax is charged or credited in the income statement, except when it relates to items recognised in other comprehensive income or directly in equity, in which case the deferred tax is also recognised in other comprehensive income or directly in equity. Deferred tax liabilities are recognised for taxable temporary differences arising on investments in subsidiaries except where the Santander UK group is able to control reversal of the temporary difference and it is probable that it will not reverse in the foreseeable future. The Santander UK group reviews the carrying amount of deferred tax assets at each balance sheet date and reduces it to the extent that it is no longer probable that sufficient taxable profits will be available to allow all or part of the asset to be recovered. Deferred tax relating to actuarial gains and losses on defined benefits is recognised in other comprehensive income. Deferred tax relating to fair value re-measurements of financial instruments accounted for at FVOCI and cash flow hedging instruments is charged or credited directly to other comprehensive income and is subsequently recognised in the income statement when the deferred fair value gain or loss is recognised in the income statement. Deferred and current tax assets and liabilities are only offset when they arise in the same tax reporting group and where there is both the legal right and the intention to settle on a net basis or to realise the asset and settle the liability simultaneously.
Cash and cash equivalents	Cash and cash equivalentsFor the purposes of the cash flow statement, cash and cash equivalents comprise balances with less than three months maturity from the date of acquisition, including cash and non-restricted balances with central banks, treasury bills and other eligible bills, loans and advances to banks and short-term investments in securities. Balances with central banks represent amounts held at the Bank of England as part of the Santander UK group’s liquidity management activities. In addition, it includes certain minimum cash balances held for regulatory purposes required to be maintained with the Bank of England.
Provisions	Provisions Provisions are recognised for present obligations arising as consequences of past events where it is more likely than not that a transfer of economic benefits will be necessary to settle the obligation, and it can be reliably estimated. Conduct provisions are made for the estimated cost of making redress payments with respect to the past sales of products, using conclusions such as the number of claims the number of those that will be upheld, the estimated average settlement per case and other related costs. Provision is made for the anticipated cost of restructuring, including redundancy costs, when an obligation exists. An obligation exists when the Santander UK group has a detailed formal plan for restructuring a business, has raised valid expectations in those affected by the restructuring, and has started to implement the plan or announce its main features. When a leasehold property ceases to be used in the business, provision is made where the unavoidable costs of the future obligations relating to the lease are expected to exceed anticipated rental income. The net costs are discounted using market rates of interest to reflect the long-term nature of the cash flows. Loan commitments are measured as the amount of the loss allowance, determined in line with IFRS 9 as set out in the Credit risk section of the Risk review. Contingent liabilities are possible obligations whose existence will be confirmed only by certain future events or present obligations where the transfer of economic benefit is uncertain or cannot be reliably measured. Contingent liabilities are not recognised but are disclosed unless they are remote.
Share capital	Share capital a) Share issue costs Incremental external costs directly attributable to the issue of new shares are deducted from equity net of related income taxes. b) Dividends Dividends on ordinary shares are recognised in equity in the period in which the right to receive payment is established.
Critical judgements and accounting estimates	CRITICAL JUDGEMENTS AND ACCOUNTING ESTIMATES The preparation of the Consolidated Financial Statements requires management to make judgements and accounting estimates that affect the reported amount of assets and liabilities at the date of the Consolidated Financial Statements and the reported amount of income and expenses during the reporting period. Management evaluates its judgements and accounting estimates, which are based on historical experience and on various other factors that are believed to be reasonable under the circumstances, on an ongoing basis. Actual results may differ from these accounting estimates under different assumptions or conditions. In preparing the Consolidated Financial Statements, no significant judgements have been made in the process of applying the accounting policies, other than those involving estimations about credit impairment losses, provisions and contingent liabilities, pensions and goodwill. a) Credit impairment allowance The application of the ECL impairment methodology for calculating credit impairment allowances is highly susceptible to change from period to period. The methodology requires management to make judgmental assumptions in determining the estimates. Any significant difference between the estimated amounts and actual amounts could have a material impact on the future financial results and financial condition. The impact of Covid-19 has increased the uncertainty around ECL impairment calculations, and has required management to make additional judgements and accounting estimates that affect the amount of assets and liabilities at the reporting date and the amount of income and expenses in the reporting period. The key additional judgements due to the impact of Covid-19 mainly reflect the increased uncertainty around forward-looking economic data and the need for additional post model adjustments. Key areas of judgement in accounting estimates The key judgements made by management in applying the ECL impairment methodology are the definition of default, forward-looking economic scenarios, probability weights, SICR thresholds, post model adjustments, internal credit risk rating for corporate borrowers and individually assessed corporate Stage 3 exposures. For more on each of these key judgements, including the impact of Covid-19 on them, see 'Management judgement applied in calculating ECL' in the ‘Credit risk – Santander UK group level – credit risk management’ section of the Risk review. Sensitivity of ECL allowance For detailed disclosures, see 'Sensitivity of ECL allowance' in the ‘Credit risk – Santander UK group level – credit risk management’ section of the Risk review. b) Provisions and contingent liabilities Significant judgment may be required when accounting for provisions, including in determining whether a present obligation exists and in estimating the probability and amount of any outflows. These judgments are based on the specific facts available and often require specialist professional advice. There can be a wide range of possible outcomes and uncertainties, particularly in relation to legal actions, and regulatory and consumer credit matters. As a result it is often not possible to make reliable estimates of the likelihood and amount of any potential outflows. The main areas of judgement relating to provisions and contingent liabilities are set out below. For more details, see Notes 29 and 31. Included in Regulatory and other provisions in Note 29 is an amount in respect of management’s best estimate of liability relating to a legal dispute regarding allocation of responsibility for a specific PPI portfolio of complaints. Note 31 provides disclosure relating to ongoing factual issues and reviews that could impact the timing and amount of any outflows. In addition, Note 31 includes disclosure relating to an investigation in relation to the historical involvement of Santander UK plc, Santander Financial Services plc and Cater Allen International Limited (all subsidiaries of Santander UK Group Holdings plc) in German dividend tax arbitrage transactions, as well as an FCA civil regulatory investigation which commenced in July 2017 into our compliance with the Money Laundering Regulations 2007 and potential breaches of FCA principles and rules relating to anti-money laundering and financial crime systems and controls. It also includes disclosure relating to certain leases in which current and former Santander UK group members were the lessor that are currently under review by HMRC in connection with claims for tax allowances. c) Pensions The Santander UK group operates a number of defined benefit pension schemes as described in Note 30 and estimates their position as described in the accounting policy ‘Pensions and other post retirement benefits ’ . Key areas of judgement in accounting estimates Accounting for defined benefit pension schemes requires management to make assumptions principally about the discount rate adopted, but also about price inflation and life expectancy. Management’s assumptions are based on past experience and current economic trends, which are not necessarily an indication of future experience. These are described in more detail in the ‘Actuarial assumptions’ section in Note 30 . Sensitivity of defined benefit pension scheme estimates Had management used different assumptions, a larger or smaller pension remeasurement gain or loss would have resulted that could have had a material impact on the Santander UK group’s reported financial position. Detailed disclosures on the actuarial assumption sensitivities of the schemes can be found in the ‘Actuarial assumption sensitivities’ section of Note 30. The carrying amount of goodwill is based on the application of judgements including the basis of goodwill impairment calculation assumptions. Santander UK undertakes an annual assessment to evaluate whether the carrying value of goodwill is impaired, carrying out this assessment more frequently if reviews identify indicators of impairment or when events or changes in circumstances dictate. Estimates include the determination of the carrying value of the Personal Financial Services Cash Generating Unit based on an allocation of regulatory capital, forecasts used for determining cash flows for Cash Generating Units and discount rates which factor in risk-free rates and applicable risk premiums, which are variables subject to fluctuations in external market rates and economic conditions beyond management’s control. The estimation of future cash flows and the level to which they are discounted is inherently uncertain and requires significant judgement and is subject to potential change over time. For more on these assumptions, including changes in the assumptions that would trigger an impairment, see Note 20.
Profit Before Tax Resegmentation Of Short Term Markets Business	The segmental basis of presentation in this Annual Report has changed following a management review of our structure. As a result, customer assets of £2.0bn and customer deposits of £3.1bn have been transferred from Business Banking (in Retail Banking) to CCB, non-core corporate mortgages of £0.4bn have been transferred from Corporate Centre to CCB, and a number of smaller business lines have been transferred from CIB to Corporate Centre. This resulted in an increase in profit before tax in Retail Banking of £18m (2019: decrease of £36m), a decrease in CCB of £9m (2019: increase of £24m), a decrease in CIB of £10m (2019: decrease of £1m), and an increase of £1m in Corporate Centre (2019: increase of £13m). The net impact for Santander UK was nil.The segmental data below is presented in a manner consistent with the internal reporting to the committee which is responsible for allocating resources and assessing performance of the segments and has been identified as the chief operating decision maker. The segmental data is prepared on a statutory basis of accounting, in line with the accounting policies set out in Note 1. Transactions between segments are on normal commercial terms and conditions. Internal charges and internal UK transfer pricing adjustments are reflected in the results of each segment. Revenue sharing agreements are used to allocate external customer revenues to a segment on a reasonable basis. Funds are ordinarily reallocated between segments, resulting in funding cost transfers disclosed in operating income. Interest charged for these funds is based on Santander UK’s cost of wholesale funding. Interest income and interest expense have not been reported separately. The majority of segment revenues are interest income in nature and net interest income is relied on primarily to assess segment performance and to make decisions on the allocation of segment resources.

Credit risk (Tables)

Credit risk (Tables)	12 Months Ended
	Dec. 31, 2020
Statement [Line Items]
Disclosure Of Base Expected Credit Losses to Probability weighted Estimated Credit Losses	Weighted Upside 1 Base case Downside 1 Downside 2 Downside 3 2020 £m £m £m £m £m £m Exposure 322,745 322,745 322,745 322,745 322,745 322,745 Retail Banking 210,251 210,251 210,251 210,251 210,251 210,251 – of which mortgages 180,006 180,006 180,006 180,006 180,006 180,006 CCB 24,503 24,503 24,503 24,503 24,503 24,503 CIB 11,646 11,646 11,646 11,646 11,646 11,646 Corporate Centre 76,345 76,345 76,345 76,345 76,345 76,345 ECL 1,377 1,129 1,222 1,300 1,613 1,802 Retail Banking 706 610 587 660 850 863 – of which mortgages 280 212 207 253 389 415 CCB 603 485 575 567 671 824 CIB 33 5 26 40 53 66 Corporate Centre 35 29 34 33 39 49 % % % % % % Proportion of assets in Stage 2 5.3 4.6 4.7 4.6 6.6 6.8 Retail Banking 5.4 4.6 4.6 4.7 7.2 7.0 – of which mortgages 5.7 4.8 4.9 4.9 7.7 7.5 CCB 22.4 20.1 20.8 20.2 24.5 28.6 CIB 1.7 1.7 1.7 1.7 1.7 1.7 Corporate Centre — — — — — — Weighted Upside 2 Upside 1 Base Case Downside 1 Downside 2 2019 £m £m £m £m £m £m Exposure 310,155 310,155 310,155 310,155 310,155 310,155 Retail Banking 204,780 204,780 204,780 204,780 204,780 204,780 – of which mortgages 178,788 178,788 178,788 178,788 178,788 178,788 CCB 24,118 24,118 24,118 24,118 24,118 24,118 CIB 13,384 13,384 13,384 13,384 13,384 13,384 Corporate Centre 67,873 67,873 67,873 67,873 67,873 67,873 ECL 863 640 680 726 855 1,542 Retail Banking 549 425 433 448 525 1,084 – of which mortgages 218 122 127 137 196 660 CCB 262 195 212 229 275 398 CIB 50 19 34 48 53 58 Corporate Centre 2 1 1 1 2 2 % % % % % % Proportion of assets in Stage 2 3.8 2.8 2.8 2.8 3.2 6.8 Retail Banking 4.5 3.2 3.3 3.3 3.7 8.3 – of which mortgages 4.6 3.1 3.1 3.1 3.6 8.7 CCB 8.7 7.4 7.4 7.4 8.5 16.3 CIB 1.5 1.5 1.5 1.5 1.5 1.5 Corporate Centre 0.1 0.1 0.1 0.1 0.1 0.3
Schedule Of Impact On Profit Before Tax Of Applying An Immediate And Permanent House Price Increase Or Decrease To Our Base Case Economic Scenario Explanatory	The table below shows the impact on profit before tax of applying an immediate and permanent house price increase / decrease to our unweighted base case economic scenario, and assumes no changes to the staging allocation of exposures. In 2019, we applied 40% weighting to the base case economic scenario, therefore a change in the basis of preparation in 2020. 2019 numbers have been restated. Increase/decrease in house prices +20% +10% -10% -20% Increase/(decrease) in profit before tax £m £m £m £m 2020 63 38 (66) (183) 2019 41 25 (39) (105)
Summary of Credit Rating of Financial Assets Subject to Credit Risk	Santander UK risk grade Loss allowance Total 9 8 7 6 5 4 3 to 1 Other (1) 2020 £bn £bn £bn £bn £bn £bn £bn £bn £bn £bn Exposures On balance sheet Cash and balances at central banks 41.3 — — — — — — — — 41.3 – Stage 1 41.3 — — — — — — — — 41.3 Financial assets at amortised cost: – Loans and advances to customers⁽²⁾ 8.0 27.7 74.4 44.1 13.5 27.0 8.2 7.2 (1.3) 208.8 – Stage 1 8.0 27.6 74.0 42.6 10.8 19.0 1.1 7.0 (0.2) 189.9 – Stage 2 — 0.1 0.4 1.5 2.7 8.0 4.2 0.2 (0.5) 16.6 – Stage 3 — — — — — — 2.9 — (0.6) 2.3 Of which mortgages: 7.9 24.3 68.0 36.8 5.6 19.6 4.5 — (0.3) 166.4 – Stage 1 7.9 24.3 67.7 35.5 3.9 14.7 0.6 — — 154.6 – Stage 2 — — 0.3 1.3 1.7 4.9 2.1 — (0.2) 10.1 – Stage 3 — — — — — — 1.8 — (0.1) 1.7 – Loans and advances to banks 0.1 0.1 0.4 — — — — 1.1 — 1.7 – Stage 1 0.1 0.1 0.4 — — — — 1.1 — 1.7 – Reverse repo agreements – non trading 12.2 3.3 1.5 2.4 — — — 0.2 — 19.6 – Stage 1 12.2 3.3 1.5 2.4 — — — 0.2 — 19.6 – Other financial assets at amortised cost 1.2 — — — — — — — — 1.2 – Stage 1 1.2 — — — — — — — — 1.2 Total financial assets at amortised cost 21.5 31.1 76.3 46.5 13.5 27.0 8.2 8.5 (1.3) 231.3 Financial assets at FVOCI: 5.3 3.4 0.2 0.1 — — — — — 9.0 – Stage 1 5.3 3.4 0.2 0.1 — — — — — 9.0 Total on balance sheet 68.1 34.5 76.5 46.6 13.5 27.0 8.2 8.5 (1.3) 281.6 Total off–balance sheet 0.4 8.8 9.5 8.8 5.1 1.6 0.5 8.5 (0.1) 43.1 – Stage 1 0.4 8.8 9.5 8.6 4.7 1.1 0.2 8.5 — 41.8 – Stage 2 — — — 0.2 0.4 0.5 0.2 — (0.1) 1.2 – Stage 3 — — — — — — 0.1 — — 0.1 Total exposures 68.5 43.3 86.0 55.4 18.6 28.6 8.7 17.0 (1.4) 324.7 ECL On balance sheet Cash and balances at central banks — — — — — — — — — – Stage 1 — — — — — — — — — Financial assets at amortised cost: – Loans and advances to customers⁽²⁾ — — — 0.1 0.1 0.2 0.9 — 1.3 – Stage 1 — — — 0.1 — — 0.1 — 0.2 – Stage 2 — — — — 0.1 0.2 0.2 — 0.5 – Stage 3 — — — — — — 0.6 — 0.6 Of which mortgages: — — — — — 0.1 0.2 — 0.3 – Stage 1 — — — — — — — — — – Stage 2 — — — — — 0.1 0.1 — 0.2 – Stage 3 — — — — — — 0.1 — 0.1 – Loans and advances to banks — — — — — — — — — – Stage 1 — — — — — — — — — – Reverse repo agreements – non trading — — — — — — — — — – Stage 1 — — — — — — — — — – Other financial assets at amortised cost — — — — — — — — — – Stage 1 — — — — — — — — — Total financial assets at amortised cost — — — 0.1 0.1 0.2 0.9 — 1.3 Financial assets at FVOCI: — — — — — — — — — – Stage 1 — — — — — — — — — Total on balance sheet — — — 0.1 0.1 0.2 0.9 — 1.3 Total off–balance sheet — — — — — — 0.1 — 0.1 – Stage 1 — — — — — — — — — – Stage 2 — — — — — — 0.1 — 0.1 – Stage 3 — — — — — — — — — Total ECL — — — 0.1 0.1 0.2 1.0 — 1.4 Santander UK risk grade Total 9 8 7 6 5 4 3 to 1 Other (1) 2020 % % % % % % % % % Coverage ratio On balance sheet Cash and balances at central banks — — — — — — — — — – Stage 1 — — — — — — — — — Financial assets at amortised cost: – Loans and advances to customers⁽²⁾ — — — 0.2 0.7 0.7 11.0 — 0.6 – Stage 1 — — — 0.2 — — 9.1 — 0.1 – Stage 2 — — — — 3.7 2.5 4.8 — 3.0 – Stage 3 — — — — — — 20.7 — 26.1 Of which mortgages: — — — — — 0.5 4.4 — 0.2 – Stage 1 — — — — — — — — — – Stage 2 — — — — — 2.0 4.8 — 2.0 – Stage 3 — — — — — — 5.6 — 5.9 – Loans and advances to banks — — — — — — — — — – Stage 1 — — — — — — — — — – Reverse repo agreements – non trading — — — — — — — — — – Stage 1 — — — — — — — — — – Other financial assets at amortised cost — — — — — — — — — – Stage 1 — — — — — — — — — Total financial assets at amortised cost — — — 0.2 0.7 0.7 11.0 — 0.6 Financial assets at FVOCI: — — — — — — — — — – Stage 1 — — — — — — — — — Total on balance sheet — — — 0.2 0.7 0.7 11.0 — 0.5 Total off–balance sheet — — — — — — 20.0 — 0.2 – Stage 1 — — — — — — — — — – Stage 2 — — — — — — 50.0 — 8.3 – Stage 3 — — — — — — — — — Total coverage ratio — — — 0.2 0.5 0.7 11.5 — 0.4 Santander UK risk grade Loss allowance 9 8 7 6 5 4 3 to 1 Other (3) Total 2019 £bn £bn £bn £bn £bn £bn £bn £bn £bn £bn Exposures On balance sheet Cash and balances at central banks 21.2 — — — — — — — — 21.2 – Stage 1 21.2 — — — — — — — — 21.2 Financial assets at amortised cost: – Loans and advances to customers⁽²⁾ 11.4 30.6 75.4 52.1 18.8 10.9 6.2 2.8 (0.8) 207.4 – Stage 1 11.4 30.6 75.0 50.9 16.1 6.2 1.2 2.8 (0.1) 194.1 – Stage 2 — — 0.4 1.2 2.7 4.7 2.7 — (0.3) 11.4 – Stage 3 — — — — — — 2.3 — (0.4) 1.9 Of which mortgages: 9.8 25.0 71.9 42.9 7.7 4.2 3.9 — (0.2) 165.2 – Stage 1 9.8 25.0 71.7 42.0 5.7 1.1 0.2 — — 155.5 – Stage 2 — — 0.2 0.9 2.0 3.1 2.0 — (0.1) 8.1 – Stage 3 — — — — — — 1.7 — (0.1) 1.6 – Loans and advances to banks — 0.2 0.4 — — — — 1.3 — 1.9 – Stage 1 — 0.2 0.4 — — — — 1.3 — 1.9 – Reverse repo agreements – non trading 15.3 2.4 4.2 1.5 — — — 0.2 — 23.6 – Stage 1 15.3 2.4 4.2 1.5 — — — 0.2 — 23.6 – Other financial assets at amortised cost 7.1 — — — — — — — — 7.1 – Stage 1 7.1 — — — — — — — — 7.1 Total financial assets at amortised cost 33.8 33.2 80.0 53.6 18.8 10.9 6.2 4.3 (0.8) 240.0 Financial assets at FVOCI: 6.1 3.2 0.4 — — — — — — 9.7 – Stage 1 6.1 3.2 0.4 — — — — — — 9.7 Total on balance sheet 61.1 36.4 80.4 53.6 18.8 10.9 6.2 4.3 (0.8) 270.9 Total off–balance sheet 0.9 8.5 8.4 8.1 5.0 1.1 0.5 9.1 (0.1) 41.5 – Stage 1 0.9 8.5 8.4 8.0 4.7 1.0 0.3 9.1 — 40.9 – Stage 2 — — — 0.1 0.3 0.1 0.2 — (0.1) 0.6 – Stage 3 — — — — — — — — — — Total exposures 62.0 44.9 88.8 61.7 23.8 12.0 6.7 13.4 (0.9) 312.4 Santander UK risk grade 9 8 7 6 5 4 3 to 1 Other (3) Total 2019 £bn £bn £bn £bn £bn £bn £bn £bn £bn ECL On balance sheet Cash and balances at central banks — — — — — — — — — – Stage 1 — — — — — — — — — Financial assets at amortised cost: – Loans and advances to customers⁽²⁾ — — — — 0.1 0.2 0.5 — 0.8 – Stage 1 — — — — — 0.1 — — 0.1 – Stage 2 — — — — 0.1 0.1 0.1 — 0.3 – Stage 3 — — — — — — 0.4 — 0.4 Of which mortgages: — — — — — 0.1 0.1 — 0.2 – Stage 1 — — — — — — — — — – Stage 2 — — — — — 0.1 — — 0.1 – Stage 3 — — — — — — 0.1 — 0.1 – Loans and advances to banks — — — — — — — — — – Stage 1 — — — — — — — — — – Reverse repo agreements – non trading — — — — — — — — — – Stage 1 — — — — — — — — — – Other financial assets at amortised cost — — — — — — — — — – Stage 1 — — — — — — — — — Total financial assets at amortised cost — — — — 0.1 0.2 0.5 — 0.8 Financial assets at FVOCI: — — — — — — — — — – Stage 1 — — — — — — — — — Total on balance sheet — — — — 0.1 0.2 0.5 — 0.8 Total off–balance sheet — — — — — — 0.1 — 0.1 – Stage 1 — — — — — — — — — – Stage 2 — — — — — — 0.1 — 0.1 – Stage 3 — — — — — — — — — Total ECL — — — — 0.1 0.2 0.6 — 0.9 2019 % % % % % % % % % Coverage ratio On balance sheet Cash and balances at central banks — — — — — — — — — – Stage 1 — — — — — — — — — Financial assets at amortised cost: – Loans and advances to customers⁽²⁾ — — — — 0.5 1.8 8.1 — 0.4 – Stage 1 — — — — — 1.6 — — 0.1 – Stage 2 — — — — 3.7 2.1 3.7 — 2.6 – Stage 3 — — — — — — 17.4 — 21.1 Of which mortgages: — — — — — 2.4 2.6 — 0.1 – Stage 1 — — — — — — — — — – Stage 2 — — — — — 3.2 — — 1.2 – Stage 3 — — — — — — 5.9 — 6.3 – Loans and advances to banks — — — — — — — — — – Stage 1 — — — — — — — — — – Reverse repo agreements – non trading — — — — — — — — — – Stage 1 — — — — — — — — — – Other financial assets at amortised cost — — — — — — — — — – Stage 1 — — — — — — — — — Total financial assets at amortised cost — — — — 0.5 1.8 8.1 — 0.3 Financial assets at FVOCI: — — — — — — — — — – Stage 1 — — — — — — — — — Total on balance sheet — — — — 0.5 1.8 8.1 — 0.3 Total off–balance sheet — — — — — — 20.0 — 0.2 – Stage 1 — — — — — — — — — – Stage 2 — — — — — — 50.0 — 16.7 – Stage 3 — — — — — — — — — Total coverage ratio — — — — 0.4 1.7 9.0 — 0.3 (1) Includes cash at hand and smaller cases mainly in the consumer (auto) finance and commercial mortgages portfolios, as well as loans written as part of the Government Covid-19 support schemes for micro-SMEs. We use scorecards for these items, rather than rating models. (2) Includes interest we have charged to the customer’s account and accrued interest we have not charged to the account yet. (3) Includes cash at hand and smaller cases mainly in the consumer (auto) finance and commercial mortgages portfolios. We use scorecards for these items, rather than rating models.
Summary of IFRS 9 Credit Quality	Total on-balance sheet exposures at 31 December 2020 comprised £207.0bn of customer loans, loans and advances to banks of £1.7bn, £20.8bn of sovereign assets measured at amortised cost, £9.0bn of assets measured at FVOCI, and £41.3bn of cash and balances at central banks. Stage 1 Stage 2 Stage 3 Total 2020 £m £m £m £m Exposures On-balance sheet Retail Banking 170,128 11,341 1,935 183,404 – of which mortgages 154,586 10,345 1,799 166,730 Corporate & Commercial Banking 11,167 5,498 961 17,626 Corporate & Investment Banking 2,587 198 — 2,785 Corporate Centre 75,743 27 — 75,770 Total on-balance sheet 259,625 17,064 2,896 279,585 Off-balance sheet Retail Banking (1) 26,550 256 41 26,847 – of which mortgages (1) 13,180 82 14 13,276 Corporate & Commercial Banking 6,050 768 59 6,877 Corporate & Investment Banking 8,630 231 — 8,861 Corporate Centre 558 17 — 575 Total off-balance sheet (2) 41,788 1,272 100 43,160 Total exposures 301,413 18,336 2,996 322,745 ECL On-balance sheet Retail Banking 100 350 218 668 – of which mortgages 15 130 132 277 Corporate & Commercial Banking 46 189 342 577 Corporate & Investment Banking 5 17 — 22 Corporate Centre 35 — — 35 Total on-balance sheet 186 556 560 1,302 Off-balance sheet Retail Banking 18 19 1 38 – of which mortgages 2 1 — 3 Corporate & Commercial Banking 8 10 8 26 Corporate & Investment Banking 4 7 — 11 Total off-balance sheet 30 36 9 75 Total ECL 216 592 569 1,377 % % % % Coverage ratio (3) On-balance sheet Retail Banking 0.1 3.1 11.3 0.4 – of which mortgages — 1.3 7.3 0.2 Corporate & Commercial Banking 0.4 3.4 35.6 3.3 Corporate & Investment Banking 0.2 8.6 — 0.8 Corporate Centre — — — — Total on-balance sheet 0.1 3.3 19.3 0.5 Off-balance sheet Retail Banking 0.1 7.4 2.4 0.1 – of which mortgages — 1.2 — — Corporate & Commercial Banking 0.1 1.3 13.6 0.4 Corporate & Investment Banking — 3.0 — 0.1 Total off-balance sheet 0.1 2.8 9.0 0.2 Total coverage 0.1 3.2 19.0 0.4 (1) Off-balance sheet exposures include £7.7bn of retail mortgage offers in the pipeline. (2) Off-balance sheet amounts consist of contingent liabilities and commitments. For more, see Note 31 to the Consolidated Financial Statements. (3) ECL as a percentage of the related exposure. Total on-balance sheet exposures at 31 December 2019 comprised £205.0bn of customer loans, loans and advances to banks of £1.9bn, £30.7bn of sovereign assets measured at amortised cost, £9.7bn of assets measured at FVOCI, and £21.2bn of cash and balances at central banks. Stage 1 Stage 2 Stage 3 Total 2019 £m £m £m £m Exposures On-balance sheet Retail Banking 167,609 9,289 1,864 178,762 – of which mortgages 155,477 8,157 1,722 165,356 Corporate & Commercial Banking 15,871 2,096 424 18,391 Corporate & Investment Banking 3,843 198 — 4,041 Corporate Centre 67,272 94 — 67,366 Total on-balance sheet 254,595 11,677 2,288 268,560 Off-balance sheet Retail Banking (1) 25,795 186 37 26,018 – of which mortgages (1) 13,353 67 12 13,432 Corporate & Commercial Banking 5,410 289 28 5,727 Corporate & Investment Banking 9,129 199 15 9,343 Corporate Centre 507 — — 507 Total off-balance sheet (2) 40,841 674 80 41,595 Total exposures 295,436 12,351 2,368 310,155 ECL On-balance sheet Retail Banking 76 246 200 522 – of which mortgages 11 100 103 214 Corporate & Commercial Banking 46 45 156 247 Corporate & Investment Banking 1 13 — 14 Corporate Centre 1 1 — 2 Total on-balance sheet 124 305 356 785 Off-balance sheet Retail Banking 13 13 1 27 – of which mortgages 3 1 — 4 Corporate & Commercial Banking 7 6 2 15 Corporate & Investment Banking 3 24 9 36 Total off-balance sheet 23 43 12 78 Total ECL 147 348 368 863 % % % % Coverage ratio (3) On-balance sheet Retail Banking — 2.6 10.7 0.3 – of which mortgages — 1.2 6.0 0.1 Corporate & Commercial Banking 0.3 2.1 36.8 1.3 Corporate & Investment Banking — 6.6 — 0.3 Corporate Centre — 1.1 — — Total on-balance sheet — 2.6 15.6 0.3 Off-balance sheet Retail Banking 0.1 7.0 2.7 0.1 – of which mortgages — 1.5 — — Corporate & Commercial Banking 0.1 2.1 7.1 0.3 Corporate & Investment Banking — 12.1 60.0 0.4 Total off-balance sheet 0.1 6.4 15.0 0.2 Total coverage — 2.8 15.5 0.3 (1) Off-balance sheet exposures include £7.6bn of retail mortgage offers in the pipeline. (2) Off-balance sheet amounts consist of contingent liabilities and commitments. For more, see Note 31 to the Consolidated Financial Statements. (3) ECL as a percentage of the related exposure.
Summary of ECL Reconciliation	Movement in total exposures and the corresponding ECL (audited) The following table shows changes in total on and off-balance sheet exposures, subject to ECL assessment, and the corresponding ECL, in the year. The table presents total gross carrying amounts and ECLs at a Santander UK group level. We present segmental views in the sections below. Stage 1 Stage 2 Stage 3 Total Exposures (1) ECL Exposures (1) ECL Exposures (1) ECL Exposures (1) ECL £m £m £m £m £m £m £m £m At 1 January 2020 295,436 147 12,351 348 2,368 368 310,155 863 Transfers from Stage 1 to Stage 2 (3) (9,815) (47) 9,815 47 — — — — Transfers from Stage 2 to Stage 1 (3) 3,178 110 (3,178) (110) — — — — Transfers to Stage 3 (3) (385) (8) (1,126) (61) 1,511 69 — — Transfers from Stage 3 (3) 12 2 326 21 (338) (23) — — Transfers of financial instruments (7,010) 57 5,837 (103) 1,173 46 — — Net ECL remeasurement on stage transfer (4) — (101) — 239 — 241 — 379 Change in economic scenarios (2) — 15 — 139 — 10 — 164 Changes to model — — — — — 25 — 25 New lending and assets purchased (5) 55,546 40 1,371 64 104 52 57,021 156 Redemptions, repayments and assets sold (7) (50,698) (30) (2,295) (42) (441) (18) (53,434) (90) Other (6) 8,141 88 1,072 (53) 185 98 9,398 133 Assets written off (7) (2) — — — (393) (253) (395) (253) At 31 December 2020 301,413 216 18,336 592 2,996 569 322,745 1,377 Net movement in the period 5,977 69 5,985 244 628 201 12,590 514 ECL charge/(release) to the Income Statement 69 244 454 767 Less: Discount unwind — — (14) (14) Less: Recoveries net of collection costs — — (108) (108) Total ECL charge/(release) to the Income Statement 69 244 332 645 At 1 January 2019 290,882 143 12,011 307 2,571 357 305,464 807 Transfers from Stage 1 to Stage 2 (3) (4,101) (11) 4,101 11 — — — — Transfers from Stage 2 to Stage 1 (3) 3,458 74 (3,458) (74) — — — — Transfers to Stage 3 (3) (361) (2) (595) (24) 956 26 — — Transfers from Stage 3 (3) 10 1 516 23 (526) (24) — — Transfers of financial instruments (994) 62 564 (64) 430 2 — — Net remeasurement of ECL on stage transfer (4) — (66) — 130 — 96 — 160 Change in economic scenarios (2) — 5 — (15) — (9) — (19) Changes to model — — — — — 13 — 13 New lending and assets purchased (5) 42,415 29 827 32 15 9 43,257 70 Redemptions, repayments and assets sold (7) (40,380) (32) (1,344) (28) (459) (42) (42,183) (102) Other (6) 3,514 6 294 (14) 172 191 3,980 183 Assets written off (7) (1) — (1) — (361) (249) (363) (249) At 31 December 2019 295,436 147 12,351 348 2,368 368 310,155 863 Net movement in the period 4,554 4 340 41 (203) 11 4,691 56 ECL charge/(release) to the Income Statement 4 41 260 305 Less: Discount unwind — — (13) (13) Less: Recoveries net of collection costs (10) (15) (46) (71) Total ECL charge/(release) to the Income Statement (6) 26 201 221 (1) Exposures that have attracted an ECL, and as reported in the Credit Quality table above. (2) Changes to assumptions in the year. Isolates the impact on ECL from changes to the economic variables for each scenario, changes to the scenarios themselves as well as changes in the probability weights from all other movements. This also includes the impact of quarterly revaluation of collateral. The impact of changes in economics on exposure Stage allocations are shown within Transfers of financial instruments. (3) Total impact of facilities that moved Stage(s) in the year. This means, for example, that where risk parameter changes (model inputs) or model changes (methodology) result in a facility moving Stage, the full impact is reflected here (rather than in Other). Stage flow analysis only applies to facilities that existed at both the start and end of the year. Transfers between Stages are based on opening balances and ECL at the start of the year. (4) Relates to the revaluation of ECL following the transfer of an exposure from one Stage to another. (5) Exposures and ECL of facilities that did not exist at the start of the year but did at the end. Amounts in Stage 2 and 3 represent assets which deteriorated in the period after origination in Stage 1. (6) Residual movements on existing facilities that did not change Stage in the year, and which were not acquired in the year. Includes the net increase or decrease in the period of cash at central banks, the impact of changes in risk parameters in the period, unwind of discount rates and increases in ECL requirements of accounts which ultimately were written off in the year. (7) Exposures and ECL for facilities that existed at the start of the year but not at the end. The following table shows changes in total on and off-balance sheet exposures subject to ECL assessment, and the corresponding ECL in the year. The footnotes to the Santander UK group level table on page 99 also apply to this table. Stage 1 Stage 2 Stage 3 Total Exposures ⁽¹⁾ ECL Exposures ⁽¹⁾ ECL Exposures ⁽¹⁾ ECL Exposures ⁽¹⁾ ECL 2020 £m £m £m £m £m £m £m £m At 1 January 2020 193,404 89 9,475 259 1,901 201 204,780 549 Transfers from Stage 1 to Stage 2 (3) (5,242) (9) 5,242 9 — — — — Transfers from Stage 2 to Stage 1 (3) 2,544 86 (2,544) (86) — — — — Transfers to Stage 3 (3) (305) (7) (540) (27) 845 34 — — Transfers from Stage 3 (3) 8 1 324 20 (332) (21) — — Transfers of financial instruments (2,995) 71 2,482 (84) 513 13 — — Net ECL remeasurement on stage transfer (4) — (79) — 144 — 42 — 107 Change in economic scenarios (2) — 8 — 81 — 10 — 99 Changes to model — — — — — 25 — 25 New lending and assets purchased (5) 34,985 28 479 37 10 6 35,474 71 Redemptions, repayments and assets sold (7) (28,864) (16) (1,181) (22) (319) 8 (30,364) (30) Other (6) 150 17 342 (46) 99 94 591 65 Assets written off (7) (2) — — — (228) (180) (230) (180) At 31 December 2020 196,678 118 11,597 369 1,976 219 210,251 706 Net movement in the year 3,274 29 2,122 110 75 18 5,471 157 Charge/(release) to the Income Statement 29 110 198 337 Less: Discount unwind — — (9) (9) Less: Recoveries net of collection costs — — (19) (19) Income statement charge/(release) for the year 29 110 170 309 2019 At 1 January 2019 181,600 88 10,357 259 2,170 203 194,127 550 Transfers from Stage 1 to Stage 2 (3) (3,078) (8) 3,078 8 — — — — Transfers from Stage 2 to Stage 1 (3) 3,254 67 (3,254) (67) — — — — Transfers to Stage 3 (3) (315) (2) (482) (19) 797 21 — — Transfers from Stage 3 (3) 9 1 500 22 (509) (23) — — Transfers of financial instruments: (130) 58 (158) (56) 288 (2) — — Net ECL remeasurement on stage transfer (4) — (60) — 75 — 56 — 71 Change in economic scenarios (2) — (5) — (26) — (9) — (40) Changes to model — — — — — 13 — 13 New lending and assets purchased (5) 36,779 23 447 24 13 9 37,239 56 Redemptions, repayments and assets sold (7) (25,809) (20) (1,207) (16) (437) (33) (27,453) (69) Other (6) 965 5 37 (1) 117 159 1,119 163 Assets written off (7) (1) — (1) — (250) (195) (252) (195) At 31 December 2019 193,404 89 9,475 259 1,901 201 204,780 549 Net movement in the year 11,804 1 (882) — (269) (2) 10,653 (1) Charge/(release) to the Income Statement 1 — 193 194 Less: Discount unwind — — (8) (8) Less: Recoveries net of collection costs — — (31) (31) Income statement charge/(release) for the year 1 — 154 155 (1) Exposures that have attracted an ECL, and as reported in the Credit Quality table above. (2) Changes to assumptions in the year. Isolates the impact on ECL from changes to the economic variables for each scenario, changes to the scenarios themselves and changes in the probability weights from all other movements. This also includes the impact of quarterly revaluation of collateral. The impact of changes in economics on exposure Stage allocations are shown in Transfers of financial instruments. (3) Total impact of facilities that moved stage(s) in the year. This means, for example, that where risk parameter changes (model inputs) or model changes (methodology) result in a facility moving Stage, the full impact is reflected here (rather than in Other). Stage flow analysis only applies to facilities that existed at both the start and end of the year. Transfers from each stage are based on opening balances, and transfers in are based on closing balances, giving rise to a net movement on transfer. (4) Relates to the revaluation of ECL following the transfer of an exposure from one Stage to another. (5) Exposures and ECL on facilities that did not exist at the start of the period but did at the end. Amounts in Stage 2 and 3 represent assets which deteriorated in the period after origination in Stage 1. (6) Residual movements on facilities that did not change Stage in the period, and which were not acquired in the year. Included the impact of changes in risk parameters in the year, unwind of discount rates and increases in ECL requirements of accounts which ultimately were written off in the year. (7) Exposures and ECL for facilities that existed at the start of the year, but not at the end. 2020 Stage 1 Stage 2 Stage 3 Total Exposures (1) ECL Exposures (1) ECL Exposures (1) ECL Exposures (1) ECL £m £m £m £m £m £m £m £m At 1 January 2020 168,830 14 8,224 101 1,734 103 178,788 218 Transfers from Stage 1 to Stage 2 (3) (4,686) (2) 4,686 2 — — — — Transfers from Stage 2 to Stage 1 (3) 1,911 17 (1,911) (17) — — — — Transfers to Stage 3 (3) (229) (3) (491) (11) 720 14 — — Transfers from Stage 3 (3) 4 — 311 15 (315) (15) — — Transfers of financial instruments (3,000) 12 2,595 (11) 405 (1) — — Net ECL remeasurement on stage transfer (4) — (15) — 49 — 17 — 51 Change in economic scenarios (2) — 7 — 13 — 10 — 30 Changes to model — — — — — 25 — 25 New lending and assets purchased (5) 26,102 4 237 5 1 — 26,340 9 Redemptions, repayments and assets sold (7) (23,707) (4) (899) (7) (297) (15) (24,903) (26) Other (6) (459) (1) 270 (19) 24 7 (165) (13) Assets written off (7) — — — — (54) (14) (54) (14) At 31 December 2020 167,766 17 10,427 131 1,813 132 180,006 280 Net movement in the year (1,064) 3 2,203 30 79 29 1,218 62 Charge/(release) to the Income Statement 3 29 43 75 Less: Discount unwind — — (2) (2) Less: Recoveries net of collection costs — — (1) (1) Income statement charge/(release) for the year 3 29 40 72 2019 At 1 January 2019 157,739 12 9,432 119 1,999 106 169,170 237 Transfers from Stage 1 to Stage 2 (3) (2,345) (1) 2,345 1 — — — — Transfers from Stage 2 to Stage 1 (3) 2,921 24 (2,921) (24) — — — — Transfers to Stage 3 (3) (231) — (429) (8) 660 8 — — Transfers from Stage 3 (3) 5 (1) 485 16 (490) (15) — — Transfers of financial instruments 350 22 (520) (15) 170 (7) — — Net ECL remeasurement on stage transfer (4) — (23) — 16 — 12 — 5 Change in economic scenarios (2) — (5) — (22) — (9) — (36) Changes to model — — — — — 13 — 13 New lending and assets purchased (5) 31,090 5 198 3 2 — 31,290 8 Redemptions, repayments and assets sold (7) (22,439) (1) (1,039) (6) (396) (12) (23,874) (19) Other (6) 2,090 4 153 6 31 14 2,274 24 Assets written off (7) — — — — (72) (14) (72) (14) At 31 December 2019 168,830 14 8,224 101 1,734 103 178,788 218 Net movement in the year 11,091 2 (1,208) (18) (265) (3) 9,618 (19) Charge/(release) to the Income Statement 2 (17) 11 (4) Less: Discount unwind — — (4) (4) Less: Recoveries net of collection costs — — — — Income statement charge/(release) for the year 2 (17) 7 (8)
Summary of Residential Mortgages by Credit Performance	Credit performance (audited) 2020 2019 £m £m Mortgage loans and advances to customers of which: 166,730 165,356 – Stage 1 154,586 155,477 – Stage 2 10,345 8,157 – Stage 3 1,799 1,722 Loss allowances(3) 280 218 % % Stage 1 ratio (1) 92.72 94.03 Stage 2 ratio (1) 6.20 4.93 Stage 3 ratio (2) 1.09 1.05 (1) Stage 1/Stage 2 exposures as a percentage of customer loans. (2) Total Stage 3 exposure as a percentage of customer loans plus undrawn Stage 3 exposures. (3) The ECL allowance is for both on and off–balance sheet exposures.
Summary of Consumer (Auto) Finance and Other Unsecured Lending By Forbearance	The balances (audited) at 31 December 2020 and 31 December 2019 were: : Other unsecured Consumer Personal loans Credit cards Overdrafts Total other Total 2020 £m £m £m £m £m £m Total 9 — 43 16 59 68 – Stage 2 4 — 10 5 15 19 – Stage 3 5 — 33 11 44 49 2019 Total 7 — 51 25 76 83 – Stage 2 5 — 10 7 17 22 – Stage 3 2 — 41 18 59 61
Customer Classification	We manage credit risk across all our business segments in line with the credit risk lifecycle that we show in the next section. We tailor the way we manage risk to the type of customer. We classify our customers as standardised or non-standardised: Standardised Non-standardised – Mainly individuals and small businesses. Their transactions are for relatively small amounts of money and share similar credit characteristics. – Mainly medium and large corporate customers. Their transactions are for larger values and have more diverse credit characteristics. – We manage risk using automated decision-making tools. These are backed by teams of expert analysts. – We manage risk through expert analysis with support from internal risk assessment models.
Key Differences Between Regulatory EL And IFRS 9 ECL Models	Key differences between regulatory EL and IFRS 9 ECL models There are differences between the regulatory EL and the IFRS 9 ECL approaches. Although our IFRS 9 models use the existing Basel advanced IRB risk components, we need to make several adjustments to ensure the outcome is in line with the IFRS 9 requirements, i.e. the financial reporting standard we use, as follows. Basel advanced IRB EL IFRS 9 ECL Rating philosophy Mix of point-in-time, through-the-cycle or hybrid Point-in-time, forward-looking. Considers a range of economic scenarios Parameters calibration Contains regulatory floors and downturn calibration Unbiased estimate, based on conditions known at the balance sheet date Calculation timing Considers aggregation of possible default events in the next 12 months Considers monthly calculation of parameters, for all possible future default dates. First 12 months are used for Stage 1, full lifetime for Stages 2 and 3 Probability of Default (PD) PD in the next 12 months Includes forward-looking economic data and removes conservatism. PD in next 12 months for Stage 1, lifetime for Stages 2 and 3 Loss Given Default (LGD) Lifetime LGD for defaults in the next 12 months Modelled without regulatory floors and exclusion of indirect costs Exposure at Default (EAD) Exposure at the point of default if the customer defaults in the next 12 months Floored at amount owed, except on some revolving facilities. Recognises ability for exposure to reduce from the balance sheet date to default date SICR Does not include SICR concept Includes SICR concept Discounting applied At the weighted average cost of capital to the default date At the effective interest rate (EIR) to the balance sheet date
Definition Of Default	Retail Banking – They have been reported bankrupt or insolvent. This excludes accounts which are up to date and are not defaulted. – Their loan term has ended, but they still owe us money more than three months later – They have had forbearance while in default, but have not caught up with the payments they had missed before that, or they have had multiple forbearance – We have suspended their fees and interest because they are in financial difficulties – We have repossessed the property. Other business segments: Corporate & Commercial Banking, Corporate & Investment Banking and Corporate Centre – They have had a winding up notice issued, or something happens that is likely to trigger insolvency – such as another lender calls in a loan – Something happens that makes them less likely to be able to pay us – such as they lose an important client or contract – They have regularly missed or delayed payments, even though they have not gone over the three-month limit for default – Their loan is unlikely to be refinanced or repaid in full on maturity – Their loan has an excessive LTV that is unlikely to be resolved, such as by a change in planning policy, pay-downs, or increase in market value.
Summary Of Sector Split Explanatory	Corporate customer sector split (1) 2020 2019 Total Total £bn £bn Social housing 5.8 6.4 Other real estate 4.9 5.5 Wholesale & retail trade 3.7 3.8 Accommodation & food 2.1 1.8 Construction 1.6 1.2 Human health & social work 1.5 1.4 Professional, scientific and technical 1.3 1.0 Administrative & support services 1.3 1.3 Manufacturing 1.0 1.2 Information & communication 0.8 0.5 Transport & storage 0.5 0.5 Electricity & gas 0.6 0.5 Arts, entertainment & recreation 0.4 0.3 Other segments 2.0 1.0 Total corporate loans 27.5 26.4 Of which government lending schemes 4.6 — (1) Total corporate loans includes £24.3bn of Corporate lending (CCB, CIB and Business Banking) and £3.2bn of non-core loans in Corporate Centre (which is mostly Social Housing).
Other business segments
Statement [Line Items]
Summary of ECL Reconciliation	Corporate & Commercial Banking Stage 1 Stage 2 Stage 3 Total Exposures (1) ECL Exposures (1) ECL Exposures (1) ECL Exposures (1) ECL £m £m £m £m £m £m £m £m At 1 January 2020 21,281 53 2,385 51 452 158 24,118 262 Transfers from Stage 1 to Stage 2 (3) (4,443) (37) 4,443 37 — — — — Transfers from Stage 2 to Stage 1 (3) 528 20 (528) (20) — — — — Transfers to Stage 3 (3) (78) (1) (543) (21) 621 22 — — Transfers from Stage 3 (3) 4 1 1 — (5) (1) — — Transfers of financial instruments (3,989) (17) 3,373 (4) 616 21 — — Net ECL remeasurement on stage transfer (4) — (18) — 92 — 198 — 272 Change in economic scenarios (2) — 7 — 44 — — — 51 New lending and assets purchased (5) 7,984 10 805 28 94 44 8,883 82 Redemptions, repayments and assets sold (7) (6,487) (13) (818) (18) (118) (25) (7,423) (56) Other (6) (1,572) 32 521 6 97 5 (954) 43 Assets written off (7) — — — — (121) (51) (121) (51) At 31 December 2020 17,217 54 6,266 199 1,020 350 24,503 603 Net movement in the year (4,064) 1 3,881 148 568 192 385 341 ECL charge/(release) to the Income Statement 1 148 243 392 Less: Discount unwind — — (5) (5) Less: Recoveries net of collection costs — — (93) (93) Income statement charge/(release) for the year 1 148 145 294 Corporate & Investment Banking £m £m £m £m £m £m £m £m At 1 January 2020 12,972 4 397 37 15 9 13,384 50 Transfers from Stage 1 to Stage 2 (3) (43) — 43 — — — — — Transfers from Stage 2 to Stage 1 (3) — — — — — — — — Transfers to Stage 3 (3) — — (40) (13) 40 13 — — Transfers from Stage 3 (3) — — — — — — — — Transfers of financial instruments (43) — 3 (13) 40 13 — — Net ECL remeasurement on stage transfer (4) — — — — — — — — Change in economic scenarios (2) — — — 8 — — — 8 New lending and assets purchased (5) 1,119 1 43 — — — 1,162 1 Redemptions, repayments and assets sold (7) (5,005) (1) (287) (1) (1) — (5,293) (2) Other (6) 2,174 5 273 (7) (12) — 2,435 (2) Assets written off (7) — — — — (42) (22) (42) (22) At 31 December 2020 11,217 9 429 24 — — 11,646 33 Net movement in the year (1,755) 5 32 (13) (15) (9) (1,738) (17) ECL charge/(release) to the Income Statement 5 (13) 13 5 Less: Discount unwind — — — — Less: Recoveries net of collection costs — — 2 2 Income statement charge/(release) for the year 5 (13) 15 7 Stage 1 Stage 2 Stage 3 Total Exposures (1) ECL Exposures (1) ECL Exposures (1) ECL Exposures (1) ECL Corporate Centre £m £m £m £m £m £m £m £m At 1 January 2020 67,779 1 94 1 — — 67,873 2 Transfers from Stage 1 to Stage 2 (3) (87) (2) 87 2 — — — — Transfers from Stage 2 to Stage 1 (3) 106 4 (106) (4) — — — — Transfers to Stage 3 (3) (2) — (3) (1) 5 1 — — Transfers from Stage 3 (3) 1 1 1 — (2) (1) — — Transfers of financial instruments 18 3 (21) (3) 3 — — — Net ECL remeasurement on stage transfer (4) — (3) — 3 — 1 — 1 Change in economic scenarios (2) — — — 6 — — — 6 New lending and assets purchased (5) 11,457 1 45 — 1 — 11,503 1 Redemptions, repayments and assets sold (7) (10,342) (1) (9) (1) (3) (1) (10,354) (3) Other (6) 7,389 34 (65) (6) 1 — 7,325 28 Assets written off (7) — — — — (2) — (2) — At 31 December 2020 76,301 35 44 — — — 76,345 35 Net movement in the year 8,522 34 (50) (1) — — 8,472 33 ECL charge/(release) to the Income Statement 34 (1) — 33 Less: Discount unwind — — — — Less: Recoveries net of collection costs — — 4 4 Income statement charge/(release) for the year 34 (1) 4 37 Stage 1 Stage 2 Stage 3 Total Exposures (1) ECL Exposures (1) ECL Exposures (1) ECL Exposures (1) ECL Corporate & Commercial Banking £m £m £m £m £m £m £m £m At 1 January 2019 23,259 47 1,422 45 375 143 25,056 235 Transfers from Stage 1 to Stage 2 (3) (801) (3) 801 3 — — — — Transfers from Stage 2 to Stage 1 (3) 156 7 (156) (7) — — — — Transfers to Stage 3 (3) (42) — (109) (5) 151 5 — — Transfers from Stage 3 (3) — — 16 1 (16) (1) — — Transfers of financial instruments (687) 4 552 (8) 135 4 — — Net ECL remeasurement on stage transfer (4) — (7) — 18 — 38 — 49 Change in economic scenarios (2) — 8 — 11 — — — 19 New lending and assets purchased (5) 3,733 6 379 7 2 1 4,114 14 Redemptions, repayments and assets sold (7) (4,062) (10) (32) (11) (7) (9) (4,101) (30) Other (6) (962) 5 64 (11) 55 35 (843) 29 Assets written off (7) — — — — (108) (54) (108) (54) At 31 December 2019 21,281 53 2,385 51 452 158 24,118 262 Net movement in the year (1,978) 6 963 6 77 15 (938) 27 ECL charge/(release) to the Income Statement 6 6 69 81 Less: Discount unwind — — (6) (6) Less: Recoveries net of collection costs (10) (8) (12) (30) Income statement charge/(release) for the year (4) (2) 51 45 Stage 1 Stage 2 Stage 3 Total Exposures (1) ECL Exposures (1) ECL Exposures (1) ECL Exposures (1) ECL Corporate & Investment Banking £m £m £m £m £m £m £m £m At 1 January 2019 17,389 5 134 3 26 10 17,549 18 Transfers from Stage 1 to Stage 2 (3) (208) — 208 — — — — — Transfers from Stage 2 to Stage 1 (3) 41 — (41) — — — — — Transfers to Stage 3 (3) 0 — 0 — — — — — Transfers from Stage 3 (3) 0 — 0 — — — — — Transfers of financial instruments (167) — 167 — — — — — Net ECL remeasurement on stage transfer (4) 0 — 0 36 — — — 36 New lending and assets purchased (5) 54 — — — — — 54 — Redemptions, repayments and assets sold (7) (5,179) (1) 50 (1) (28) — (5,157) (2) Other (6) 875 — 46 (1) 17 (1) 938 (2) Assets written off (7) — — — — — — — — At 31 December 2019 12,972 4 397 37 15 9 13,384 50 Net movement in the year (4,417) (1) 263 34 (11) (1) (4,165) 32 ECL charge/(release) to the Income Statement (1) 34 (1) 32 Less: Discount unwind — — — — Less: Recoveries net of collection costs — (8) (2) (10) Income statement charge/(release) for the year (1) 26 (3) 22 Corporate Centre £m £m £m £m £m £m £m £m At 1 January 2019 68,635 3 97 1 (1) 1 68,731 5 Transfers from Stage 1 to Stage 2 (3) (15) — 15 — — — — — Transfers from Stage 2 to Stage 1 (3) 7 — (7) — — — — — Transfers to Stage 3 (3) (4) — (4) — 8 — — — Transfers from Stage 3 (3) — — — — — — — — Transfers of financial instruments (12) — 4 — 8 — — — Net ECL remeasurement on stage transfer (4) — — — — — 1 — 1 Change in economic scenarios (2) — 1 — — — — — 1 New lending and assets purchased (5) 1,849 — — — — — 1,849 — Redemptions, repayments and assets sold (7) (5,328) — (155) — 14 — (5,469) — Other (6) 2,635 (3) 148 — (18) (2) 2,765 (5) Assets written off (7) — — — — (3) — (3) — At 31 December 2019 67,779 1 94 1 — — 67,873 2 Net movement in the year (856) (2) (3) — 1 (1) (858) (3) ECL charge/(release) to the Income Statement (2) — (1) (3) Less: Discount unwind — — 1 1 Less: Recoveries net of collection costs — — (1) (1) Income statement charge/(release) for the year (2) — (1) (3)

Market risk (Tables)

Market risk (Tables)	12 Months Ended
	Dec. 31, 2020
Text block [abstract]
Summary of NIM and EVE Sensitivity of Interest Rate Risk	The table below shows how our base case income and valuation would be affected by a 50 basis point parallel shift (both up and down) applied instantaneously to the yield curve at 31 December 2020 and 31 December 2019. Sensitivity to parallel shifts represents the amount of risk in a way that we think is both simple and scalable. 50 basis points is the stress we typically focus on for banking market risk controls, although we also monitor sensitivities to other parallel and non-parallel shifts as well as scenarios. 2020 2019 +50bps -50bps +50bps -50bps £m £m £m £m NIM sensitivity (audited) 225 (15) 99 56 EVE sensitivity 367 (585) 10 (88)
Disclosure Of Detailed Information About Risk Metrics Covering The Portfolio Of Securities Held For Liquidity And Investment Purpose Explanatory	The table below shows the risk metrics covering the portfolios of securities we hold for liquidity and investment purposes. 2020 2019 £m £m VaR 7 3 Worst three month stressed loss 93 102

Liquidity risk (Tables)

Liquidity risk (Tables)	12 Months Ended
	Dec. 31, 2020
Text block [abstract]
Summary of Detailed Information About Liquidity Coverage Ratio Explanatory	This table shows our LCR and LRA at 31 December 2020 and 31 December 2019. The LRA data reflect the stress testing methodology in place at that time. LCR RFB DoLSub (1) LRA RFB (2) 2020 2019 2020 2019 £bn £bn £bn £bn Eligible liquidity pool (liquidity value) (3) 51.2 41.6 47.2 40.6 Net stress outflows (34.1) (29.3) (34.4) (31.7) Surplus 17.1 12.3 12.8 8.9 Eligible liquidity pool as a percentage of anticipated net cash flows 150 % 142 % 137 % 128 % (1) The RFB LCR was 152% (2019:146%). (2) The LRA is calculated for the Santander UK plc group (the RFB Group) and is a three-month Santander UK specific requirement. (3) The liquidity value is calculated as applying an applicable haircut to the carrying value.
Summary of Detailed Information About For Liquidity Pool of Carrying value And Weighted Average Explanatory	This table shows the carrying value of our eligible liquidity pool assets at 31 December 2020 and 31 December 2019. It also shows the weighted average carrying value in the year. RFB DoLSub Carrying value Weighted average carrying 2020 2019 2020 2019 Level 1 Level 2 Total Level 1 Level 2 Total Total Total £bn £bn £bn £bn £bn £bn £bn £bn Cash and balances at central banks 39.4 — 39.4 19.3 — 19.3 26.8 19.1 Government bonds 8.9 0.1 9.0 16.7 1.2 17.9 15.5 20.8 Supranational bonds and multilateral development banks 1.5 — 1.5 2.9 — 2.9 2.9 2.9 Covered bonds 1.0 — 1.0 1.4 0.1 1.5 1.2 2.4 Asset-backed securities — 0.6 0.6 — 0.4 0.4 0.6 1.4 50.8 0.7 51.5 40.3 1.7 42.0 47.0 46.6
Summary of Detailed Information About Weighted Average Carrying Value In The Year The Composition Of The Pool Is Consistent With The Currency Profile Of Our Net Liquidity Outflows Explanatory	This table shows the carrying value of our eligible liquidity pool by major currencies at 31 December 2020 and 31 December 2019. The composition of the pool is consistent with the currency profile of our net liquidity outflows. RFB DoLSub US Dollar Euro Sterling Other Total £bn £bn £bn £bn £bn 2020 2.0 1.2 48.1 0.2 51.5 2019 3.6 1.2 36.1 1.1 42.0
Summary of Reconciliation of Wholesale Funding to Balance Sheet	This table reconciles our wholesale funding to our balance sheet at 31 December 2020 and 31 December 2019. . Balance sheet line item Funding Deposits by banks (3) Deposits by customers (1) Repurchase agreements - non trading Financial Debt Subordinated Other equity instruments (2) 2020 £bn £bn £bn £bn £bn £bn £bn £bn Deposits by banks — — — — — — — — Certificates of deposit and commercial paper 5.7 — — — — 5.7 — — Senior unsecured – public benchmark 15.4 — 7.8 — — 7.6 — — – privately placed 1.1 — 0.1 — 0.9 0.1 — — Covered bonds 17.9 — — — — 17.9 — — Securitisation and structured issuance 2.8 — — — 0.5 2.3 — — Term Funding Scheme 6.3 6.3 — — — — — — TFSME 11.7 11.7 — — — — — — Subordinated liabilities and equity 4.4 — — — — — 2.2 2.2 Total wholesale funding 65.3 18.0 7.9 — 1.4 33.6 2.2 2.2 Repos 15.8 — — 15.8 — — — — Foreign exchange and hedge accounting 2.5 — 0.2 — — 2.0 0.3 — Other 3.0 3.0 — — — — — — Balance sheet total 86.6 21.0 8.1 15.8 1.4 35.6 2.5 2.2 2019 Deposits by banks 0.3 0.3 — — — — — — Certificates of deposit and commercial paper 5.8 — — — — 5.8 — — Senior unsecured – public benchmark 18.8 — 8.6 — — 10.2 — – privately placed 2.7 — 0.1 — 1.0 1.6 — — Covered bonds 18.2 — — — — 18.2 — — Securitisation and structured issuance 5.6 — — 1.4 0.5 3.7 — — Term Funding Scheme 10.8 10.8 — — — — — — TFSME — — — — — — — — Subordinated liabilities and equity 5.2 — — — — — 3.0 2.2 Total wholesale funding 67.4 11.1 8.7 1.4 1.5 39.5 3.0 2.2 Repos 16.9 — — 16.9 — — — — Foreign exchange and hedge accounting 2.5 — 0.4 — — 1.6 0.5 — Other 3.5 3.3 — — 0.2 — — — Balance sheet total 90.3 14.4 9.1 18.3 1.7 41.1 3.5 2.2 (1) This is included in our balance sheet total of £195,135m (2019: £181,883m). (2) Consists of £nil (2019: £nil ) fixed/floating rate non-cumulative callable preference shares, £235m (2019: £235m) Step-up Callable Perpetual Reserve Capital Instruments and £1,956m (2019: £1,956m) Perpetual Capital Securities . See Notes 33 and 34 to the Consolidated Financial Statements. (3) Other consists of items in the course of transmission and other deposits, excluding the TFS. See Note 24 to the Consolidated Financial Statements.
Summary of Sources of Wholesale Funding by Maturity	This table shows our main sources of wholesale funding. It does not include securities finance agreements. The table is based on exchange rates at issue and scheduled repayments and call dates. It does not reflect the final contractual maturity of the funding. ≤ 1 >1 and ≤ 3 months >3 and ≤ 6 months >6 and ≤ 9 months >9 and ≤ 12 months Sub-total >1 and >2 and >5 years Total 2020 £bn £bn £bn £bn £bn £bn £bn £bn £bn £bn Downstreamed from Santander UK Group Holdings plc to Santander UK plc (1) Senior unsecured – public benchmark 0.7 — — 1.1 — 1.8 1.2 4.8 1.3 9.1 – privately placed — — — — — — — — 0.1 0.1 Subordinated liabilities and equity (incl. AT1) — — — — — — 0.8 1.5 0.4 2.7 0.7 — — 1.1 — 1.8 2.0 6.3 1.8 11.9 Other Santander UK plc Certificates of deposit and commercial paper 0.7 3.7 1.2 — 0.1 5.7 — — — 5.7 Senior unsecured – public benchmark 0.4 — 1.6 — 0.8 2.8 0.6 2.6 0.3 6.3 – privately placed — 0.3 — — 0.1 0.4 — 0.2 0.4 1.0 Covered bonds — — 3.7 1.7 — 5.4 1.7 6.4 4.4 17.9 Securitisation & structured issuance (2) 0.2 — — 0.2 — 0.4 0.9 0.3 — 1.6 Term Funding Scheme — 1.5 1.5 1.0 — 4.0 2.3 — — 6.3 TFSME — — — — — — — 11.7 — 11.7 Subordinated liabilities — — — — — — — 0.5 1.2 1.7 1.3 5.5 8.0 2.9 1.0 18.7 5.5 21.7 6.3 52.2 Other group entities Securitisation & structured issuance (3) 0.2 0.1 — 0.1 0.2 0.6 0.4 0.2 — 1.2 Total at 31 December 2020 2.2 5.6 8.0 4.1 1.2 21.1 7.9 28.2 8.1 65.3 Of which: – Secured 0.4 1.6 5.2 3.0 0.2 10.4 5.3 18.6 4.4 38.7 – Unsecured 1.8 4.0 2.8 1.1 1.0 10.7 2.6 9.6 3.7 26.6 2019 Total at 31 December 2019 1.7 5.5 4.7 2.1 8.5 22.5 16.6 18.5 9.8 67.4 Of which: – Secured 0.2 0.1 2.7 0.3 5.8 9.1 11.5 10.4 3.6 34.6 – Unsecured 1.5 5.4 2.0 1.8 2.7 13.4 5.1 8.1 6.2 32.8 (1) 95% of Senior Unsecured debt issued from Santander UK Group Holdings plc has been downstreamed to Santander UK plc as ‘secondary non-preferential debt’ in line with the guidelines from the Bank of England for Internal MREL. (2) Includes funding from mortgage-backed securitisation vehicles where Santander UK plc is the asset originator. (3) Includes funding from asset-backed securitisation vehicles where entities other than Santander UK plc are the asset originator.
Summary of Wholesale Funding by Currency	This table shows our wholesale funding by major currency at 31 December 2020 and 31 December 2019. 2020 2019 Sterling US Dollar Euro Other Sterling US Dollar Euro Other % % % % % % % % Downstreamed from Santander UK Group Holdings plc to Santander UK plc Senior unsecured – public benchmark 10 62 28 — 11 65 22 2 – privately placed — — — 100 — — — 100 Subordinated liabilities and equity (incl. AT1) 73 27 — — 67 33 — — 24 53 22 1 25 56 16 3 Other Santander UK plc Deposits by banks — — — — 3 97 — — Certificates of deposit and commercial paper 51 44 4 1 45 54 1 — Senior unsecured – public benchmark 10 73 17 — 14 54 32 — – privately placed 41 37 10 12 21 15 59 5 Covered bonds 48 5 46 1 54 — 45 1 Securitisation & structured issuance 77 23 — — 72 28 — — Term Funding Scheme 100 — — — 100 — — — TFSME 100 — — — — — — — Subordinated liabilities 63 37 — — 49 51 — — 63 18 19 — 54 22 24 — Other group entities Securitisation & structured issuance 100 — — — 95 5 — — Total 57 24 19 — 50 27 22 1
Summary of External Term Issuance (Sterling Equivalent)	In 2020, our external term issuance (sterling equivalent) was: Sterling US Dollar Euro Total 2020 Total 2019 £bn £bn £bn £bn £bn Downstreamed from Santander UK Group Holdings plc to Santander UK plc Senior unsecured – public benchmark — 0.8 0.6 1.4 — Subordinated debt and equity (inc. AT1) — — — — 0.5 — 0.8 0.6 1.4 0.5 Other Santander UK plc Covered bonds 1.0 0.9 1.1 3.0 2.9 Senior unsecured – public benchmark — 1.0 — 1.0 0.9 TFSME 11.7 — — 11.7 — 12.7 1.9 1.1 15.7 3.8 Other group entities Securitisations — — — — 0.2 Total gross issuances 12.7 2.7 1.7 17.1 4.5
Summary of Balance Sheet Encumbered And Unencumbered Assets Explanatory	On-balance sheet encumbered and unencumbered assets (audited) Encumbered with counterparties other than central banks Unencumbered assets not pre-positioned with central banks Covered Securitis- Other Total Assets positioned at central banks (3) Readily Other Cannot be Total Total 2020 £m £m £m £m £m £m £m £m £m £m Cash and balances at central banks (1)(2) — — 985 985 854 39,411 — — 40,265 41,250 Financial assets at FVTPL: – Derivative financial instruments — — — — — — — 3,406 3,406 3,406 – Other financial assets at FVTPL — — — — — — — 208 208 208 Financial assets at amortised cost: – Loans and advances to customers 23,669 7,469 184 31,322 61,292 74,758 19,801 21,577 177,428 208,750 – Loans and advances to banks — — 804 804 — — — 878 878 1,682 – Repurchase agreements – non trading — — — — — — — 19,599 19,599 19,599 – Other financial assets at amortised cost — — 648 648 — 515 — — 515 1,163 Financial assets at FVOCI — — 5,581 5,581 — 3,369 — — 3,369 8,950 Interests in other entities — — — — — — — 172 172 172 Intangible assets — — — — — — — 1,646 1,646 1,646 Property, plant and equipment — — — — — — 1,734 — 1,734 1,734 Current tax assets — — — — — — — 264 264 264 Retirement benefit assets — — — — — — — 495 495 495 Other assets — — — — — — — 3,013 3,013 3,013 Total assets 23,669 7,469 8,202 39,340 62,146 118,053 21,535 51,258 252,992 292,332 2019 Cash and balances at central banks (1)(2) — — 1,080 1,080 707 19,393 — — 20,100 21,180 Financial assets at FVTPL: – Derivative financial instruments — — — — — — — 3,316 3,316 3,316 – Other financial assets at FVTPL — — — — — — — 386 386 386 Financial assets at amortised cost: – Loans and advances to customers 23,310 12,915 332 36,557 55,273 76,567 22,875 16,015 170,730 207,287 – Loans and advances to banks — — 403 403 — — — 1,452 1,452 1,855 – Repurchase agreements – non trading — — — — — — — 23,636 23,636 23,636 – Other financial assets at amortised cost — — 3,026 3,026 — 4,030 — — 4,030 7,056 Financial assets at FVOCI — — 6,009 6,009 — 3,738 — — 3,738 9,747 Interests in other entities — — — — — — — 117 117 117 Intangible assets — — — — — — — 1,766 1,766 1,766 Property, plant and equipment — — — — — — 1,967 — 1,967 1,967 Current tax assets — — — — — — — 200 200 200 Retirement benefit assets — — — — — — — 669 669 669 Other assets — — — — — — — 2,520 2,520 2,520 Total assets 23,310 12,915 10,850 47,075 55,980 103,728 24,842 50,077 234,627 281,702 (1) Encumbered cash and balances at central banks include minimum cash balances we have to hold at central banks for regulatory purposes. (2) Readily realisable cash and balances at central banks are amounts held at central banks as part of our liquidity management activities. (3) Comprises pre-positioned assets and encumbered assets.

Accounting Policies (Tables)

Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of significant accounting policies [Abstract]
Schedule of Useful Lives or Depreciation Rates Used for Property, Plant and Equipment	Classes of property, plant and equipment are depreciated on a straight-line basis over their useful life, as follows: Owner-occupied properties Not exceeding 50 years Office fixtures and equipment 3 to 15 years Computer software 3 to 7 years Right-of-use assets (see ‘Leases – The Santander UK group as lessee’ below) Shorter of the lease term or the useful life of the underlying asset

Segments (Tables)

Segments (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of operating segments [abstract]
Summary of Results by Segment	Results by segment Retail Banking Corporate & Commercial Banking Corporate & Investment Banking Corporate Centre Total 2020 £m £m £m £m £m Net interest income/(expense) 3,100 364 55 (76) 3,443 Non-interest income 375 93 68 (4) 532 Total operating income/(expense) 3,475 457 123 (80) 3,975 Operating expenses before credit impairment losses, provisions and charges (1,913) (316) (114) (109) (2,452) Credit impairment losses (308) (294) (7) (36) (645) Provisions for other liabilities and (charges)/release (173) (12) (10) (78) (273) Total operating credit impairment losses, provisions and charges (481) (306) (17) (114) (918) Profit/(loss) before tax 1,081 (165) (8) (303) 605 Revenue from external customers 4,160 549 123 (857) 3,975 Inter-segment revenue (685) (92) — 777 — Total operating income/(expense) 3,475 457 123 (80) 3,975 Revenue from external customers includes the following fee and commission income disaggregated by income type: (1) – Current account and debit card fees 441 41 10 — 492 – Insurance, protection and investments 65 — — — 65 – Credit cards 67 — — — 67 – Non-banking and other fees (2) 15 47 66 4 132 Total fee and commission income 588 88 76 4 756 Fee and commission expense (335) (19) (10) (7) (371) Net fee and commission income/(expense) 253 69 66 (3) 385 Customer loans 183,404 17,626 2,784 3,196 207,010 Total assets(3) 192,070 17,626 2,784 79,852 292,332 Customer deposits 152,167 24,985 6,506 2,049 185,707 Total liabilities 152,715 25,011 6,517 92,153 276,396 Average number of full-time equivalent staff 19,151 2,092 716 39 21,998 (1) The disaggregation of fees and commission income as shown above is not included in reports provided to the chief operating decision maker but is provided to show the split by reportable segments. (2) Non-banking and other fees include mortgages (except mortgage account fees), consumer finance, commitment commission, asset finance, invoice finance and trade finance. (3) Includes customer loans, net of credit impairment loss allowances. Retail Banking Corporate & Commercial Banking Corporate & Investment Banking Corporate Centre Total 2019 £m £m £m £m £m Net interest income/(expense) 2,827 422 62 (19) 3,292 Non-interest income 691 109 70 11 881 Total operating income/(expense) 3,518 531 132 (8) 4,173 Operating expenses before credit impairment losses, provisions and charges (1,980) (324) (130) (65) (2,499) Credit impairment (losses)/releases (156) (45) (22) 2 (221) Provisions for other liabilities and charges (290) (22) (16) (113) (441) Total operating credit impairment losses, provisions and charges (446) (67) (38) (111) (662) Profit/(loss) before tax 1,092 140 (36) (184) 1,012 Revenue from external customers 4,255 633 138 (853) 4,173 Inter-segment revenue (737) (102) (6) 845 — Total operating income/(expense) 3,518 531 132 (8) 4,173 Revenue from external customers includes the following fee and commission income disaggregated by income type: (1) – Current account and debit card fees 696 49 13 — 758 – Insurance, protection and investments 76 — — 1 77 – Credit cards 86 — — — 86 – Non-banking and other fees (2) 61 58 67 5 191 Total fee and commission income 919 107 80 6 1,112 Fee and commission expense (373) (23) (17) (13) (426) Net fee and commission income/(expense) 546 84 63 (7) 686 Customer loans 178,762 18,391 4,041 3,814 205,008 Total assets (3) 185,920 18,391 4,046 73,345 281,702 Customer deposits 142,735 20,546 6,102 2,332 171,715 Total liabilities 143,602 20,572 6,233 95,278 265,685 Average number of full-time equivalent staff 20,594 2,151 804 21 23,570 2018 Net interest income 3,076 470 69 (12) 3,603 Non-interest income 632 115 132 52 931 Total operating income 3,708 585 201 40 4,534 Operating expenses before credit impairment losses, provisions and charges (1,914) (318) (203) (144) (2,579) Credit impairment (losses)/ releases (135) (8) (14) 4 (153) Provisions for other liabilities and charges (228) (16) (8) (5) (257) Total operating credit impairment losses, provisions and (charges)/releases (363) (24) (22) (1) (410) Profit/(loss) before tax 1,431 243 (24) (105) 1,545 Revenue from external customers 4,361 750 245 (822) 4,534 Inter-segment revenue (653) (165) (44) 862 — Total operating income 3,708 585 201 40 4,534 Revenue from external customers includes the following fee and commission income disaggregated by income type: (1) – Current account and debit card fees 690 49 14 — 753 – Insurance, protection and investments 105 — — — 105 – Credit card fees 85 — — — 85 – Non-banking and other fees (2) 76 65 81 5 227 Total fee and commission income 956 114 95 5 1,170 Fee and commission expense (382) (25) (14) — (421) Net fee and commission income 574 89 81 5 749 Customer loans 171,134 19,858 4,543 4,051 199,586 Total assets (3) 177,959 19,858 3,244 82,311 283,372 Customer deposits 139,976 19,695 4,853 2,791 167,315 Total liabilities 140,750 19,723 4,400 102,590 267,463 Average number of full-time equivalent staff 20,964 2,102 985 154 24,205 (1) The disaggregation of fees and commission income as shown above is not included in reports provided to the chief operating decision maker but is provided to show the split by reportable segments. (2) Non-banking and other fees include mortgages (except mortgage account fees), consumer finance, commitment commission, asset finance, invoice finance and trade finance. (3) Includes customer loans, net of credit impairment loss allowances. Geographical information is not provided, as substantially all of Santander UK’s activities are in the UK.

Net Interest Income (Tables)

Net Interest Income (Tables)	12 Months Ended
	Dec. 31, 2020
Analysis of income and expense [abstract]
Summary of Net Interest Income	Group 2020 2019 2018 £m £m £m Interest and similar income: Loans and advances to customers (3) 4,819 5,230 5,458 Loans and advances to banks 49 137 202 Reverse repurchase agreements – non trading 118 244 124 Other 119 306 282 Total interest and similar income (1) 5,105 5,917 6,066 Interest expense and similar charges: Deposits by customers (1,011) (1,540) (1,433) Deposits by banks (47) (134) (117) Repurchase agreements – non trading (43) (126) (42) Debt securities in issue (440) (678) (721) Subordinated liabilities (111) (137) (142) Other (10) (10) (8) Total interest expense and similar charges (2) (1,662) (2,625) (2,463) Net interest income 3,443 3,292 3,603 (1) This includes £38m (2019: £155m) of interest income on financial assets at fair value through other comprehensive income. (2) This includes £451m (2019: £310m) of interest expense on derivatives hedging debt issuances and £3m (2019: £4m) of interest expense on lease liabilities. (3) During 2020, we revised the accounting treatment for certain items of mortgage income to better align our policy to current practice. The net impact of these changes is not material and comparatives have not been restated. Further details are provided in Note 1 Accounting Policies.

Net Fee and Commission Income (

Net Fee and Commission Income (Tables)	12 Months Ended
	Dec. 31, 2020
Fee and commission income (expense) [abstract]
Summary of Net Fee and Commission Income	Group 2020 2019 2018 £m £m £m Fee and commission income: Current account and debit card fees 492 758 753 Insurance, protection and investments 65 77 105 Credit cards 67 86 85 Non-banking and other fees (1) 132 191 227 Total fee and commission income 756 1,112 1,170 Total fee and commission expense (371) (426) (421) Net fee and commission income 385 686 749 (1) Non-banking and other fees include mortgages (except mortgage account fees), consumer finance, commitment commission, asset finance, invoice finance and trade finance.

Other Operating Income (Tables)

Other Operating Income (Tables)	12 Months Ended
	Dec. 31, 2020
Net Trading And Other Income [abstract]
Summary of Net Trading and Other Income	Group 2020 2019 2018 £m £m £m Net losses on financial instruments designated at fair value through profit or loss (75) (134) (86) Net gains on financial instruments mandatorily at fair value through profit or loss 46 70 36 Hedge ineffectiveness 20 8 34 Net profit on sale of financial assets at fair value through other comprehensive income 17 15 19 Income from operating lease assets 126 124 86 Other 13 112 93 147 195 182

Operating Expenses Before Cre_2

Operating Expenses Before Credit Impairment Losses, Provisions and Charges (Tables)	12 Months Ended
	Dec. 31, 2020
Expenses by nature [abstract]
Summary of Operating Expenses Before Credit Impairment Losses, Provisions and Charges	Group 2020 2019 2018 £m £m £m Staff costs: Wages and salaries 831 852 898 Performance-related payments 97 159 159 Social security costs 101 111 111 Pensions costs: – defined contribution plans 66 66 67 defined benefit plans 38 35 79 Other share-based payments — — 3 Other personnel costs 33 40 52 1,166 1,263 1,369 Other administration expenses 724 693 835 Depreciation, amortisation and impairment 562 543 375 Total 2,452 2,499 2,579
Deferred Performance Awards	Costs recognised in 2020 Costs expected to be recognised in 2021 or later Arising from awards in current year Arising from awards in prior year Total Arising from awards in current year Arising from awards in prior year Total £m £m £m £m £m £m Cash 2 7 9 2 9 11 Shares 2 6 8 2 9 11 4 13 17 4 18 22
Disclosure of Amount of Bonus Awarded to Employees	Expenses charged in the year Expenses deferred to future periods Total 2020 2019 2020 2019 2020 2019 £m £m £m £m £m £m Cash award – not deferred 77 130 — — 77 130 – deferred 9 10 11 17 20 27 Shares award – not deferred 3 10 — — 3 10 – deferred 8 9 11 15 19 24 Total discretionary bonus 97 159 22 32 119 191

Audit and Other Services (Table

Audit and Other Services (Tables)	12 Months Ended
	Dec. 31, 2020
Audit And Other Services [Abstract]
Summary of Audit and Other Services	Group 2020 2019 2018 £m £m £m Audit fees: Fees payable to the Company's auditor and its associates for the audit of the Santander UK group's annual accounts 10.0 8.0 7.2 Fees payable to the Company’s auditor and its associates for other services to the Santander UK group: – Audit of the Santander UK group's subsidiaries 1.4 1.3 1.1 Total audit fees (1) 11.4 9.3 8.3 Non-audit fees: Audit-related assurance services (1) 0.8 0.8 0.7 Other assurance services 0.3 0.2 0.1 Other non-audit services 0.0 0.2 1.0 Total non-audit fees 1.1 1.2 1.8 (1) 2020 audit fees included £0.8m (2019: £0.1m) which related to the prior year. (2) 2020 audit-related assurance services included £nil (2019: £0.1m) which related to the prior year.

Credit Impairment Losses and _2

Credit Impairment Losses and Provisions (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of credit impairment loss and provisions [abstract]
Summary of Impairment Losses and Provisions	Group 2020 2019 2018 £m £m £m Credit impairment losses: Loans and advances to customers 672 239 189 Recoveries of loans and advances, net of collection costs (24) (40) (42) Off-balance sheet exposures (See Note 29) (3) 22 6 645 221 153 Provisions for other liabilities and charges (excluding off-balance sheet credit exposures) (See Note 29) 267 435 257 Provisions for residual value and voluntary termination 6 6 — 273 441 257 918 662 410

Taxation (Tables)

Taxation (Tables)	12 Months Ended
	Dec. 31, 2020
Major components of tax expense (income) [abstract]
Disclosure Of Income Tax And Deferred Taxes Explanatory	Group 2020 2019 2018 £m £m £m Current tax: UK corporation tax on profit for the year 120 265 408 Adjustments in respect of prior years (24) (25) (20) Total current tax 96 240 388 Deferred tax: Charge for the year 34 46 16 Adjustments in respect of prior years 4 (7) (5) Total deferred tax 38 39 11 Tax on profit 134 279 399 The standard rate of UK corporation tax was 27% for banking entities and 19% for non-banking entities (2019: 27% for banking entities and 19% for non-banking entities; 2018: 27% for banking entities and 19% for non-banking entities) following the introduction of an 8% surcharge to be applied to banking companies from 1 January 2016. Taxation for other jurisdictions is calculated at the rates prevailing in the relevant jurisdictions. Finance Act 2016 introduced a reduction in the standard rate of corporation tax rate to 17% from 2020 but this was reversed in the UK Budget in March 2020. As a result, the standard rate of corporation tax remains at 19% and the effect of the increase of 2% over that expected at 31 December 2019 has been reflected in the opening deferred tax balance at 1 January 2020.
Schedule of Tax on Profit Before Tax Differs from Theoretical Amount that Arise Using Basic Corporation Tax Rate	Group 2020 2019 2018 £m £m £m Profit before tax 605 1,012 1,545 Tax calculated at a tax rate of 19% (2019: 19%, 2018: 19%) 115 192 294 Bank surcharge on profits 31 65 109 Non-deductible preference dividends paid 8 8 8 Non-deductible UK Bank Levy 19 24 20 Non-deductible conduct remediation, fines and penalties (4) 44 6 Other non-deductible costs and non-taxable income 25 31 30 Effect of change in tax rate on deferred tax provision 6 (14) (1) Tax relief on dividends in respect of other equity instruments (40) (39) (42) Adjustment to prior year provisions (26) (32) (25) Tax charge 134 279 399
Disclosure Of Movement In Current Tax Assets And Liabilities	Current tax assets and liabilities Movements in current tax assets and liabilities during the year were as follows: Group 2020 2019 £m £m Assets 200 153 Liabilities — — At 1 January 200 153 Income statement charge (96) (240) Other comprehensive income (charge)/credit 1 (4) Corporate income tax paid 159 292 Other movements — (1) 264 200 Assets 264 200 Liabilities — — At 31 December 264 200
Schedule of Deferred Taxes	The table below shows the deferred tax assets and liabilities including the movement in the deferred tax account during the year. Deferred tax balances are presented in the balance sheet after offsetting assets and liabilities where the Santander UK group has the legal right to offset and intends to settle on a net basis. Group Fair value of financial instruments Pension remeasurement Cash flow hedges Fair value Tax losses carried forward Accelerated tax depreciation Other temporary differences Total £m £m £m £m £m £m £m £m At 1 January 2020 (52) (96) (58) (8) 13 17 35 (149) Income statement (charge)/ credit (13) (63) — — 2 21 15 (38) Transfers/reclassifications — — 12 (1) — — (13) (2) Credited/(charged) to other comprehensive income — 133 (53) (2) — — — 78 At 31 December 2020 (65) (26) (99) (11) 15 38 37 (111) At 1 January 2019 (51) (183) (43) (13) 20 (6) 53 (223) Income statement (charge)/credit (1) (44) — — (7) 23 (10) (39) Transfers/reclassifications — — 22 5 — — (27) — Credited/(charged) to other comprehensive income — 131 (37) — — — 19 113 At 31 December 2019 (52) (96) (58) (8) 13 17 35 (149)

Dividends on Ordinary Shares (T

Dividends on Ordinary Shares (Tables)	12 Months Ended
	Dec. 31, 2020
Dividends On Ordinary Shares [abstract]
Disclosure of Dividends on Ordinary Shares Declared and Paid	Dividends on ordinary shares declared and paid in the year were as follows: Group Group 2020 2019 2018 2020 2019 2018 Pence per share Pence per share Pence per share £m £m £m In respect of current year – first interim 0.42 0.53 0.81 129 164 250 – second interim — 0.49 2.15 — 151 668 – third interim — — 0.71 — — 221 0.42 1.02 3.67 129 315 1,139

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of nature and extent of risks arising from financial instruments [abstract]
Notional Amounts and Fair Values of Derivatives	Group 2020 2019 Fair value Fair value Notional amount Assets Liabilities Notional amount Assets Liabilities £m £m £m £m £m £m Derivatives held for trading: Exchange rate contracts 14,951 395 418 14,149 134 200 Interest rate contracts 40,160 888 542 46,564 718 315 Equity and credit contracts 1,140 123 55 2,474 283 160 Total derivatives held for trading 56,251 1,406 1,015 63,187 1,135 675 Derivatives held for hedging Designated as fair value hedges: Exchange rate contracts 789 84 6 1,482 166 2 Interest rate contracts 93,748 1,225 1,885 94,550 1,022 1,488 Equity derivative contracts — — — — — — 94,537 1,309 1,891 96,032 1,188 1,490 Designated as cash flow hedges: Exchange rate contracts 27,020 1,978 409 28,502 2,023 462 Interest rate contracts 19,407 467 23 17,451 184 35 Equity derivative contracts — — — — — — 46,427 2,445 432 45,953 2,207 497 Total derivatives held for hedging 140,964 3,754 2,323 141,985 3,395 1,987 Derivative netting (1) (1,754) (1,754) (1,214) (1,214) Total derivatives 197,215 3,406 1,584 205,172 3,316 1,448 (1) Derivative netting excludes the effect of cash collateral, which is offset against the gross derivative position. The amount of cash collateral received that had been offset against the gross derivative assets was £330m (2019: £222m) and the amount of cash collateral paid that had been offset against the gross derivative liabilities was £651m (2019: £629m).
Disclosure of Analysis of the Notional and Fair Values of Derivatives by Trading and Settlement Method	The table below analyses the notional and fair values of derivatives by trading and settlement method. Notional Traded over the counter Asset Liability Traded on recognised exchanges Settled by central counterparties Not settled by central counterparties Total Traded on recognised exchanges Traded over the counter Traded on recognised exchanges Traded over the counter 2020 £m £m £m £m £m £m £m £m Exchange rate contracts — — 42,760 42,760 — 2,457 — 833 Interest rate contracts — 144,343 8,972 153,315 — 826 — 696 Equity and credit contracts — — 1,140 1,140 — 123 — 55 — 144,343 52,872 197,215 — 3,406 — 1,584 2019 Exchange rate contracts — — 44,133 44,133 — 2,324 — 663 Interest rate contracts — 131,538 27,027 158,565 — 709 — 625 Equity and credit contracts — — 2,474 2,474 — 283 — 160 — 131,538 73,634 205,172 — 3,316 — 1,448
Disclosure of Maturity Profile and Average Price/Rate of Hedging Instruments Used in Hedging Strategies	The following table sets out the maturity profile and average price/rate of the hedging instruments used in the Santander UK group’s hedging strategies: Group 2020 Hedging Instruments ≤1 month >1 and ≤3 months >3 and ≤12 months >1 and ≤5 years >5 years Total Fair value hedges: Interest rate risk Interest rate contracts- Nominal Amount (£m) 2,429 7,617 27,791 47,749 7,889 93,475 Average fixed interest rate - GBP 0.69 % 0.65 % 0.82 % 0.73 % 3.61 % Average fixed interest rate - EUR 1.18 % 0.23 % 3.02 % 0.98 % 2.34 % Average fixed interest rate - USD 1.87 % 1.72 % 2.89 % 2.49 % 4.16 % Interest rate/FX risk Exchange rate contracts - Nominal Amount (£m) — — 132 461 196 789 Interest rate contracts - Nominal Amount (£m) — — — 236 37 273 Average GBP - EUR exchange rate — — 1.1405 1.1700 1.1669 Average GBP - USD exchange rate — — — — — Average fixed interest rate - EUR — — 4.64 % 1.78 % 3.56 % Average fixed interest rate - USD — — — — — Cash flow hedges: Interest rate risk Interest rate contracts – Notional amount (£m) — 897 2,528 7,964 1,061 12,450 Average fixed interest rate - GBP — 0.46 % 0.57 % 1.45 % 1.33 % FX risk Exchange rate contracts- Nominal Amount (£m) 1,439 2,015 3,877 7,113 1,119 15,563 Interest rate contracts- Nominal Amount (£m) — — — 366 — 366 Average GBP - JPY exchange rate — 137.9769 135.6073 132.2714 — Average GBP - EUR exchange rate — — — 1.1629 1.1787 Average GBP - USD exchange rate 1.2931 1.3164 1.3231 1.3039 — Interest rate/FX risk Exchange rate contracts - Nominal Amount (£m) 732 — 2,583 6,550 1,592 11,457 Interest rate contracts - Nominal Amount (£m) 732 — 882 4,062 915 6,591 Average GBP - EUR exchange rate — — 1.3543 1.2525 1.1965 Average GBP - USD exchange rate 1.4649 — — 1.6085 1.3811 Average fixed interest rate – GBP 2.01 % — 3.18 % 2.48 % 3.39 % 2019 Fair value hedges: Interest rate risk Interest rate contracts- Nominal Amount (£m) 4,354 5,804 27,405 43,652 13,099 94,314 Average fixed interest rate - GBP 0.77 % 0.90 % 0.88 % 1.33 % 3.00 % Average fixed interest rate - EUR (0.41) % 0.29 % 2.21 % 1.36 % 2.36 % Average fixed interest rate - USD — 1.54 % 1.99 % 2.69 % 4.56 % Interest rate/FX risk Exchange rate contracts - Nominal Amount (£m) — 755 — 317 410 1,482 Interest rate contracts - Nominal Amount (£m) — — — 18 218 236 Average GBP - EUR exchange rate — — — 1.1781 1.1603 Average GBP - USD exchange rate — 1.5110 — — — Average fixed interest rate - EUR — — — 3.52 % 2.12 % Average fixed interest rate - USD — 2.38 % — — — Cash flow hedges: Interest rate risk Interest rate contracts - Nominal Amount (£m) — 339 1,066 4,671 500 6,576 Average fixed interest rate - GBP (%) — 0.76 % 0.82 % 1.46 % 0.40 % — FX risk Exchange rate contracts- Nominal Amount (£m) 1,187 2,119 3,758 5,217 — 12,281 Interest rate contracts- Nominal Amount (£m) — — — 755 — 755 Average GBP - JPY exchange rate — 145.9275 143.0857 140.8152 — Average GBP - EUR exchange rate — 1.1444 1.1167 1.1526 — Average GBP - USD exchange rate 1.2856 1.2624 1.2925 1.2991 — Interest rate/FX risk Exchange rate contracts - Nominal Amount (£m) 812 — 3,367 8,009 4,033 16,221 Interest rate contracts - Nominal Amount (£m) — — 3,121 4,829 2,170 10,120 Average GBP - EUR exchange rate 1.2742 — 1.1689 1.3114 1.2090 Average GBP - USD exchange rate — — 1.5357 1.5811 1.4499 Average fixed interest rate - GBP (%) 2.49 % — 2.16 % 2.87 % 2.96 %
Net Gains or Losses Arising From Hedges Included in Operating Income	Net gains or losses arising from fair value and cash flow hedges included in other operating income Group 2020 2019 2018 £m £m £m Fair value hedging: (Losses)/gains on hedging instruments (299) (360) 4 Gains/(losses) on hedged items attributable to hedged risks 365 414 75 Fair value hedging ineffectiveness 66 54 79 Cash flow hedging ineffectiveness (46) (46) (45) 20 8 34
Disclosure Of Information About Hedging Ineffectiveness by Risk Category Explanatory	Hedge ineffectiveness can be analysed by risk category as follows: Group 2020 2019 Change in FV of hedging instruments Change in FV of hedged items Recognised in income statement Change in FV of hedging instruments Change in FV of hedged items Recognised in income statement £m £m £m £m £m £m Fair value hedges: Interest rate risk (358) 385 27 (264) 284 20 Interest rate/FX risk 59 (20) 39 (96) 130 34 (299) 365 66 (360) 414 54 Group 2020 2019 Hedging Instruments Hedging Instruments Income statement line item affected by reclassification Change in FV Recognised in OCI Recognised in Income Statement Reclassified from reserves to income Change in FV Recognised in OCI Recognised in Income Statement Reclassified from reserves to income £m £m £m £m £m £m £m £m Cash flow hedges: Interest rate risk Net interest income 185 (179) 6 33 34 (33) 1 13 FX risk Net interest income/other operating income (42) 38 (4) 2 (333) 329 (4) (316) Equity risk Operating expenses — — — — — — — — Interest rate/FX risk Net interest income/other operating income 782 (830) (48) 773 (604) 561 (43) (709) 925 (971) (46) 808 (903) 857 (46) (1,012)
Disclosure Of Reconciliation by Risk Category of Components of Equity and Analysis of Other Comprehensive Income Items Resulting from Hedge Accounting Explanatory	Group 2020 2019 £m £m Balance at 1 January 481 326 Effective portion of changes in fair value: – Interest rate risk 179 33 – Foreign currency risk (38) (329) – Equity risk — — – Interest rate/foreign currency risk 830 (561) 971 (857) Income statement transfers – Interest rate risk (33) (13) – Foreign currency risk (2) 316 – Equity risk — — – Interest rate/foreign currency risk (773) 709 (808) 1,012 Balances at 31 December 644 481 Hedged exposures Santander UK hedges its exposures to various risks, including interest rate risk and foreign currency risk, as set out in the following table. Group 2020 2019 Accumulated amount of FV hedge adjustments Change in value to calculate hedge ineffectiveness Accumulated amount of FV hedge adjustments Change in value to calculate hedge ineffectiveness Carrying value Hedged item Portfolio hedge of interest rate risks Of which Discontinued hedges Carrying value Hedged item Portfolio hedge of interest rate risks Of which Discontinued hedges £m £m £m £m £m £m £m £m £m £m Fair value hedges Interest rate risk: Loans and advances to customers 54,118 — 1,189 892 334 43,098 — 870 630 258 Other financial assets at amortised cost 772 — 36 13 121 6,627 — 142 121 83 Reverse repos – non trading 12,149 — 1 — 3 17,121 — (2) — (2) Other financial assets at FVOCI 5,129 155 — 74 88 5,944 102 — 82 125 Deposits by customers (7,309) (158) (10) (77) (73) (9,944) (85) 4 (11) (110) Deposits by banks — — — — — (517) (16) — (17) (1) Debt securities in issue (5,885) (375) (137) (239) (61) (8,099) (303) (166) (300) (50) Subordinated liabilities (636) (185) (41) (166) (27) (707) (181) (48) (204) (19) Interest rate/FX risk: Other financial assets at FVOCI 299 5 — — 15 241 3 — — (4) Debt securities in issue (621) (94) — (76) (34) (1,396) (135) — (122) 136 Subordinated liabilities 3 3 — 3 (1) 7 7 — 7 (2) 58,019 (649) 1,038 424 365 52,375 (608) 800 186 414 Group 2020 2019 Change in value to calculate hedge ineffectiveness Cash flow hedge reserve Balances on cash flow hedge reserve for discontinued hedges Change in value to calculate hedge ineffectiveness Cash flow hedge reserve Balances on cash flow hedge reserve for discontinued hedges Hedged item balance sheet line item £m £m £m £m £m £m Cash flow hedges: Interest rate risk: Loans and advances to customers (183) 165 1 (34) 21 (11) Loans and advances to banks (2) 1 — (2) — — Reverse repurchase agreements – non trading (2) 1 — — — — Deposits by banks 7 (2) — 3 (2) — Debt securities in issue — — — — — — Repurchase agreements – non trading 1 (1) — — — — FX risk: Other financial assets at FVOCI 40 6 — (122) 3 — Not applicable – highly probable forecast transactions 33 3 — 267 2 — Deposits by customers (5) 14 — 3 — — Deposits by banks — — — 4 — — Debt securities in issue (15) (60) — 177 (3) — Repurchase agreements – non trading (15) — — — — — Equity risk: Other liabilities — — — — — (1) Interest rate/FX risk: Debt securities in issue/loans and advances to customers (569) 236 (2) 630 280 20 Deposits by customers (132) 87 — — — — Subordinated liabilities/loans and advances to customers (130) 194 — (69) 180 — (972) 644 (1) 857 481 8

Other Financial Assets At Fai_2

Other Financial Assets At Fair Value Through Profit Or Loss (Tables)	12 Months Ended
	Dec. 31, 2020
Financial assets at fair value through profit or loss [abstract]
Summary of Other Financial Assets at Fair Value Through Profit or Loss	Group 2020 2019 £m £m Loans and advances to customers: Loans to housing associations 13 12 Other loans 86 80 99 92 Debt securities 109 294 Equity securities — — Reverse repurchase agreements - non trading — — 208 386

Loans and Advances to Custome_2

Loans and Advances to Customers (Tables)	12 Months Ended
	Dec. 31, 2020
Net Loans and Advances to Customers [abstract]
Summary of Net Loans and Advances to Customers	Group 2020 2019 £m £m Loans secured on residential properties 166,714 165,356 Corporate loans 24,474 27,043 Finance leases 6,554 6,264 Secured advances — — Other unsecured loans 9,933 7,096 Amounts due from fellow Banco Santander subsidiaries and joint ventures 2,425 2,366 Amounts due from Santander UK Group Holdings plc 7 8 Amounts due from subsidiaries — — Loans and advances to customers 210,107 208,133 Credit impairment loss allowances on loans and advances to customers (1,303) (785) RV and voluntary termination provisions on finance leases (54) (61) Net loans and advances to customers 208,750 207,287
Disclosure of finance lease and hire purchase contract receivables	Finance lease and hire purchase contract receivables may be analysed as follows: Group 2020 2019 Gross investment Unearned finance income Net investment Gross investment Unearned finance income Net investment £m £m £m £m £m £m No later than one year 3,468 (297) 3,171 3,060 (303) 2,757 Later than one year and not later than two years 1,829 (173) 1,656 2,046 (207) 1,839 Later than two years and not later than three years 1,099 (106) 993 1,157 (119) 1,038 Later than three years and not later than four years 575 (55) 520 541 (54) 487 Later than four years and not later than five years 231 (25) 206 41 (4) 37 Later than five years 8 — 8 147 (41) 106 7,210 (656) 6,554 6,992 (728) 6,264

Securitisations and Covered B_2

Securitisations and Covered Bonds (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of securitisations and covered bonds [abstract]
Analysis of Securitisations and Covered Bonds	Gross assets External notes in issue Notes issued to Santander UK plc/subsidiaries as collateral 2020 2019 2020 2019 2020 2019 £m £m £m £m £m £m Mortgage-backed master trust structures: – Holmes 3,073 4,262 829 1,931 334 463 – Fosse 2,258 3,708 290 295 1,402 1,404 – Langton 2,782 3,076 — — 2,355 2,354 8,113 11,046 1,119 2,226 4,091 4,221 Other asset-backed securitisation structures: – Motor 189 490 104 324 97 197 – Auto ABS UK Loans 1,460 1,532 1,107 1,229 361 368 1,649 2,022 1,211 1,553 458 565 Total securitisation programmes 9,762 13,068 2,330 3,779 4,549 4,786 Covered bond programmes – Euro 35bn Global Covered Bond Programme 23,670 23,323 19,285 19,004 — — Total securitisation and covered bond programmes 33,432 36,391 21,615 22,783 4,549 4,786 Less: held by Santander UK group: – Euro 35bn Global Covered Bond Programme — — Total securitisation and covered bond programmes (See Note 26) 21,615 22,783
Summary of Issuances and Redemptions of Securitisation and Covered Bond Programme	The following table sets out the internal and external issuances and redemptions in 2020 and 2019 for each securitisation and covered bond programme. Internal issuances External issuances Internal redemptions External redemptions 2020 2019 2020 2019 2020 2019 2020 2019 £bn £bn £bn £bn £bn £bn £bn £bn Mortgage-backed master trust structures: – Holmes — — — — 0.3 — 0.9 1.1 – Fosse — 1.4 — 0.1 — — — — – Langton — — — — — — — — Other asset-backed securitisation structures: – Motor — — — — 0.1 0.2 0.2 0.4 – Auto ABS UK Loans — 0.1 0.3 0.2 — 0.1 0.1 0.2 Covered bond programme — — 3.0 2.9 — 0.5 2.7 1.5 — 1.5 3.3 3.2 0.4 0.8 3.9 3.2

Transfers of Financial Assets_2

Transfers of Financial Assets Not Qualifying for Derecognition (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of transferred financial assets that are not derecognised in their entirety [abstract]
Carrying Amount of Financial Assets that Did Not Qualify for Derecognition and their Associated Financial Liabilities	The following table analyses the carrying amount of financial assets that did not qualify for derecognition and their associated financial liabilities: Group 2020 2019 Assets Liabilities Assets Liabilities Nature of transaction £m £m £m £m Sale and repurchase agreements 1,597 (1,340) 7,592 (6,739) Securities lending agreements 918 (752) 195 (143) Securitisations (See Notes 14 and 26) 6,980 (2,330) 9,992 (3,779) 9,495 (4,422) 17,779 (10,661)

Reverse Repurchase Agreements_2

Reverse Repurchase Agreements - Non Trading (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of reserve repurchase agreements non-trading [abstract]
Summary of Reverse Repurchase Agreements - Non Trading	Group 2020 2019 £m £m Agreements with banks 1,258 2,161 Agreements with customers 18,341 21,475 19,599 23,636

Other Financial Assets At Amo_2

Other Financial Assets At Amortised Cost (Tables)	12 Months Ended
	Dec. 31, 2020
Financial assets at fair value through profit or loss [abstract]
Summary of Other Financial Assets at Amortised Cost	Group 2020 2019 £m £m Asset backed securities 491 532 Debt securities 672 6,524 1,163 7,056

Financial Assets At Fair Valu_2

Financial Assets At Fair Value Through Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2020
Financial assets at fair value through other comprehensive income [abstract]
Summary of Financial Assets at Fair Value Through Other Comprehensive Income	Group 2020 2019 £m £m Debt securities 8,929 9,691 Loans and advances to customers 21 56 8,950 9,747

Interests in Other Entities (Ta

Interests in Other Entities (Tables)	12 Months Ended
	Dec. 31, 2020
Investments accounted for using equity method [abstract]
Schedule of Interests in Other Entities	Group 2020 2019 £m £m Subsidiaries — — Joint ventures 172 117 0 172 117
Schedule of Subsidiaries With Significant Non-Controlling Interests	2020 2019 £m £m Profit attributable to non-controlling interests 19 19 Accumulated non-controlling interests of the subsidiary 162 160 Dividends paid to non-controlling interests 15 12 Summarised financial information: Total assets 3,451 3,228 Total liabilities 3,127 2,905 Profit for the year 38 40 Total comprehensive income for the year 33 40

Intangible Assets (Tables)

Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of detailed information about intangible assets [abstract]
Schedule of Goodwill	a) Goodwill Group Cost Accumulated impairment Net book value £m £m £m At 31 December 2019, 1 January 2020 and 31 December 2020 1,269 (66) 1,203
Schedule of Goodwill for Cash Generating Units	Goodwill Discount rate Growth rate beyond initial cash flow projections 2020 2019 2020 2019 2020 2019 CGU £m £m % % % % Personal financial services 1,169 1,169 13.6 9.8 1.6 1.6 Private banking 30 30 8.9 9.8 1.6 1.6 Other 4 4 13.6 9.8 1.6 1.6 1,203 1,203
Reasonably Possible Changes In Key Cash Flow Projection Assumptions	Reasonably possible changes in key assumptions CGU Input Key assumptions Associated risks Reasonably possible change Personal Financial Services – Cash flow projections – BoE Bank Rate – UK house price growth – UK mortgage loan market growth – UK unemployment rate – Position in the market – Regulatory capital levels. – Uncertain market outlook – Persistent low interest rate environment – Customer remediation and regulatory action outcomes – Uncertain regulatory capital requirements. – Cash flow projections decrease by 5%. – Discount rate – Discount rate used is a reasonable estimate of a suitable market rate for the profile of the business. – Market rates of interest rise. – Discount rate increases by 100 basis points. Carrying value Value in use Headroom Increase in post tax discount rate Decrease in cash flows CGU £m £m £m bps % Personal Financial Services 6,758 8,602 1,844 239 22
Disclosure of Intangible Assets	b) Other intangibles Group Accumulated Net book value Cost £m £m £m At 1 January 2020 1,249 (686) 563 Additions 102 — 102 Disposals (47) 47 — Charge — (197) (197) Impairment — (25) (25) At 31 December 2020 1,304 (861) 443 At 1 January 2019 1,090 (485) 605 Additions 173 — 173 Disposals (14) — (14) Charge — (191) (191) Impairment — (10) (10) At 31 December 2019 1,249 (686) 563

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	12 Months Ended
	Dec. 31, 2020
Property, plant and equipment [abstract]
Disclosure of detailed information about investment property	Group Property Office fixtures and equipment Computer software Operating lease assets Right-of-use assets Total (3) £m £m £m £m £m £m Cost: At 1 January 2020 1,270 1,436 439 738 212 4,095 Additions 61 43 2 185 8 299 Disposals (59) (104) (5) (203) (2) (373) At 31 December 2020 1,272 1,375 436 720 218 4,021 Accumulated depreciation: At 1 January 2020 454 1,016 434 164 60 2,128 Charge for the year (2) 79 111 — 92 58 340 Disposals (44) (59) — (78) — (181) At 31 December 2020 489 1,068 434 178 118 2,287 Net book value 783 307 2 542 100 1,734 Cost: At 1 January 2019 (1) 1,291 1,398 438 604 209 3,940 Additions 5 70 1 251 4 331 Disposals (26) (32) — (117) (1) (176) At 31 December 2019 1,270 1,436 439 738 212 4,095 Accumulated depreciation: At 1 January 2019 429 903 434 134 — 1,900 Charge for the year 39 161 — 81 60 341 Disposals (14) (48) — (51) — (113) At 31 December 2019 454 1,016 434 164 60 2,128 Net book value 816 420 5 574 152 1,967 (1) Represents the value of the right-of-use assets, principally premises, recognised on 1 January 2019 upon adoption of IFRS 16. (2) Following a review of the estimated useful lives of property as part of Santander UK's transformation program, the charge for the year includes accelerated property depreciation of £9m (2019: £2m). (3) Property, plant and equipment includes assets under construction of £55m (2019: £16m).

Other Financial Liabilities a_2

Other Financial Liabilities at Fair Value Through Profit or Loss (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of financial liabilities [abstract]
Summary of Other Financial Liabilities at Fair Value Through Profit or Loss	Group 2020 2019 £m £m US$30bn Euro Medium Term Note Programme 102 159 Structured Notes Programmes 805 809 Eurobonds 150 137 Structured deposits 375 435 Collateral and associated financial guarantees 2 173 Repurchase agreements - non trading — — 1,434 1,713

Deposits by Customers (Tables)

Deposits by Customers (Tables)	12 Months Ended
	Dec. 31, 2020
Deposits from customers [abstract]
Schedule of Deposits by Customers	Group 2020 2019 £m £m Demand and time deposits (1) 185,879 171,736 Amounts due to other Santander UK Group Holdings plc subsidiaries 59 44 Amounts due to Santander UK Group Holdings plc (2) 7,883 8,869 Amounts due to fellow Banco Santander subsidiaries and joint ventures 1,314 1,234 195,135 181,883 (1) Includes equity index-linked deposits of £577m (2019: £1,139m). The capital amount guaranteed/protected and the amount of return guaranteed in respect of the equity index-linked deposits were £577m and £2m (2019: £1,139m and £18m) respectively. (2) Includes downstreamed funding from our immediate parent company Santander UK Group Holdings plc.

Deposits by Banks (Tables)

Deposits by Banks (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of deposits from banks [abstract]
Schedule of Deposits by Banks	Group 2020 2019 £m £m Items in the course of transmission 375 337 Deposits held as collateral 2,063 2,169 Other deposits (1) 18,519 11,847 Amounts due to Santander UK subsidiaries 1 — 20,958 14,353 (1) Includes drawdown from the TFS of £6.3bn (2019: £10.8bn) and drawdown from the TFSME of £11.7bn (2019:£nil).

Repurchase Agreements - Non T_2

Repurchase Agreements - Non Trading (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure Of Repurchase Agreement [Abstract]
Summary of Repurchase Agreements - Non Trading	Group 2020 2019 £m £m Agreements with banks 6,358 10,227 Agreements with customers 9,490 8,059 15,848 18,286

Debt Securities in Issue (Table

Debt Securities in Issue (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Securities In Issue [Abstract]
Summary of Debt Securities in Issue	Group 2020 2019 £m £m Medium-term notes: – US$30bn Euro Medium Term Note Programme 1,694 4,679 – Euro 30bn Euro Medium Term Note Programme 388 1,896 – US SEC-registered Debt Programme – Santander UK plc 4,723 5,891 – Euro 750m Senior Unsecured Notes 673 — – US$1bn Senior Unsecured Notes 734 — 8,212 12,466 Euro 35bn Global Covered Bond Programme (See Note 14) 19,285 19,004 US$20bn Commercial Paper Programmes 2,824 3,014 Certificates of deposit 2,858 2,806 Credit linked notes 57 60 Securitisation programmes (See Note 14) 2,330 3,779 35,566 41,129

Subordinated Liabilities (Table

Subordinated Liabilities (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of Subordinated Liabilities [Abstract]
Schedule of Subordinated Liabilities	Group 2020 2019 £m £m £325m Sterling preference shares 344 344 Undated subordinated liabilities 557 581 Dated subordinated liabilities 1,655 2,603 2,556 3,528
Disclosure Of Undated Subordinated Liabilities Explanatory	Group 2020 2019 First call date £m £m 10.0625% Exchangeable capital securities n/a 205 205 7.375% 20 Year Step-up perpetual callable subordinated notes 2020 — 15 7.125% 30 Year Step-up perpetual callable subordinated notes 2030 352 361 557 581
Disclosure Of Dated Subordinated Liabilities Explanatory	Group 2020 2019 Maturity £m £m 5% Subordinated notes (US$1,500m) 2023 542 1,132 4.75% Subordinated notes (US$1,000m) 2025 536 763 7.95% Subordinated notes (US$1,000m) 2029 242 280 6.50% Subordinated notes 2030 31 40 5.875% Subordinated notes 2031 10 9 5.625% Subordinated notes (US$500m) 2045 294 379 1,655 2,603

Other Liabilities (Tables)

Other Liabilities (Tables) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Subclassifications of assets, liabilities and equities [abstract]
Disclosure Of Other Liabilities Explanatory	Group 2020 2019 £m £m Lease liabilities 97 137 Other 2,240 2,207 2,337 2,344
Lease liabilities	£ 97	£ 137
Other	2,240	2,207
Other liabilities	£ 2,337	£ 2,344

Provisions (Tables)

Provisions (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of other provisions [abstract]
Summary of Provisions Reconciliation	Group Conduct remediation PPI Other products Bank Levy Property Off balance sheet ECL Regulatory and other Total £m £m £m £m £m £m £m At 1 January 2020 189 25 46 59 78 175 572 Additional provisions (See Note 8) — — 72 9 — 208 289 Provisions released (See Note 8) — (15) — (6) (3) (1) (25) Utilisation and other (1) (113) (2) (94) (16) — (157) (382) Recharge (2) — — 10 — — — 10 At 31 December 2020 76 8 34 46 75 225 464 To be settled: – Within 12 months 76 2 34 25 75 111 323 – In more than 12 months — 6 — 21 — 114 141 76 8 34 46 75 225 464

Retirement Benefit Plans (Table

Retirement Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of net defined benefit liability (asset) [abstract]
Summary of Retirement Benefit Plans	The amounts recognised in the balance sheet were as follows: Group 2020 2019 £m £m Assets/(liabilities) Funded defined benefit pension scheme - surplus 495 669 Funded defined benefit pension scheme - deficit (361) (239) Unfunded pension and post retirement medical benefits (42) (41) Total net assets 92 389
Disclosure of Pension Remeasurements Included in Other Comprehensive income	Remeasurement losses/(gains) recognised in other comprehensive income during the year were as follows: Group 2020 2019 2018 £m £m £m Pension remeasurement 505 522 (470)
Disclosure of Total Defined Benefit Plan Amount Charged to the Income Statement	The total amount charged to the income statement was as follows: Group 2020 2019 2018 £m £m £m Net interest income (10) (23) (7) Current service cost 36 34 41 Past service and GMP costs 1 1 41 Administration costs 8 8 8 35 20 83
Summary of Amounts Recognised in Other Comprehensive Income	The amounts recognised in other comprehensive income were as follows: Group 2020 2019 2018 £m £m £m Return on plan assets (excluding amounts included in net interest expense) (1,328) (855) 246 Actuarial losses/(gains) arising from changes in demographic assumptions 34 42 (56) Actuarial (gains)/losses arising from experience adjustments (141) (42) 15 Actuarial losses/(gains) arising from changes in financial assumptions 1,940 1,377 (675) Pension remeasurement 505 522 (470)
Disclosure Of Movements In The Present Value Of Defined Benefit Obligations	Movements in the present value of defined benefit scheme obligations were as follows: Group 2020 2019 £m £m At 1 January (12,158) (10,804) Current service cost paid by Santander UK plc (24) (22) Current service cost paid by subsidiaries (12) (12) Interest cost (253) (308) Employer salary sacrifice contributions (2) (9) Past service cost (1) (1) Remeasurement due to actuarial movements arising from: – Changes in demographic assumptions (34) (42) – Experience adjustments 141 42 – Changes in financial assumptions (1,940) (1,377) Benefits paid 396 375 At 31 December (13,887) (12,158)
Disclosure of Movements in Fair Value of Schemes' Assets	Movements in the fair value of the schemes’ assets were as follows: Group 2020 2019 £m £m At 1 January 12,547 11,532 Interest income 263 331 Contributions paid by employer and scheme members 245 212 Contributions paid by fellow Banco Santander subsidiaries — — Administration costs paid (8) (8) Return on plan assets (excluding amounts included in net interest expense) 1,328 855 Benefits paid (396) (375) At 31 December 13,979 12,547
Summary of Composition and Fair Value of Plan Assets	The composition and fair value of the schemes’ assets by category was: Group Quoted prices in active markets Prices not quoted in active markets Total 2020 £m % £m % £m % UK equities 40 — — — 40 — Overseas equities 1,271 9 1,004 7 2,275 16 Corporate bonds 1,121 8 457 3 1,578 11 Government fixed interest bonds 1,618 12 — — 1,618 12 Government index-linked bonds 6,695 48 — — 6,695 48 Property — — 1,454 10 1,454 10 Derivatives — — 312 2 312 2 Cash — — 1,161 8 1,161 8 Repurchase agreements (1) — — (2,198) (15) (2,198) (15) Other — — 1,044 8 1,044 8 10,745 77 3,234 23 13,979 100 2019 UK equities 128 1 — — 128 1 Overseas equities 1,742 14 933 7 2,675 21 Corporate bonds 1,333 11 444 4 1,777 15 Government fixed interest bonds 2,710 22 — — 2,710 22 Government index-linked bonds 4,543 35 — — 4,543 35 Property — — 1,332 11 1,332 11 Derivatives — — 94 1 94 1 Cash — — 984 8 984 8 Repurchase agreements (1) — — (3,263) (26) (3,263) (26) Other — — 1,567 12 1,567 12 10,456 83 2,091 17 12,547 100 (1) Sale and repurchase agreements net of purchase and resale agreements.
Summary of Principal Actuarial Assumptions Used for Defined Benefit Schemes	The principal actuarial assumptions used for the defined benefit schemes were: Group 2020 2019 2018 % % % To determine benefit obligations: – Discount rate for scheme liabilities 1.3 2.1 2.9 – General price inflation 3.0 3.0 3.2 – General salary increase 1.0 1.0 1.0 – Expected rate of pension increase 2.9 2.9 2.9 Years Years Years Longevity at 60 for current pensioners, on the valuation date: – Males 27.5 27.3 27.3 – Females 30.0 29.8 30.1 Longevity at 60 for future pensioners currently aged 40, on the valuation date: – Males 29.0 28.9 28.7 – Females 31.5 31.3 31.6
Summary of Actuarial Assumption Sensitivities	The sensitivity analyses below have been determined based on reasonably possible changes of the respective assumptions occurring at the end of the reporting period, while holding all other assumptions constant. (Decrease)/increase 2020 2019 Assumption Change in pension obligation at period end from £m £m Discount rate 25 bps increase (662) (564) General price inflation 25 bps increase 365 407 Mortality Each additional year of longevity assumed 515 419
Disclosure of Defined Benefit Plan Expected Future Benefit Payments	The benefits expected to be paid in each of the next five years, and in the aggregate for the five years thereafter are: Year ending 31 December £m 2021 359 2022 327 2023 336 2024 359 2025 382 Five years ending 2030 2,139

Contingent Liabilities and Co_2

Contingent Liabilities and Commitments (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of contingent liabilities [abstract]
Summary of Contingent Liabilities and Commitments	Group 2020 2019 £m £m Guarantees given to third parties 939 1,198 Formal standby facilities, credit lines and other commitments 42,221 40,397 43,160 41,595

Share Capital (Tables)

Share Capital (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of classes of share capital [abstract]
Schedule of Share Capital Issued and Fully Paid	Group Ordinary shares £300m Preference shares of £1,000 each Total Issued and fully paid share capital No. £m No. £m £m At 1 January 2020 31,051,768,866 3,105 — — 3,105 Redemption of preference shares — — — — — At 31 December 2020 31,051,768,866 3,105 — — 3,105 At 1 January 2018, 31 December 2018 and 1 January 2019 31,051,768,866 3,105 13,780 14 3,119 Redemption of preference shares — — (13,780) (14) (14) At 31 December 2019 31,051,768,866 3,105 — — 3,105
Summary of Share Premium	Group 2020 2019 Share premium £m £m At 1 January and 31 December 5,620 5,620

Other Equity Instruments (Table

Other Equity Instruments (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of classes of share capital [abstract]
Summary of other equity instruments	Group Interest rate 2020 2019 % Next call date £m £m £300m Step-up Callable Perpetual Reserve Capital Instruments 7.037 February 2026 235 235 AT1 securities: - £500m Fixed Rate Reset Perpetual AT1 Capital Securities 6.75 June 2024 496 496 - £750m Fixed Rate Reset Perpetual AT1 Capital Securities 7.375 June 2022 750 750 - £500m Fixed Rate Reset Perpetual AT1 Capital Securities 5.18 March 2021 210 210 - £500m Fixed Rate Reset Perpetual AT1 Capital Securities 6.30 March 2025 500 500 2,191 2,191

Non-Controlling Interests (Tabl

Non-Controlling Interests (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure Of Non Controlling Interests [Abstract]
Schedule of Amounts Attributable to Non-Controlling Interests	2020 2019 £m £m PSA Finance UK Limited 162 160 162 160

Changes in Liabilities Arisin_2

Changes in Liabilities Arising from Financing Activities (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of reconciliation of liabilities arising from financing activities [abstract]
Summary of Changes in Liabilities Arising from Financing Activities	The table below shows the changes in liabilities arising from financing activities. Group 2020 2019 Balance sheet line item Balance sheet line item Debt securities in issue Subordinated liabilities Other equity instruments Dividends paid Total Debt securities in issue Subordinated liabilities Other equity instruments Dividends paid Total £m £m £m £m £m £m £m £m £m £m At 1 January 41,129 3,528 2,191 — 46,848 46,692 3,601 1,991 — 52,284 Cash flows from financing activities (7,201) (659) — (292) (8,152) (3,839) — 196 (469) (4,112) Cash flows from operating activities 1,201 — — — 1,201 (529) — — — (529) Non-cash changes: – Unrealised foreign exchange 376 22 — — 398 (1,320) (50) — — (1,370) – Other changes 61 (335) — 292 18 125 (23) 4 469 575 At 31 December 35,566 2,556 2,191 — 40,313 41,129 3,528 2,191 — 46,848

Assets Charged As Security Fo_2

Assets Charged As Security For Liabilities And Collateral Accepted As Security For Assets (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of assets pledged as security [abstract]
Financial Assets Charged as Security Under On-Balance Sheet and Off-Balance	The financial assets below are analysed between those assets accounted for on-balance sheet and off-balance sheet. Group 2020 2019 £m £m On-balance sheet: Cash and balances at central banks 985 1,080 Loans and advances to banks 550 403 Loans and advances to customers – securitisations and covered bonds (See Note 14) 31,138 36,225 Loans and advances to customers – other 23,655 16,282 Other financial assets at amortised cost 648 3,026 Financial assets at fair value through other comprehensive income 5,581 6,009 Total on-balance sheet 62,557 63,025 Total off-balance sheet 24,701 15,098
Schedule of Collateral Held as Security For Assets	The collateral held as security for assets, analysed between those liabilities accounted for on balance sheet and off-balance sheet, was: Group 2020 2019 £m £m On-balance sheet: Deposits by banks 2,063 2,169 Total on-balance sheet 2,063 2,169 Total off-balance sheet 30,510 25,120

Share-Based Compensation (Table

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of terms and conditions of share-based payment arrangement [abstract]
Summary of Movement in Share Options	The table below summarises movements in the number of options, and changes in weighted average exercise price over the same period. 2020 2019 Number of options Weighted average exercise price Number of options Weighted average exercise price ‘000 £ ‘000 £ Outstanding at 1 January 23,373 3.03 26,838 3.12 Granted 11,642 1.65 9,594 2.83 Exercised (860) 2.75 (7,978) 2.83 Forfeited/expired (12,993) 2.96 (5,081) 3.42 Outstanding at 31 December 21,162 2.32 23,373 3.03 Exercisable at 31 December 1,805 3.59 2,519 3.62
Summary of Range of Exercise Prices and Weighted Average Remaining Contractual Life of the Options Outstanding	The following table summarises the range of exercise prices and weighted average remaining contractual life of the options at 31 December 2020 and 2019. 2020 2019 Range of exercise prices Weighted average remaining contractual life Weighted average exercise price Weighted average remaining contractual life Weighted average exercise price Years £ Years £ £1 to £2 4 1.65 — — £2 to £3 2 2.81 3 2.80 £3 to £4 1 3.38 2 3.38 £4 to £5 1 4.02 2 4.13
Summary of Movement in Value of Conditional Awards in LTIPs	The following table summarises the movement in the value of conditional awards in the LTIP in 2020 and 2019: 2015 LTIP 2020 2019 £000 £000 Outstanding at 1 January — 6,374 Payments made — (4,578) Forfeited/cancelled — (1,796) Outstanding at 31 December — —

Transactions With Directors a_2

Transactions With Directors and Other Key Management Personnel (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure Of Remuneration Of Directors And Other Key Management Personnel [Abstract]
Summary of Remuneration of the Directors and Other Key Management Personnel	The remuneration of the Directors and Other Key Management Personnel of the Santander UK group is set out in aggregate below. 2020 2019 2018 Directors’ remuneration £ £ £ Salaries and fees 5,353,980 5,025,665 5,028,434 Performance-related payments (1) 1,458,911 3,864,965 5,194,317 Other fixed remuneration (pension and other allowances & non-cash benefits) 1,107,348 1,367,069 1,467,011 Expenses 6,772 42,526 25,198 Total remuneration 7,927,011 10,300,225 11,714,960 2020 2019 2018 Directors' and Other Key Management Personnel compensation £ £ £ Short-term employee benefits (2) 18,432,698 21,925,975 24,445,189 Post-employment benefits (3) 2,084,645 3,590,466 2,399,261 Total Compensation 20,517,343 25,516,441 26,844,450 (1) In line with the Code, a proportion of the performance-related payment was deferred. Further details can be found in Note 37. (2) In addition to the remuneration in the table above, grants of shares in Banco Santander SA were made as buy-outs of deferred performance-related payments of shares in connection with previous employment for two individuals. One to a director of £1,293,678 of which £242,605 vested in the year and one to Key Management Personnel of £924,133 of which £60,500 vested in the year (2019: £nil). (3) No Termination payments were paid in 2020 to key management persons (2019: £1,076,435 for one individual; 2018: £847,388 for two individuals).
Summary of Transactions with Directors, Other Key Management Personnel	Directors, Other Key Management Personnel (defined as the Executive Committee of Santander UK plc who served during the year) and their connected persons have undertaken the following transactions with the Santander UK group in the ordinary course of business. 2020 2019 No. £000 No. £000 Secured loans, unsecured loans and overdrafts At 1 January 18 4,920 16 3,035 Net movements (6) (1,280) 2 1,885 At 31 December 12 3,640 18 4,920 Deposit, bank and instant access accounts and investments At 1 January 32 11,975 30 10,963 Net movements (9) (3,780) 2 1,012 At 31 December 23 8,195 32 11,975

Related Party Disclosures (Tabl

Related Party Disclosures (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of transactions between related parties [abstract]
Summary of Transactions With Related Parties	Transactions with related parties during the year and balances outstanding at the year-end: Group Interest, fees and Interest, fees and Amounts owed by Amounts owed to 2020 2019 2018 2020 2019 2018 2020 2019 2020 2019 £m £m £m £m £m £m £m £m £m £m Ultimate parent (119) (130) (72) 105 266 217 1,557 1,560 (2,151) (2,143) Immediate parent (7) (7) (3) 316 317 275 8 8 (10,121) (10,012) Fellow subsidiaries (58) (66) (86) 157 173 178 223 204 (559) (544) Associates & joint ventures (29) (29) (28) — — — 2,234 2,194 (1,034) (930) (213) (232) (189) 578 756 670 4,022 3,966 (13,865) (13,629)

Financial Instruments (Tables)

Financial Instruments (Tables) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of financial assets [abstract]
Fair Values of Financial Instruments Carried at Amortised Cost	Group 2020 2019 Fair value Carrying Fair value Carrying Level 1 Level 2 Level 3 Total value Level 1 Level 2 Level 3 Total value £m £m £m £m £m £m £m £m £m £m Assets Loans and advances to customers — — 211,279 211,279 208,750 — — 211,796 211,796 207,287 Loans and advances to banks — 1,682 — 1,682 1,682 — 1,739 116 1,855 1,855 Reverse repurchase agreements - non trading — 19,382 226 19,608 19,599 — 23,634 — 23,634 23,636 Other financial assets at amortised cost 799 393 — 1,192 1,163 6,575 535 — 7,110 7,056 799 21,457 211,505 233,761 231,194 6,575 25,908 211,912 244,395 239,834 Liabilities Deposits by customers — 108 195,134 195,242 195,135 — 95 181,918 182,013 181,883 Deposits by banks — 20,951 16 20,967 20,958 — 13,956 407 14,363 14,353 Repurchase agreements - non trading — 15,847 — 15,847 15,848 — 18,292 — 18,292 18,286 Debt securities in issue — 34,967 1,430 36,397 35,566 — 42,694 — 42,694 41,129 Subordinated liabilities — 2,830 239 3,069 2,556 — 4,220 — 4,220 3,528 — 74,703 196,819 271,522 270,063 — 79,257 182,325 261,582 259,179
Fair Values of Financial Instruments Measured at Fair Value on a Recurring Basis	The following tables summarise the fair values of the financial assets and liabilities accounted for at fair value at 31 December 2020 and 31 December 2019, analysed by their levels in the fair value hierarchy - Level 1, Level 2 and Level 3. Group 2020 2019 Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Total Valuation £m £m £m £m £m £m £m £m technique Assets Derivative financial instruments Exchange rate contracts — 2,455 2 2,457 — 2,317 6 2,323 A Interest rate contracts — 2,566 14 2,580 — 1,915 9 1,924 A & C Equity and credit contracts — 71 52 123 — 223 60 283 B & D Netting — (1,754) — (1,754) — (1,214) — (1,214) — 3,338 68 3,406 — 3,241 75 3,316 Other financial assets at FVTPL Loans and advances to customers — — 99 99 — — 92 92 A Debt securities — — 109 109 — — 294 294 A, B & D Equity securities B Reverse repurchase agreements – non trading A — — 208 208 — — 386 386 Financial assets at FVOCI Debt securities 8,501 428 — 8,929 9,209 482 — 9,691 D Loans and advances to customers — — 21 21 — — 56 56 D 8,501 428 21 8,950 9,209 482 56 9,747 Total assets at fair value 8,501 3,766 297 12,564 9,209 3,723 517 13,449 Liabilities Derivative financial instruments Exchange rate contracts — 833 — 833 — 660 4 664 A Interest rate contracts — 2,447 3 2,450 — 1,836 2 1,838 A & C Equity and credit contracts — 26 29 55 — 134 26 160 B & D Netting — (1,754) — (1,754) — (1,214) — (1,214) — 1,552 32 1,584 — 1,416 32 1,448 Other financial liabilities at FVTPL Debt securities in issue — 1,051 6 1,057 — 1,099 6 1,105 A Structured deposits — 375 — 375 — 406 29 435 A Repurchase agreements – non trading A Collateral and associated financial guarantees — — 2 2 — 147 26 173 D — 1,426 8 1,434 — 1,652 61 1,713 Total liabilities at fair value — 2,978 40 3,018 — 3,068 93 3,161
Summary of Fair Value Adjustment	The magnitude and types of fair value adjustment are listed in the following table: 2020 2019 £m £m Risk-related: - Bid-offer and trade specific adjustments (8) (12) - Uncertainty 23 17 - Credit risk adjustment 11 6 - Funding fair value adjustment 3 6 29 17 Model-related — — Day One profit — — 29 17
Disclosure Of Analysis Of Financial Instruments Valued Using Internal Models Based On Information Other Than Market Data	The table below provides an analysis of financial instruments valued using internal models based on information other than market data together with further details on the valuation techniques used for each type of instrument. Each instrument is initially valued at transaction price: Balance sheet value Fair value movements recognised in profit/(loss) 2020 2019 2020 2019 2018 Balance sheet line item Category Financial instrument product type £m £m £m £m £m 1. Derivative assets Equity and credit contracts Reversionary property interests 51 52 3 2 30 2. FVTPL assets Loans and advances to customers Roll-up mortgage portfolio 56 51 6 — 8 3. FVTPL assets Loans and advances to customers Other loans 43 41 3 1 2 4. FVTPL assets Debt securities Reversionary property securities 107 120 6 (17) (28) 5. FVTPL assets Debt securities Credit linked notes 2 174 (16) 7 13 6. FVOCI assets Loans and advances to customers Other loans 21 56 (4) (2) (5) 7. Derivative liabilities Equity contracts Property options and forwards (29) (26) (3) — — 8. FVTPL liabilities Financial guarantees Credit protection guarantee (2) (26) 16 (7) (13) 249 442 11 (16) 7 Other Level 3 assets 17 23 7 16 (2) Other Level 3 liabilities (9) (41) (1) (5) 1 Total net assets 257 424 — — — Total income/(expense) 17 (5) 6
Reconciliation of fair value measurement in Level 3 of the fair value hierarchy	The following table sets out the movements in Level 3 financial instruments in 2020 and 2019: Assets Liabilities Derivatives Other financial assets at FVTPL Financial assets at FVOCI Total Derivatives Other financial liabilities at FVTPL Total £m £m £m £m £m £m £m At 1 January 2020 75 386 56 517 (32) (61) (93) Total (losses)/gains recognised: - Fair value movements 10 (1) (4) 5 (6) 18 12 - Foreign exchange and other movements — (5) — (5) — 8 8 Transfers in 1 — — 1 — — — Transfers out — — — — — 28 28 Netting (1) — (42) — (42) — — — Additions 2 — — 2 — (2) (2) Sales — (19) (19) (38) — — — Settlements (20) (111) (12) (143) 6 1 7 At 31 December 2020 68 208 21 297 (32) (8) (40) Gains/(losses) recognised in profit or loss/other comprehensive income relating to assets and liabilities held at the end of the period 10 (6) (4) — (6) 26 20 At 1 January 2019 94 976 73 1,143 (66) (49) (115) Total gains/(losses) recognised: - Fair value movements 18 (9) (2) 7 (6) (6) (12) - Foreign exchange and other movements — 6 — 6 — (6) (6) Transfers in — 11 — 11 — — — Netting (1) — (430) — (430) — — — Additions 2 188 — 190 — (3) (3) Sales — — — — — — — Settlements (39) (356) (15) (410) 40 3 43 At 31 December 2019 75 386 56 517 (32) (61) (93) Gains/(losses) recognised in profit or loss/other comprehensive income relating to assets and liabilities held at the end of the period 18 (3) (2) 13 (6) (12) (18) (1) This relates to the effect of netting on the fair value of the credit linked notes due to a legal right of set-off between the principal amounts of the senior notes and the associated cash deposits. For more, see ‘ii) Credit protection entities’ in Note 19.
Effects Of Changes In Significant Unobservable Assumptions To Reasonably Possible Alternatives Level Three	Significant unobservable input Sensitivity Assumption value Favourable changes Unfavourable changes Fair value Range (1) Weighted average Shift 2020 £m Assumption description £m £m 1. Derivative assets – Equity and credit contracts: 51 HPI Forward growth rate 0% - 5% 2.57 % 1 % 8 (8) – Reversionary property derivatives HPI Spot rate (2) n/a 445 10 % 7 (7) 2. FVTPL – Loans and advances to customers: 56 HPI Forward growth rate 0% - 5% 2.69 % 1 % 2 (2) – Roll-up mortgage portfolio 3. FVTPL – Loans and advances to customers: 43 Credit spreads 0.07% - 1.55% 0.44 % 0.2 % — — – Other loans 4. FVTPL – Debt securities: 107 HPI Forward growth rate 0% - 5% 2.57 % 1 % 1 (1) – Reversionary property securities HPI Spot rate (2) n/a 445 10 % 5 (5) 6. FVOCI - Loans and advances to customers: 21 Credit spreads 0.15% - 0.53% 0.32 % 20 % — — – Other loans 7. Derivative liabilities – Equity contracts: (29) HPI Forward growth rate 0% - 5% 2.42 % 1 % 2 (2) –Property-related options and forwards HPI Spot rate (2) n/a 433 10 % 3 (3) 2019 1. Derivative assets – Equity and credit contracts: 52 HPI Forward growth rate 0% - 5% 2.57 % 1 % 8 (8) – Reversionary property derivatives HPI Spot rate (2) n/a 802 10 % 7 (7) 2. FVTPL – Loans and advances to customers: 51 HPI Forward growth rate 0% - 5% 2.69 % 1 % 2 (2) – Roll-up mortgage portfolio 3. FVTPL – Loans and advances to customers: 41 Credit spreads 0% - 1% 0.35 % 20 % — — – Other loans 4. FVTPL – Debt securities: 120 HPI Forward growth rate 0% - 5% 2.57 % 1 % — — – Reversionary property securities HPI Spot rate (2) n/a 802 10 % 6 (6) 6. FVOCI - Loans and advances to customers: 56 Credit spreads 0% - 1% 0.51 % 20 % — — – Other loans 7. Derivative liabilities – Equity contracts: (26) HPI Forward growth rate 0% - 5% 2.44 % 1 % 2 (2) –Property-related options and forwards HPI Spot rate (2) n/a 758 10 % 3 (3) (1) The range of actual assumption values used to calculate the weighted average disclosure. (2) The HPI spot rate in the weighted average column represents the HPI spot rate index level at 31 December 2020 and 2019.
Maturities of Undiscounted Cash Flows for Financial Liabilities and Off Balance Sheet Commitments	Group On demand Not later than 3 months Later than 3 months and not later than 1 year Later than 1 year and not later than 5 years Later than 5 years Total 2020 £m £m £m £m £m £m Financial liabilities Derivative financial instruments — 279 133 582 638 1,632 Other financial liabilities at fair value through profit or loss — 1 107 570 759 1,437 Deposits by customers 176,911 3,443 5,453 7,483 2,273 195,563 Deposits by banks 2,410 1,912 2,596 14,230 212 21,360 Repurchase agreements – non trading — 15,350 500 — — 15,850 Debt securities in issue — 4,993 9,897 14,083 7,540 36,513 Subordinated liabilities — 30 92 1,515 2,424 4,061 Lease liabilities — — 17 56 36 109 Total financial liabilities 179,321 26,008 18,795 38,519 13,882 276,525 Off-balance sheet commitments given 21,054 4,493 4,974 11,493 1,146 43,160 2019 Financial liabilities Derivative financial instruments 9 243 293 418 532 1,495 Other financial liabilities at fair value through profit or loss 1 6 203 617 969 1,796 Deposits by customers 160,833 2,531 6,476 9,770 3,045 182,655 Deposits by banks 2,711 486 4,764 6,338 220 14,519 Repurchase agreements – non trading 6 15,878 1,578 846 — 18,308 Debt securities in issue — 7,129 8,702 18,100 8,897 42,828 Subordinated liabilities — 239 131 1,539 3,961 5,870 Lease liabilities — — 45 58 52 155 Total financial liabilities 163,560 26,512 22,192 37,686 17,676 267,626 Off-balance sheet commitments given (1) 20,158 2,927 5,742 11,490 1,278 41,595
Lease liabilities	£ 97	£ 137
Other financial liabilities at fair value through profit or loss	1,434	1,713
Deposits by customers	195,135	181,883
Deposits by banks	20,958	14,353
Repurchase agreements – non trading	15,848	18,286
Debt securities in issue	35,566	41,129
Subordinated liabilities	£ 2,556	£ 3,528

Offsetting Financial Assets a_2

Offsetting Financial Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure of offsetting of financial liabilities [abstract]
Summary of Information about Impact of Offsetting of Financial Assets and Liabilities	Group Amounts subject to enforceable netting arrangements Assets not subject to enforceable netting arrangements (2) Effects of offsetting on balance sheet Related amounts not offset Gross Amounts Net amounts Financial Financial collateral (1) Net Balance sheet total (3) 2020 £m £m £m £m £m £m £m £m Assets Derivative financial assets 5,071 (1,754) 3,317 (782) (1,840) 695 89 3,406 Reverse repurchase, securities borrowing & similar agreements: – Amortised cost 26,084 (6,485) 19,599 (129) (19,470) — — 19,599 – Fair value — — — — — — — — Loans and advances to customers and banks⁽⁴⁾ 7,454 (1,073) 6,381 — — 6,381 204,051 210,432 38,609 (9,312) 29,297 (911) (21,310) 7,076 204,140 233,437 Liabilities Derivative financial liabilities 3,261 (1,754) 1,507 (782) (551) 174 77 1,584 Repurchase, securities lending & similar agreements: – Amortised cost 22,333 (6,485) 15,848 (129) (15,719) — — 15,848 – Fair value — — — — — — — — Deposits by customers and banks⁽⁴⁾ 11,159 (1,073) 10,086 — (502) 9,584 206,007 216,093 36,753 (9,312) 27,441 (911) (16,772) 9,758 206,084 233,525 2019 Assets Derivative financial assets 4,446 (1,214) 3,232 (768) (1,915) 549 84 3,316 Reverse repurchase, securities borrowing & similar agreements: – Amortised cost 25,312 (1,676) 23,636 (537) (23,099) — — 23,636 – Fair value — — — — — — — — Loans and advances to customers and banks⁽⁴⁾ 5,247 (1,248) 3,999 — — 3,999 205,143 209,142 35,005 (4,138) 30,867 (1,305) (25,014) 4,548 205,227 236,094 Liabilities Derivative financial liabilities 2,616 (1,214) 1,402 (768) (572) 62 46 1,448 Repurchase, securities lending & similar agreements: – Amortised cost 19,962 (1,676) 18,286 (537) (17,749) — — 18,286 – Fair value — — — — — — — — Deposits by customers and banks⁽⁴⁾ 11,395 (1,248) 10,147 — (502) 9,645 186,089 196,236 33,973 (4,138) 29,835 (1,305) (18,823) 9,707 186,135 215,970 (1) Financial collateral is reflected at its fair value, but has been limited to the net balance sheet exposure so as not to include any over-collateralisation. (2) This column includes contractual rights of set-off that are subject to uncertainty under the laws of the relevant jurisdiction. (3) The balance sheet total is the sum of ‘Net amounts reported on the balance sheet’ that are subject to enforceable netting arrangements and ‘Amounts not subject to enforceable netting arrangements’. ’.

Interest Rate Benchmark Reform

Interest Rate Benchmark Reform (Tables)	12 Months Ended
	Dec. 31, 2020
Interest Rate Benchmark Reform [Abstract]
Schedule of Amounts Affected by IBOR Reform	Group GBP (2) LIBOR USD (2) LIBOR Other (2) Total £m £m £m £m Assets Derivatives (1) 33,486 4,514 2,149 40,149 Other financial assets at fair value through profit and loss 968 22 — 990 Financial assets at amortised cost 15,062 1,191 90 16,343 Financial assets at fair value through comprehensive income 428 — — 428 49,944 5,727 2,239 57,910 Liabilities Derivatives (1) 35,217 5,205 88 40,510 Other financial liabilities at fair value through profit and loss 1,129 69 — 1,198 Financial liabilities at amortised cost 2,354 1,319 — 3,673 38,700 6,593 88 45,381 Off-balance sheet commitments given 11,400 2,126 573 14,099 (1) Many of the Santander UK group’s derivatives subject to IBOR reform are governed by ISDA definitions. In October 2020 ISDA issued an IBOR fallbacks supplement setting out how the amendments to new alternative benchmark rates will be accomplished, the effect of which is to create fallback provisions in derivatives that describe what floating rates will apply on the permanent discontinuation of certain key IBORs or upon ISDA declaring a non-representative determination of an IBOR. The Santander UK group has adhered to the protocol to implement the fallbacks to derivative contracts that were entered into before the effective date of the supplement (25 January 2021). If derivative counterparties also adhere to the protocol, new fallbacks will automatically be implemented in existing derivative contracts when the supplement becomes effective. (2) Cessation dates are :- GBP, JPY, NOK LIBOR 31/12/2021, USD LIBOR 30/06/23, EONIA 03/01/2022
Schedule of Derivatives Directly Affected by IBOR Reform Uncertainties	The following tables show the notional amount of derivatives in hedging relationships directly affected by uncertainties related to IBOR reform. Group GBP USD Other Total 2020 £m £m £m £m Total notional value of hedging instruments: – Cash flow hedges 15,198 5,119 — 20,317 – Fair value hedges 32,223 1,077 778 34,078 47,421 6,196 778 54,395 Maturing after cessation date (1) – Cash flow hedges 10,553 2,562 — 13,115 – Fair value hedges 12,477 162 720 13,359 23,030 2,724 720 26,474 (1) Cessation dates are :- GBP, JPY, NOK LIBOR 31/12/2021, USD LIBOR 30/06/23, EONIA 03/01/2022

Credit Risk - Summary of Exposu

Credit Risk - Summary of Exposures to Credit Risk in Business Segments (Detail) - Credit risk	12 Months Ended
	Dec. 31, 2020
Retail banking
Disclosure of credit risk exposure [line items]
Description of exposure to risk in business segments	–Residential mortgages, business banking, consumer (auto) finance and other unsecured lending (credit cards, personal loans and overdrafts).–We provide these to individuals and small businesses.
Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Description of exposure to risk in business segments	–Loans, bank accounts, treasury services, invoice discounting, cash transmission, trade finance and asset finance.–We provide these to SMEs and mid corporates, Commercial Real Estate and Social Housing associations.
Corporate and investment banking
Disclosure of credit risk exposure [line items]
Description of exposure to risk in business segments	–Loans, bank accounts, treasury services, treasury markets activities, trade finance, receivables discounting and cash transmission.–We provide these to large corporates and financial institutions.
Corporate centre
Disclosure of credit risk exposure [line items]
Description of exposure to risk in business segments	–Asset and liability management of our balance sheet, as well as our non-core and Legacy Portfolios being run down.–Exposures include sovereign and other international organisation assets that we hold for liquidity.

Credit Risk - Summary of Key Me

Credit Risk - Summary of Key Metrics to Measure and Control Credit Risk (Detail) - Credit risk	12 Months Ended
	Dec. 31, 2020
Expected credit losses [member]
Disclosure of credit risk exposure [line items]
Metric	Expected Credit Loss (ECL)
Description	ECL tells us what credit risk is likely to cost us either over the next 12 months on qualifying exposures, or defaults over the lifetime of the exposure where there is evidence of a SICR since origination. We explain how we calculate ECL below.
Stages 1, 2 and 3 [member]
Disclosure of credit risk exposure [line items]
Metric	Stages 1, 2 and 3
Description	We assess each facility’s credit risk profile to determine which stage to allocate them to, and we monitor where there is a SICR and transfers between the Stages including monitoring of coverage ratios for each stage. We explain how we allocate a facility to Stage 1, 2 or 3 below.
Stage 3 ratio [member]
Disclosure of credit risk exposure [line items]
Metric	Stage 3 ratio
Description	The Stage 3 ratio is total Stage 3 exposure as a percentage of customer loans plus undrawn Stage 3 exposures. The Stage 3 ratio is the main indicator of credit quality performance.
Expected Loss [member]
Disclosure of credit risk exposure [line items]
Metric	Expected Loss (EL)
Description	EL is based on the regulatory capital rules of CRD IV and gives us another view of credit risk. It is the product of the probability of default, exposure at default and loss given default. We calculate each factor in accordance with CRD IV and include direct and indirect costs. We base them on our risk models and our assessment of each customer’s credit quality. There are differences between regulatory EL and IFRS 9 ECL, which we set out below. The rest of our Risk review, impairments, losses and loss allowances refer to calculations in accordance with IFRS, unless we specifically say they relate to CRD IV. For our IFRS accounting policy on impairment, see Note 1 to the Consolidated Financial Statements.

Credit Risk - Rate for each of

Credit Risk - Rate for each of the Five Scenarios (Detail)	Dec. 31, 2020
Upside 1
Disclosure of probability weightings of scenarios [line items]
House Price Growth Rate, Year One	3.70%
House Price Growth Rate, Year Two	(4.60%)
GDP Growth Rate, Year One	(10.50%)
GDP Growth Rate, Year Two	4.80%
GDP Growth Rate, Year Three	4.90%
Unemployment Rate, Year One	6.30%
Unemployment Rate, Year Two	6.10%
Bank Of England Base Rate, Year One	0.10%
Bank Of England Base Rate, Year Two	0.25%
Bank Of England Base Rate, Year Three	0.75%
Base case
Disclosure of probability weightings of scenarios [line items]
House Price Growth Rate, Year One	3.50%
House Price Growth Rate, Year Two	(2.00%)
GDP Growth Rate, Year One	(11.50%)
GDP Growth Rate, Year Two	4.50%
GDP Growth Rate, Year Three	6.10%
Unemployment Rate, Year One	6.80%
Unemployment Rate, Year Two	7.50%
Bank Of England Base Rate, Year One	0.10%
Bank Of England Base Rate, Year Two	0.10%
Bank Of England Base Rate, Year Three	0.10%
Downside 1
Disclosure of probability weightings of scenarios [line items]
House Price Growth Rate, Year One	3.70%
House Price Growth Rate, Year Two	(5.40%)
GDP Growth Rate, Year One	(10.50%)
GDP Growth Rate, Year Two	4.00%
GDP Growth Rate, Year Three	3.60%
Unemployment Rate, Year One	6.30%
Unemployment Rate, Year Two	6.50%
Bank Of England Base Rate, Year One	0.10%
Bank Of England Base Rate, Year Two	0.10%
Bank Of England Base Rate, Year Three	0.10%
Downside 2
Disclosure of probability weightings of scenarios [line items]
House Price Growth Rate, Year One	3.70%
House Price Growth Rate, Year Two	(11.30%)
GDP Growth Rate, Year One	(11.10%)
GDP Growth Rate, Year Two	(0.80%)
GDP Growth Rate, Year Three	3.20%
Unemployment Rate, Year One	6.30%
Unemployment Rate, Year Two	8.50%
Bank Of England Base Rate, Year One	0.10%
Bank Of England Base Rate, Year Two	0.75%
Bank Of England Base Rate, Year Three	1.75%
Downside 3
Disclosure of probability weightings of scenarios [line items]
House Price Growth Rate, Year One	3.50%
House Price Growth Rate, Year Two	(19.70%)
GDP Growth Rate, Year One	(11.50%)
GDP Growth Rate, Year Two	(8.00%)
GDP Growth Rate, Year Three	3.10%
Unemployment Rate, Year One	6.80%
Unemployment Rate, Year Two	11.40%
Bank Of England Base Rate, Year One	0.10%
Bank Of England Base Rate, Year Two	(0.50%)
Bank Of England Base Rate, Year Three	0.00%

Credit Risk - Summary of Compou

Credit Risk - Summary of Compound Annual Growth Rates Over 5 Year Forecast (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Upside 2
Disclosure of forward looking information used in the determination of expected credit losses [line items]
House Price Index Five Year Average Increase Decrease		4.90%
House Price Growth Peak/Trough		8.10%
Gross Domestic product Five Year Average Increase Decrease		2.40%
Gross Domestic Product Peak/Trough Rate		1.50%
Unemployement Five Year Average end period		1.90%
Unemployment Peak/Trough Rate		1.88%
Bank of England Base Rate Five Year end period		2.00%
Bank of England Base Rate Peak/Trough		2.00%
Upside 1
Disclosure of forward looking information used in the determination of expected credit losses [line items]
House Price Index Five Year Average Increase Decrease	0.49%	3.70%
House Price Growth Peak/Trough	2.45%	5.80%
Gross Domestic product Five Year Average Increase Decrease	0.75%	2.00%
Gross Domestic Product Peak/Trough Rate	3.82%	1.00%
Unemployement Five Year Average end period	4.14%	2.70%
Unemployment Peak/Trough Rate	6.28%	2.73%
Bank of England Base Rate Five Year end period	1.75%	2.00%
Bank of England Base Rate Peak/Trough	1.75%	2.00%
Base case
Disclosure of forward looking information used in the determination of expected credit losses [line items]
House Price Index Five Year Average Increase Decrease	1.38%	1.60%
House Price Growth Peak/Trough	7.11%	2.00%
Gross Domestic product Five Year Average Increase Decrease	0.39%	1.60%
Gross Domestic Product Peak/Trough Rate	1.96%	0.70%
Unemployement Five Year Average end period	5.50%	4.00%
Unemployment Peak/Trough Rate	7.90%	4.10%
Bank of England Base Rate Five Year end period	0.25%	0.75%
Bank of England Base Rate Peak/Trough	0.25%	0.75%
Downside 1
Disclosure of forward looking information used in the determination of expected credit losses [line items]
House Price Index Five Year Average Increase Decrease	(2.01%)	(1.20%)
House Price Growth Peak/Trough	(9.65%)	(2.80%)
Gross Domestic product Five Year Average Increase Decrease	(0.38%)	0.70%
Gross Domestic Product Peak/Trough Rate	(1.88%)	(1.10%)
Unemployement Five Year Average end period	5.84%	5.60%
Unemployment Peak/Trough Rate	6.51%	5.64%
Bank of England Base Rate Five Year end period	0.25%	2.00%
Bank of England Base Rate Peak/Trough	0.25%	2.00%
Downside 2
Disclosure of forward looking information used in the determination of expected credit losses [line items]
House Price Index Five Year Average Increase Decrease	(4.54%)	(9.30%)
House Price Growth Peak/Trough	(20.72%)	(13.50%)
Gross Domestic product Five Year Average Increase Decrease	(0.98%)	0.20%
Gross Domestic Product Peak/Trough Rate	(4.80%)	(5.60%)
Unemployement Five Year Average end period	6.52%	7.40%
Unemployment Peak/Trough Rate	8.78%	7.84%
Bank of England Base Rate Five Year end period	2.75%	2.25%
Bank of England Base Rate Peak/Trough	3.00%	3.00%
Downside 3
Disclosure of forward looking information used in the determination of expected credit losses [line items]
House Price Index Five Year Average Increase Decrease	(4.44%)
House Price Growth Peak/Trough	(20.32%)
Gross Domestic product Five Year Average Increase Decrease	(2.82%)
Gross Domestic Product Peak/Trough Rate	(13.33%)
Unemployement Five Year Average end period	7.40%
Unemployment Peak/Trough Rate	11.90%
Bank of England Base Rate Five Year end period	0.00%
Bank of England Base Rate Peak/Trough	(0.50%)

Credit Risk - Summary of Probab

Credit Risk - Summary of Probability Weightings Applied to Scenarios for CIB Portfolio (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Base case
Disclosure of probability weightings of scenarios [line items]
Probability %	15.00%	40.00%
Upside 2
Disclosure of probability weightings of scenarios [line items]
Probability %	5.00%	5.00%
Upside 1
Disclosure of probability weightings of scenarios [line items]
Probability %	45.00%	10.00%
Downside 1
Disclosure of probability weightings of scenarios [line items]
Probability %	25.00%	30.00%
Downside 2
Disclosure of probability weightings of scenarios [line items]
Probability %	10.00%	15.00%
Corporate and investment banking | Upside Scenario [member]
Disclosure of probability weightings of scenarios [line items]
Probability %		30.00%
Corporate and investment banking | Base case
Disclosure of probability weightings of scenarios [line items]
Probability %		40.00%
Corporate and investment banking | Downside Scenario [member]
Disclosure of probability weightings of scenarios [line items]
Probability %		30.00%

Credit Risk - Summary of Annual

Credit Risk - Summary of Annual Growth Rates over 4 Year Forecast for CIB portfolio (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Upside Scenario [member] | 5 year average increase/decrease
Disclosure of forward looking information used in the determination of expected credit losses [line items]
GDP		3.70%
Upside Scenario [member] | Cumulative growth/(fall) to peak/(trough)
Disclosure of forward looking information used in the determination of expected credit losses [line items]
GDP		0.30%
Base case | 5 year average increase/decrease
Disclosure of forward looking information used in the determination of expected credit losses [line items]
GDP		3.50%
Base case | Cumulative growth/(fall) to peak/(trough)
Disclosure of forward looking information used in the determination of expected credit losses [line items]
GDP		0.50%
Downside Scenario [member] | 5 year average increase/decrease
Disclosure of forward looking information used in the determination of expected credit losses [line items]
GDP	1.30%	3.00%
Downside Scenario [member] | Cumulative growth/(fall) to peak/(trough)
Disclosure of forward looking information used in the determination of expected credit losses [line items]
GDP		(1.20%)

Credit Risk - Summary of Prob_2

Credit Risk - Summary of Probability Weights Applied for Each Scenario of Expected Credit Losses Allowance (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 208,750	£ 207,287
Weighted
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	322,745	310,155
ECL	£ 1,377	£ 863
Proportion of assets in Stage 2	5.30%	3.80%
Weighted | Retail banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 210,251	£ 204,780
Exposure - of which: mortgages	180,006	178,788
ECL	706	549
ECL - of which: mortgages	£ 280	£ 218
Proportion of assets in Stage 2	5.40%	4.50%
Proportion of assets in Stage 2 - of which: mortgages	5.70%	4.60%
Weighted | Corporate and commercial banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 24,503	£ 24,118
ECL	£ 603	£ 262
Proportion of assets in Stage 2	22.40%	8.70%
Weighted | Corporate and investment banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 11,646	£ 13,384
ECL	£ 33	£ 50
Proportion of assets in Stage 2	1.70%	1.50%
Weighted | Corporate centre
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 76,345	£ 67,873
ECL	£ 35	£ 2
Proportion of assets in Stage 2	0.00%	0.10%
Upside 2
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 322,745	£ 310,155
ECL	£ 1,129	£ 640
Proportion of assets in Stage 2	4.60%	2.80%
Upside 2 | Retail banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 210,251	£ 204,780
Exposure - of which: mortgages	180,006	178,788
ECL	610	425
ECL - of which: mortgages	£ 212	£ 122
Proportion of assets in Stage 2	4.60%	3.20%
Proportion of assets in Stage 2 - of which: mortgages	4.80%	3.10%
Upside 2 | Corporate and commercial banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 24,503	£ 24,118
ECL	£ 485	£ 195
Proportion of assets in Stage 2	20.10%	7.40%
Upside 2 | Corporate and investment banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 11,646	£ 13,384
ECL	£ 5	£ 19
Proportion of assets in Stage 2	1.70%	1.50%
Upside 2 | Corporate centre
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 76,345	£ 67,873
ECL	£ 29	£ 1
Proportion of assets in Stage 2	0.00%	0.10%
Upside 1
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 322,745	£ 310,155
ECL	£ 1,222	£ 680
Proportion of assets in Stage 2	4.70%	2.80%
Upside 1 | Retail banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 210,251	£ 204,780
Exposure - of which: mortgages	180,006	178,788
ECL	587	433
ECL - of which: mortgages	£ 207	£ 127
Proportion of assets in Stage 2	4.60%	3.30%
Proportion of assets in Stage 2 - of which: mortgages	4.90%	3.10%
Upside 1 | Corporate and commercial banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 24,503	£ 24,118
ECL	£ 575	£ 212
Proportion of assets in Stage 2	20.80%	7.40%
Upside 1 | Corporate and investment banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 11,646	£ 13,384
ECL	£ 26	£ 34
Proportion of assets in Stage 2	1.70%	1.50%
Upside 1 | Corporate centre
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 76,345	£ 67,873
ECL	£ 34	£ 1
Proportion of assets in Stage 2	0.00%	0.10%
Base case
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 322,745	£ 310,155
ECL	£ 1,300	£ 726
Proportion of assets in Stage 2	4.60%	2.80%
Base case | Retail banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 210,251	£ 204,780
Exposure - of which: mortgages	180,006	178,788
ECL	660	448
ECL - of which: mortgages	£ 253	£ 137
Proportion of assets in Stage 2	4.70%	3.30%
Proportion of assets in Stage 2 - of which: mortgages	4.90%	3.10%
Base case | Corporate and commercial banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 24,503	£ 24,118
ECL	£ 567	£ 229
Proportion of assets in Stage 2	20.20%	7.40%
Base case | Corporate and investment banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 11,646	£ 13,384
ECL	£ 40	£ 48
Proportion of assets in Stage 2	1.70%	1.50%
Base case | Corporate centre
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 76,345	£ 67,873
ECL	£ 33	£ 1
Proportion of assets in Stage 2	0.00%	0.10%
Downside 1
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 322,745	£ 310,155
ECL	£ 1,613	£ 855
Proportion of assets in Stage 2	6.60%	3.20%
Downside 1 | Retail banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 210,251	£ 204,780
Exposure - of which: mortgages	180,006	178,788
ECL	850	525
ECL - of which: mortgages	£ 389	£ 196
Proportion of assets in Stage 2	7.20%	3.70%
Proportion of assets in Stage 2 - of which: mortgages	7.70%	3.60%
Downside 1 | Corporate and commercial banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 24,503	£ 24,118
ECL	£ 671	£ 275
Proportion of assets in Stage 2	24.50%	8.50%
Downside 1 | Corporate and investment banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 11,646	£ 13,384
ECL	£ 53	£ 53
Proportion of assets in Stage 2	1.70%	1.50%
Downside 1 | Corporate centre
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 76,345	£ 67,873
ECL	£ 39	£ 2
Proportion of assets in Stage 2	0.00%	0.10%
Downside 2
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 322,745	£ 310,155
ECL	£ 1,802	£ 1,542
Proportion of assets in Stage 2	6.80%	6.80%
Downside 2 | Retail banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 210,251	£ 204,780
Exposure - of which: mortgages	180,006	178,788
ECL	863	1,084
ECL - of which: mortgages	£ 415	£ 660
Proportion of assets in Stage 2	7.00%	8.30%
Proportion of assets in Stage 2 - of which: mortgages	7.50%	8.70%
Downside 2 | Corporate and commercial banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 24,503	£ 24,118
ECL	£ 824	£ 398
Proportion of assets in Stage 2	28.60%	16.30%
Downside 2 | Corporate and investment banking
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 11,646	£ 13,384
ECL	£ 66	£ 58
Proportion of assets in Stage 2	1.70%	1.50%
Downside 2 | Corporate centre
Disclosure of probability weightings of scenarios [line items]
Loans and advances to customers	£ 76,345	£ 67,873
ECL	£ 49	£ 2
Proportion of assets in Stage 2	0.00%	0.30%

Credit Risk - Disclosure of Qua

Credit Risk - Disclosure of Quantitative Criteria of Credit Risk Exposure (Detail) - Probability of default	12 Months Ended
	Dec. 31, 2020
Corporate and commercial banking
Disclosure of credit risk exposure [line items]
PD threshold for measuring significant increase in credit risk	0.30%
Mortgages | Retail banking
Disclosure of credit risk exposure [line items]
PD threshold for measuring significant increase in credit risk	0.30%
Consumer (auto) finance | Retail banking
Disclosure of credit risk exposure [line items]
PD threshold for measuring significant increase in credit risk	3.00%
Personal loans | Retail banking
Disclosure of credit risk exposure [line items]
PD threshold for measuring significant increase in credit risk	0.30%
Credit cards | Retail banking
Disclosure of credit risk exposure [line items]
PD threshold for measuring significant increase in credit risk	3.40%
Overdrafts | Retail banking
Disclosure of credit risk exposure [line items]
PD threshold for measuring significant increase in credit risk	2.60%

Credit Risk - Disclosure of Q_2

Credit Risk - Disclosure of Qualitative Criteria of Credit Risk Exposure (Detail)	12 Months Ended
	Dec. 31, 2020
Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Qualitative Criteria For Measuring SICR	- In forbearance- Default in last 12m- Watchlist: proactive management- Default at proxy origination
Corporate and investment banking
Disclosure of credit risk exposure [line items]
Qualitative Criteria For Measuring SICR	- Watchlist: proactive management
Mortgages | Retail banking
Disclosure of credit risk exposure [line items]
Qualitative Criteria For Measuring SICR	- In forbearance- Default in last 24m- >30 Days past due (DPD) in last 12m- Bankrupt- £100+ arrears
Consumer (auto) finance | Retail banking
Disclosure of credit risk exposure [line items]
Qualitative Criteria For Measuring SICR	- In forbearance- Deceased or Insolvent- Court ‘Return of goods’ order or Police watchlist- Agreement terminated- Payment holiday- Cash Collection
Personal loans | Retail banking
Disclosure of credit risk exposure [line items]
Qualitative Criteria For Measuring SICR	- In Collections- Default in last 12m- £50+ arrears
Credit cards | Retail banking
Disclosure of credit risk exposure [line items]
Qualitative Criteria For Measuring SICR	- In forbearance- Default in last 12m- In Collections- £100+ arrears
Overdrafts | Retail banking
Disclosure of credit risk exposure [line items]
Qualitative Criteria For Measuring SICR	- Fees suspended- Default in last 12m- Debit dormant >35 days- Any excess in month

Credit Risk - Disclosure of Pos

Credit Risk - Disclosure of Post Model Adjustments Explanatory (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Interest-only maturity default risk
Statement [Line Items]
Post model adjustments	£ 49	£ 51
Buy-To-Let
Statement [Line Items]
Post model adjustments	24	21
Long-term indeterminate arrears
Statement [Line Items]
Post model adjustments	29	19
12+ months in arrears
Statement [Line Items]
Post model adjustments	30	23
Corporate Covid-19 Affected Segments
Statement [Line Items]
Post model adjustments	193	0
Payment Holidays
Statement [Line Items]
Post model adjustments	27	0
Corporate: Tall Tree
Statement [Line Items]
Post model adjustments	£ 35	£ 0

Credit Risk - Summary of Differ

Credit Risk - Summary of Difference Between Maximum and Net Exposure to Credit Risk - IFRS 9 (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	£ 95,156	£ 80,918
Collateral cash	(16,772)	(18,823)
Netting	(911)	(1,305)
Net exposure	322,745	310,155
Credit risk
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	281,600	270,900
Collateral cash	(200)	(200)
Collateral non-cash	(215,700)	(217,500)
Netting	(100)	(500)
Net exposure	108,700	94,200
Credit risk | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	282,900	271,700
Credit risk | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	(1,300)	(800)
Credit risk | Cash and balances at central banks
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	41,300	21,200
Collateral cash	0	0
Collateral non-cash	0	0
Netting	0	0
Net exposure	41,300	21,200
Credit risk | Cash and balances at central banks | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	41,300	21,200
Credit risk | Cash and balances at central banks | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Credit risk | Financial assets at FVOCI
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	9,000	9,700
Collateral cash	0	0
Collateral non-cash	0	0
Netting	0	0
Net exposure	9,000	9,700
Credit risk | Financial assets at FVOCI | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	9,000	9,700
Credit risk | Financial assets at FVOCI | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Off-Balance Sheet | Credit risk
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	43,100	41,500
Off-Balance Sheet | Credit risk | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	43,200	41,600
Off-Balance Sheet | Credit risk | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	(100)	(100)
Off-Balance Sheet | Credit risk | Cash and balances at central banks
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Off-Balance Sheet | Credit risk | Cash and balances at central banks | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Off-Balance Sheet | Credit risk | Cash and balances at central banks | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Off-Balance Sheet | Credit risk | Financial assets at FVOCI
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Off-Balance Sheet | Credit risk | Financial assets at FVOCI | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Off-Balance Sheet | Credit risk | Financial assets at FVOCI | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Financial assets at amortised cost | Credit risk
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	231,300	240,000
Collateral cash	(200)	(200)
Collateral non-cash	(215,700)	(217,500)
Netting	(100)	(500)
Net exposure	58,400	63,300
Financial assets at amortised cost | Credit risk | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	232,600	240,800
Financial assets at amortised cost | Credit risk | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	(1,300)	(800)
Financial assets at amortised cost | Credit risk | Loans and advances to customers
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	208,800	207,400
Collateral cash	(200)	(200)
Collateral non-cash	(196,200)	(194,400)
Netting	0	0
Net exposure	54,500	53,100
Financial assets at amortised cost | Credit risk | Loans and advances to customers | Advances secured on residential property [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	166,400	165,200
Collateral cash	0	0
Collateral non-cash	(170,000)	(168,700)
Netting	0	0
Net exposure	9,700	9,900
Financial assets at amortised cost | Credit risk | Loans and advances to customers | Corporate loans
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	23,900	26,800
Collateral cash	(100)	(100)
Collateral non-cash	(20,400)	(19,400)
Netting	0	0
Net exposure	18,700	21,500
Financial assets at amortised cost | Credit risk | Loans and advances to customers | Finance leases [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	6,500	6,100
Collateral cash	(100)	(100)
Collateral non-cash	(5,800)	(6,300)
Netting	0	0
Net exposure	800	0
Financial assets at amortised cost | Credit risk | Loans and advances to customers | Other unsecured advances [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	9,600	6,900
Collateral cash	0	0
Collateral non-cash	0	0
Netting	0	0
Net exposure	22,900	19,300
Financial assets at amortised cost | Credit risk | Loans and advances to customers | Amounts due from fellow banco santander group subsidiaries and joint ventures [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	2,400	2,400
Collateral cash	0	0
Collateral non-cash	0	0
Netting	0	0
Net exposure	2,400	2,400
Financial assets at amortised cost | Credit risk | Loans and advances to customers | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	210,100	208,200
Financial assets at amortised cost | Credit risk | Loans and advances to customers | Gross carrying amount | Advances secured on residential property [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	166,700	165,400
Financial assets at amortised cost | Credit risk | Loans and advances to customers | Gross carrying amount | Corporate loans
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	24,500	27,000
Financial assets at amortised cost | Credit risk | Loans and advances to customers | Gross carrying amount | Finance leases [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	6,600	6,300
Financial assets at amortised cost | Credit risk | Loans and advances to customers | Gross carrying amount | Other unsecured advances [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	9,900	7,100
Financial assets at amortised cost | Credit risk | Loans and advances to customers | Gross carrying amount | Amounts due from fellow banco santander group subsidiaries and joint ventures [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	2,400	2,400
Financial assets at amortised cost | Credit risk | Loans and advances to customers | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	(1,300)	(800)
Financial assets at amortised cost | Credit risk | Loans and advances to customers | Accumulated impairment | Advances secured on residential property [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	(300)	(200)
Financial assets at amortised cost | Credit risk | Loans and advances to customers | Accumulated impairment | Corporate loans
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	(600)	(200)
Financial assets at amortised cost | Credit risk | Loans and advances to customers | Accumulated impairment | Finance leases [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	(100)	(200)
Financial assets at amortised cost | Credit risk | Loans and advances to customers | Accumulated impairment | Other unsecured advances [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	(300)	(200)
Financial assets at amortised cost | Credit risk | Loans and advances to customers | Accumulated impairment | Amounts due from fellow banco santander group subsidiaries and joint ventures [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Financial assets at amortised cost | Credit risk | Loans and advances to banks
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	1,700	1,900
Collateral cash	0	0
Collateral non-cash	0	0
Netting	0	0
Net exposure	2,700	3,100
Financial assets at amortised cost | Credit risk | Loans and advances to banks | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	1,700	1,900
Financial assets at amortised cost | Credit risk | Loans and advances to banks | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Financial assets at amortised cost | Credit risk | Reverse repurchase agreements - non-trading
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	19,600	23,600
Collateral cash	0	0
Collateral non-cash	(19,500)	(23,100)
Netting	(100)	(500)
Net exposure	0	0
Financial assets at amortised cost | Credit risk | Reverse repurchase agreements - non-trading | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	19,600	23,600
Financial assets at amortised cost | Credit risk | Reverse repurchase agreements - non-trading | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Financial assets at amortised cost | Credit risk | Other financial assets at amortised cost
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	1,200	7,100
Collateral cash	0	0
Collateral non-cash	0	0
Netting	0	0
Net exposure	1,200	7,100
Financial assets at amortised cost | Credit risk | Other financial assets at amortised cost | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	1,200	7,100
Financial assets at amortised cost | Credit risk | Other financial assets at amortised cost | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Financial assets at amortised cost | Off-Balance Sheet | Credit risk
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	43,100	41,500
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	43,200	41,600
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	(100)	(100)
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	42,100	40,300
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers | Advances secured on residential property [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	13,300	13,400
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers | Corporate loans
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	15,300	14,200
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers | Finance leases [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	200	300
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers | Other unsecured advances [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	13,300	12,400
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers | Amounts due from fellow banco santander group subsidiaries and joint ventures [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	42,200	40,400
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers | Gross carrying amount | Advances secured on residential property [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	13,300	13,400
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers | Gross carrying amount | Corporate loans
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	15,400	14,300
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers | Gross carrying amount | Finance leases [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	200	300
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers | Gross carrying amount | Other unsecured advances [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	13,300	12,400
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers | Gross carrying amount | Amounts due from fellow banco santander group subsidiaries and joint ventures [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	(100)	(100)
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers | Accumulated impairment | Advances secured on residential property [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers | Accumulated impairment | Corporate loans
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	(100)	(100)
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers | Accumulated impairment | Finance leases [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers | Accumulated impairment | Other unsecured advances [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to customers | Accumulated impairment | Amounts due from fellow banco santander group subsidiaries and joint ventures [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to banks
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	1,000	1,200
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to banks | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	1,000	1,200
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Loans and advances to banks | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Reverse repurchase agreements - non-trading
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Reverse repurchase agreements - non-trading | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Reverse repurchase agreements - non-trading | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Other financial assets at amortised cost
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Other financial assets at amortised cost | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Financial assets at amortised cost | Off-Balance Sheet | Credit risk | Other financial assets at amortised cost | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Loans and advances to customers | Credit risk | Financial assets at FVOCI
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	100
Collateral cash	0	0
Collateral non-cash	0	0
Netting	0	0
Net exposure	0	100
Loans and advances to customers | Credit risk | Financial assets at FVOCI | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	100
Loans and advances to customers | Credit risk | Financial assets at FVOCI | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Loans and advances to customers | Off-Balance Sheet | Credit risk | Financial assets at FVOCI
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Loans and advances to customers | Off-Balance Sheet | Credit risk | Financial assets at FVOCI | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Loans and advances to customers | Off-Balance Sheet | Credit risk | Financial assets at FVOCI | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Debt securities | Credit risk | Financial assets at FVOCI
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	9,000	9,600
Collateral cash	0	0
Collateral non-cash	0	0
Netting	0	0
Net exposure	9,000	9,600
Debt securities | Credit risk | Financial assets at FVOCI | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	9,000	9,600
Debt securities | Credit risk | Financial assets at FVOCI | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Debt securities | Off-Balance Sheet | Credit risk | Financial assets at FVOCI
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Debt securities | Off-Balance Sheet | Credit risk | Financial assets at FVOCI | Gross carrying amount
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	0	0
Debt securities | Off-Balance Sheet | Credit risk | Financial assets at FVOCI | Accumulated impairment
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	£ 0	£ 0

Credit Risk - Summary of Diff_2

Credit Risk - Summary of Difference Between Maximum and Net Exposure to Credit Risk (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	£ 95,156	£ 80,918
Financial collateral, liabilities	16,772	18,823
Netting	(911)	(1,305)
Net exposure	322,745	310,155
Credit risk
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	281,600	270,900
Financial collateral, liabilities	200	200
Collateral non-cash	(215,700)	(217,500)
Netting	(100)	(500)
Net exposure	108,700	94,200
Derivative Financial Instruments | Credit risk
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	3,400	3,300
Financial collateral, liabilities	0	0
Collateral non-cash	(1,800)	(1,900)
Netting	(800)	(800)
Net exposure	800	600
Other financial assets at fair value through profit loss [member] | Credit risk
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	200	400
Financial collateral, liabilities	0	0
Collateral non-cash	0	0
Netting	0	0
Net exposure	200	400
Financial assets at fair value through profit loss [member] | Credit risk
Disclosure of credit risk exposure [line items]
Maximum exposure to balance sheet	3,600	3,700
Financial collateral, liabilities	0	0
Collateral non-cash	(1,800)	(1,900)
Netting	(800)	(800)
Net exposure	£ 1,000	£ 1,000

Credit Risk - Disclosure of Equ

Credit Risk - Disclosure of Equivalent Credit Rating Grade used by Standard and Poors Ratings Services (Detail)	12 Months Ended
	Dec. 31, 2020
Santander UK risk grade 9 [member]
Statement [Line Items]
Description of rating agencies used	AAA to AA+
Santander UK risk grade 8 [member]
Statement [Line Items]
Description of rating agencies used	AA to AA-
Santander UK risk grade 7 [member]
Statement [Line Items]
Description of rating agencies used	A+ to BBB
Santander UK risk grade 6 [member]
Statement [Line Items]
Description of rating agencies used	BBB- to BB
Santander UK risk grade 5 [member]
Statement [Line Items]
Description of rating agencies used	BB-
Santander UK risk grade 4 [member]
Statement [Line Items]
Description of rating agencies used	B+ to B
Santander UK risk grade 3 [member]
Statement [Line Items]
Description of rating agencies used	B-
Santander UK risk grade 2 [member]
Statement [Line Items]
Description of rating agencies used	CCC to C
Santander UK risk grade 1 [member]
Statement [Line Items]
Description of rating agencies used	D
Mid range [member] | Santander UK risk grade 9 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	0.01%
Mid range [member] | Santander UK risk grade 8 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	0.032%
Mid range [member] | Santander UK risk grade 7 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	0.10%
Mid range [member] | Santander UK risk grade 6 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	0.316%
Mid range [member] | Santander UK risk grade 5 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	1.00%
Mid range [member] | Santander UK risk grade 4 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	3.162%
Mid range [member] | Santander UK risk grade 3 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	10.00%
Mid range [member] | Santander UK risk grade 2 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	31.623%
Mid range [member] | Santander UK risk grade 1 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	100.00%
Lower range | Santander UK risk grade 9 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	0.00%
Lower range | Santander UK risk grade 8 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	0.021%
Lower range | Santander UK risk grade 7 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	0.066%
Lower range | Santander UK risk grade 6 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	0.208%
Lower range | Santander UK risk grade 5 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	0.658%
Lower range | Santander UK risk grade 4 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	2.081%
Lower range | Santander UK risk grade 3 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	6.581%
Lower range | Santander UK risk grade 2 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	20.811%
Lower range | Santander UK risk grade 1 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	100.00%
Top of range | Santander UK risk grade 9 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	0.021%
Top of range | Santander UK risk grade 8 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	0.066%
Top of range | Santander UK risk grade 7 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	0.208%
Top of range | Santander UK risk grade 6 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	0.658%
Top of range | Santander UK risk grade 5 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	2.081%
Top of range | Santander UK risk grade 4 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	6.581%
Top of range | Santander UK risk grade 3 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	20.811%
Top of range | Santander UK risk grade 2 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	99.999%
Top of range | Santander UK risk grade 1 [member]
Statement [Line Items]
Standard and poors rating probability of default percent	100.00%

Credit Risk - Summary of Credit

Credit Risk - Summary of Credit Rating of Financial Assets Subject to Credit Risk (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of internal credit grades [line items]
Impairment loss allowances	£ 1,303	£ 785
Coverage Ratio	0.40%	0.30%
Loss allowance	£ (1,303)	£ (785)
Coverage	0.40%	0.30%
Credit risk | Mortgages
Disclosure of internal credit grades [line items]
Impairment loss allowances	£ 280	£ 218
Loss allowance	(280)	(218)
Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	324,700	312,400
Impairment loss allowances	£ 1,400	£ 900
Coverage Ratio	0.40%	0.30%
Loss allowance	£ (1,400)	£ (900)
Financial assets	£ 324,700	£ 312,400
Coverage	0.40%	0.30%
Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,400	£ 900
Financial assets	1,400	900
Internal grade 9 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 68,500	£ 62,000
Coverage Ratio	0.00%	0.00%
Financial assets	£ 68,500	£ 62,000
Coverage	0.00%	0.00%
Internal grade 9 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Internal grade 8 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 43,300	£ 44,900
Coverage Ratio	0.00%	0.00%
Financial assets	£ 43,300	£ 44,900
Coverage	0.00%	0.00%
Internal grade 8 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Internal grade 7 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 86,000	£ 88,800
Coverage Ratio	0.00%	0.00%
Financial assets	£ 86,000	£ 88,800
Coverage	0.00%	0.00%
Internal grade 7 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Internal grade 6 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 55,400	£ 61,700
Coverage Ratio	0.20%	0.00%
Financial assets	£ 55,400	£ 61,700
Coverage	0.20%	0.00%
Internal grade 6 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 0
Financial assets	100	0
Internal grade 5 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 18,600	£ 23,800
Coverage Ratio	0.50%	0.40%
Financial assets	£ 18,600	£ 23,800
Coverage	0.50%	0.40%
Internal grade 5 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 100
Financial assets	100	100
Internal grade 4 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 28,600	£ 12,000
Coverage Ratio	0.70%	1.70%
Financial assets	£ 28,600	£ 12,000
Coverage	0.70%	1.70%
Internal grade 4 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 200	£ 200
Financial assets	200	200
Internal grade 3 to 1 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 8,700	£ 6,700
Coverage Ratio	11.50%	9.00%
Financial assets	£ 8,700	£ 6,700
Coverage	11.50%	9.00%
Internal grade 3 to 1 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,000	£ 600
Financial assets	1,000	600
Other | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 17,000	£ 13,400
Coverage Ratio	0.00%	0.00%
Financial assets	£ 17,000	£ 13,400
Coverage	0.00%	0.00%
Other | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	281,600	270,900
Impairment loss allowances	£ 1,300	£ 800
Coverage Ratio	0.50%	0.30%
Loss allowance	£ (1,300)	£ (800)
Financial assets	£ 281,600	£ 270,900
Coverage	0.50%	0.30%
Balance sheet exposures [member] | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,300	£ 800
Financial assets	1,300	800
Balance sheet exposures [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	231,300	240,000
Impairment loss allowances	£ 1,300	£ 800
Coverage Ratio	0.60%	0.30%
Loss allowance	£ (1,300)	£ (800)
Financial assets	£ 231,300	£ 240,000
Coverage	0.60%	0.30%
Balance sheet exposures [member] | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 166,400	£ 165,200
Impairment loss allowances	£ 300	£ 200
Coverage Ratio	0.20%	0.10%
Loss allowance	£ (300)	£ (200)
Financial assets	£ 166,400	£ 165,200
Coverage	0.20%	0.10%
Balance sheet exposures [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,300	£ 800
Financial assets	1,300	800
Balance sheet exposures [member] | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	300	200
Financial assets	300	200
Balance sheet exposures [member] | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	41,300	21,200
Impairment loss allowances	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Loss allowance	£ 0	£ 0
Financial assets	£ 41,300	£ 21,200
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	208,800	207,400
Impairment loss allowances	£ 1,300	£ 800
Coverage Ratio	0.60%	0.40%
Loss allowance	£ (1,300)	£ (800)
Financial assets	£ 208,800	£ 207,400
Coverage	0.60%	0.40%
Balance sheet exposures [member] | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,300	£ 800
Financial assets	1,300	800
Balance sheet exposures [member] | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	1,700	1,900
Impairment loss allowances	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Loss allowance	£ 0	£ 0
Financial assets	£ 1,700	£ 1,900
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	19,600	23,600
Impairment loss allowances	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Loss allowance	£ 0	£ 0
Financial assets	£ 19,600	£ 23,600
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	1,200	7,100
Impairment loss allowances	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Loss allowance	£ 0	£ 0
Financial assets	£ 1,200	£ 7,100
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	9,000	9,700
Impairment loss allowances	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Loss allowance	£ 0	£ 0
Financial assets	£ 9,000	£ 9,700
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 9 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 68,100	£ 61,100
Coverage Ratio	0.00%	0.00%
Financial assets	£ 68,100	£ 61,100
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 9 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 9 | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 21,500	£ 33,800
Coverage Ratio	0.00%	0.00%
Financial assets	£ 21,500	£ 33,800
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 9 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 7,900	£ 9,800
Coverage Ratio	0.00%	0.00%
Financial assets	£ 7,900	£ 9,800
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 9 | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 9 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	0	0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 9 | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 41,300	£ 21,200
Coverage Ratio	0.00%	0.00%
Financial assets	£ 41,300	£ 21,200
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 9 | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 9 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 8,000	£ 11,400
Coverage Ratio	0.00%	0.00%
Financial assets	£ 8,000	£ 11,400
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 9 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 9 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 100	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 9 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 9 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 12,200	£ 15,300
Coverage Ratio	0.00%	0.00%
Financial assets	£ 12,200	£ 15,300
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 9 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 9 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,200	£ 7,100
Coverage Ratio	0.00%	0.00%
Financial assets	£ 1,200	£ 7,100
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 9 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 9 | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 5,300	£ 6,100
Coverage Ratio	0.00%	0.00%
Financial assets	£ 5,300	£ 6,100
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 9 | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 8 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 34,500	£ 36,400
Coverage Ratio	0.00%	0.00%
Financial assets	£ 34,500	£ 36,400
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 8 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 8 | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 31,100	£ 33,200
Coverage Ratio	0.00%	0.00%
Financial assets	£ 31,100	£ 33,200
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 8 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 24,300	£ 25,000
Coverage Ratio	0.00%	0.00%
Financial assets	£ 24,300	£ 25,000
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 8 | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 8 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	0	0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 8 | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 8 | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 8 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 27,700	£ 30,600
Coverage Ratio	0.00%	0.00%
Financial assets	£ 27,700	£ 30,600
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 8 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 8 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 200
Coverage Ratio	0.00%	0.00%
Financial assets	£ 100	£ 200
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 8 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 8 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 3,300	£ 2,400
Coverage Ratio	0.00%	0.00%
Financial assets	£ 3,300	£ 2,400
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 8 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 8 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 8 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 8 | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 3,400	£ 3,200
Coverage Ratio	0.00%	0.00%
Financial assets	£ 3,400	£ 3,200
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 8 | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 7 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 76,500	£ 80,400
Coverage Ratio	0.00%	0.00%
Financial assets	£ 76,500	£ 80,400
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 7 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 7 | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 76,300	£ 80,000
Coverage Ratio	0.00%	0.00%
Financial assets	£ 76,300	£ 80,000
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 7 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 68,000	£ 71,900
Coverage Ratio	0.00%	0.00%
Financial assets	£ 68,000	£ 71,900
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 7 | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 7 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	0	0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 7 | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 7 | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 7 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 74,400	£ 75,400
Coverage Ratio	0.00%	0.00%
Financial assets	£ 74,400	£ 75,400
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 7 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 7 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 400	£ 400
Coverage Ratio	0.00%	0.00%
Financial assets	£ 400	£ 400
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 7 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 7 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,500	£ 4,200
Coverage Ratio	0.00%	0.00%
Financial assets	£ 1,500	£ 4,200
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 7 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 7 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 7 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 7 | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 200	£ 400
Coverage Ratio	0.00%	0.00%
Financial assets	£ 200	£ 400
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 7 | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 6 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 46,600	£ 53,600
Coverage Ratio	0.20%	0.00%
Financial assets	£ 46,600	£ 53,600
Coverage	0.20%	0.00%
Balance sheet exposures [member] | Internal grade 6 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 0
Financial assets	100	0
Balance sheet exposures [member] | Internal grade 6 | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 46,500	£ 53,600
Coverage Ratio	0.20%	0.00%
Financial assets	£ 46,500	£ 53,600
Coverage	0.20%	0.00%
Balance sheet exposures [member] | Internal grade 6 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 36,800	£ 42,900
Coverage Ratio	0.00%	0.00%
Financial assets	£ 36,800	£ 42,900
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 6 | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 0
Financial assets	100	0
Balance sheet exposures [member] | Internal grade 6 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	0	0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 6 | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 6 | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 6 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 44,100	£ 52,100
Coverage Ratio	0.20%	0.00%
Financial assets	£ 44,100	£ 52,100
Coverage	0.20%	0.00%
Balance sheet exposures [member] | Internal grade 6 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 0
Financial assets	100	0
Balance sheet exposures [member] | Internal grade 6 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 6 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 6 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 2,400	£ 1,500
Coverage Ratio	0.00%	0.00%
Financial assets	£ 2,400	£ 1,500
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 6 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 6 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 6 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 6 | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 100	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 6 | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 5 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 13,500	£ 18,800
Coverage Ratio	0.70%	0.50%
Financial assets	£ 13,500	£ 18,800
Coverage	0.70%	0.50%
Balance sheet exposures [member] | Internal grade 5 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 100
Financial assets	100	100
Balance sheet exposures [member] | Internal grade 5 | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 13,500	£ 18,800
Coverage Ratio	0.70%	0.50%
Financial assets	£ 13,500	£ 18,800
Coverage	0.70%	0.50%
Balance sheet exposures [member] | Internal grade 5 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 5,600	£ 7,700
Coverage Ratio	0.00%	0.00%
Financial assets	£ 5,600	£ 7,700
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 5 | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 100
Financial assets	100	100
Balance sheet exposures [member] | Internal grade 5 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	0	0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 5 | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 5 | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 5 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 13,500	£ 18,800
Coverage Ratio	0.70%	0.50%
Financial assets	£ 13,500	£ 18,800
Coverage	0.70%	0.50%
Balance sheet exposures [member] | Internal grade 5 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 100
Financial assets	100	100
Balance sheet exposures [member] | Internal grade 5 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 5 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 5 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 5 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 5 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 5 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 5 | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 5 | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 4 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 27,000	£ 10,900
Coverage Ratio	0.70%	1.80%
Financial assets	£ 27,000	£ 10,900
Coverage	0.70%	1.80%
Balance sheet exposures [member] | Internal grade 4 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 200	£ 200
Financial assets	200	200
Balance sheet exposures [member] | Internal grade 4 | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 27,000	£ 10,900
Coverage Ratio	0.70%	1.80%
Financial assets	£ 27,000	£ 10,900
Coverage	0.70%	1.80%
Balance sheet exposures [member] | Internal grade 4 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 19,600	£ 4,200
Coverage Ratio	0.50%	2.40%
Financial assets	£ 19,600	£ 4,200
Coverage	0.50%	2.40%
Balance sheet exposures [member] | Internal grade 4 | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 200	£ 200
Financial assets	200	200
Balance sheet exposures [member] | Internal grade 4 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	100	100
Financial assets	100	100
Balance sheet exposures [member] | Internal grade 4 | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 4 | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 4 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 27,000	£ 10,900
Coverage Ratio	0.70%	1.80%
Financial assets	£ 27,000	£ 10,900
Coverage	0.70%	1.80%
Balance sheet exposures [member] | Internal grade 4 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 200	£ 200
Financial assets	200	200
Balance sheet exposures [member] | Internal grade 4 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 4 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 4 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 4 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 4 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 4 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 4 | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 4 | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 3 to 1 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 8,200	£ 6,200
Coverage Ratio	11.00%	8.10%
Financial assets	£ 8,200	£ 6,200
Coverage	11.00%	8.10%
Balance sheet exposures [member] | Internal grade 3 to 1 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 900	£ 500
Financial assets	900	500
Balance sheet exposures [member] | Internal grade 3 to 1 | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 8,200	£ 6,200
Coverage Ratio	11.00%	8.10%
Financial assets	£ 8,200	£ 6,200
Coverage	11.00%	8.10%
Balance sheet exposures [member] | Internal grade 3 to 1 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 4,500	£ 3,900
Coverage Ratio	4.40%	2.60%
Financial assets	£ 4,500	£ 3,900
Coverage	4.40%	2.60%
Balance sheet exposures [member] | Internal grade 3 to 1 | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 900	£ 500
Financial assets	900	500
Balance sheet exposures [member] | Internal grade 3 to 1 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	200	100
Financial assets	200	100
Balance sheet exposures [member] | Internal grade 3 to 1 | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 3 to 1 | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 3 to 1 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 8,200	£ 6,200
Coverage Ratio	11.00%	8.10%
Financial assets	£ 8,200	£ 6,200
Coverage	11.00%	8.10%
Balance sheet exposures [member] | Internal grade 3 to 1 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 900	£ 500
Financial assets	900	500
Balance sheet exposures [member] | Internal grade 3 to 1 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 3 to 1 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 3 to 1 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 3 to 1 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 3 to 1 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 3 to 1 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Internal grade 3 to 1 | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Internal grade 3 to 1 | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Other | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 8,500	£ 4,300
Coverage Ratio	0.00%	0.00%
Financial assets	£ 8,500	£ 4,300
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Other | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Other | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 8,500	£ 4,300
Coverage Ratio	0.00%	0.00%
Financial assets	£ 8,500	£ 4,300
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Other | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Other | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Other | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	0	0
Financial assets	0	0
Balance sheet exposures [member] | Other | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Other | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Other | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 7,200	£ 2,800
Coverage Ratio	0.00%	0.00%
Financial assets	£ 7,200	£ 2,800
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Other | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Other | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,100	£ 1,300
Coverage Ratio	0.00%	0.00%
Financial assets	£ 1,100	£ 1,300
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Other | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Other | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 200	£ 200
Coverage Ratio	0.00%	0.00%
Financial assets	£ 200	£ 200
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Other | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Other | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Other | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Balance sheet exposures [member] | Other | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Balance sheet exposures [member] | Other | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Off-Balance Sheet | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	43,100	41,500
Impairment loss allowances	£ 100	£ 100
Coverage Ratio	0.20%	0.20%
Loss allowance	£ (100)	£ (100)
Financial assets	£ 43,100	£ 41,500
Coverage	0.20%	0.20%
Off-Balance Sheet | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 100
Financial assets	100	100
Off-Balance Sheet | Internal grade 9 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 400	£ 900
Coverage Ratio	0.00%	0.00%
Financial assets	£ 400	£ 900
Coverage	0.00%	0.00%
Off-Balance Sheet | Internal grade 9 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Off-Balance Sheet | Internal grade 8 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 8,800	£ 8,500
Coverage Ratio	0.00%	0.00%
Financial assets	£ 8,800	£ 8,500
Coverage	0.00%	0.00%
Off-Balance Sheet | Internal grade 8 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Off-Balance Sheet | Internal grade 7 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 9,500	£ 8,400
Coverage Ratio	0.00%	0.00%
Financial assets	£ 9,500	£ 8,400
Coverage	0.00%	0.00%
Off-Balance Sheet | Internal grade 7 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Off-Balance Sheet | Internal grade 6 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 8,800	£ 8,100
Coverage Ratio	0.00%	0.00%
Financial assets	£ 8,800	£ 8,100
Coverage	0.00%	0.00%
Off-Balance Sheet | Internal grade 6 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Off-Balance Sheet | Internal grade 5 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 5,100	£ 5,000
Coverage Ratio	0.00%	0.00%
Financial assets	£ 5,100	£ 5,000
Coverage	0.00%	0.00%
Off-Balance Sheet | Internal grade 5 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Off-Balance Sheet | Internal grade 4 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,600	£ 1,100
Coverage Ratio	0.00%	0.00%
Financial assets	£ 1,600	£ 1,100
Coverage	0.00%	0.00%
Off-Balance Sheet | Internal grade 4 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Off-Balance Sheet | Internal grade 3 to 1 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 500	£ 500
Coverage Ratio	20.00%	20.00%
Financial assets	£ 500	£ 500
Coverage	20.00%	20.00%
Off-Balance Sheet | Internal grade 3 to 1 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 100
Financial assets	100	100
Off-Balance Sheet | Other | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 8,500	£ 9,100
Coverage Ratio	0.00%	0.00%
Financial assets	£ 8,500	£ 9,100
Coverage	0.00%	0.00%
Off-Balance Sheet | Other | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	41,800
Impairment loss allowances	£ 0
Coverage Ratio	0.00%
Loss allowance	£ 0
Financial assets	£ 41,800
Coverage	0.00%
Stage 1 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 1 | Internal grade 9 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 400
Coverage Ratio	0.00%
Financial assets	£ 400
Coverage	0.00%
Stage 1 | Internal grade 9 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 1 | Internal grade 8 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 8,800
Coverage Ratio	0.00%
Financial assets	£ 8,800
Coverage	0.00%
Stage 1 | Internal grade 8 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 1 | Internal grade 7 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 9,500
Coverage Ratio	0.00%
Financial assets	£ 9,500
Coverage	0.00%
Stage 1 | Internal grade 7 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 1 | Internal grade 6 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 8,600
Coverage Ratio	0.00%
Financial assets	£ 8,600
Coverage	0.00%
Stage 1 | Internal grade 6 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 1 | Internal grade 5 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 4,700
Coverage Ratio	0.00%
Financial assets	£ 4,700
Coverage	0.00%
Stage 1 | Internal grade 5 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 1 | Internal grade 4 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,100
Coverage Ratio	0.00%
Financial assets	£ 1,100
Coverage	0.00%
Stage 1 | Internal grade 4 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 1 | Internal grade 3 to 1 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 200
Coverage Ratio	0.00%
Financial assets	£ 200
Coverage	0.00%
Stage 1 | Internal grade 3 to 1 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 1 | Other | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 8,500
Coverage Ratio	0.00%
Financial assets	£ 8,500
Coverage	0.00%
Stage 1 | Other | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 1 | Balance sheet exposures [member] | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	154,600	155,500
Impairment loss allowances	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Loss allowance	£ 0	£ 0
Financial assets	£ 154,600	£ 155,500
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	41,300	21,200
Impairment loss allowances	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Loss allowance	£ 0	£ 0
Financial assets	£ 41,300	£ 21,200
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	189,900	194,100
Impairment loss allowances	£ 200	£ 100
Coverage Ratio	0.10%	0.10%
Loss allowance	£ (200)	£ (100)
Financial assets	£ 189,900	£ 194,100
Coverage	0.10%	0.10%
Stage 1 | Balance sheet exposures [member] | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 200	£ 100
Financial assets	200	100
Stage 1 | Balance sheet exposures [member] | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	1,700	1,900
Impairment loss allowances	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Loss allowance	£ 0	£ 0
Financial assets	£ 1,700	£ 1,900
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	19,600	23,600
Impairment loss allowances	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Loss allowance	£ 0	£ 0
Financial assets	£ 19,600	£ 23,600
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	1,200	7,100
Impairment loss allowances	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Loss allowance	£ 0	£ 0
Financial assets	£ 1,200	£ 7,100
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	9,000	9,700
Impairment loss allowances	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Loss allowance	£ 0	£ 0
Financial assets	£ 9,000	£ 9,700
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 9 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 7,900	£ 9,800
Coverage Ratio	0.00%	0.00%
Financial assets	£ 7,900	£ 9,800
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 9 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 9 | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 41,300	£ 21,200
Coverage Ratio	0.00%	0.00%
Financial assets	£ 41,300	£ 21,200
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 9 | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 9 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 8,000	£ 11,400
Coverage Ratio	0.00%	0.00%
Financial assets	£ 8,000	£ 11,400
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 9 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 9 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 100	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 9 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 9 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 12,200	£ 15,300
Coverage Ratio	0.00%	0.00%
Financial assets	£ 12,200	£ 15,300
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 9 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 9 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,200	£ 7,100
Coverage Ratio	0.00%	0.00%
Financial assets	£ 1,200	£ 7,100
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 9 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 9 | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 5,300	£ 6,100
Coverage Ratio	0.00%	0.00%
Financial assets	£ 5,300	£ 6,100
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 9 | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 8 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 24,300	£ 25,000
Coverage Ratio	0.00%	0.00%
Financial assets	£ 24,300	£ 25,000
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 8 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 8 | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 8 | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 8 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 27,600	£ 30,600
Coverage Ratio	0.00%	0.00%
Financial assets	£ 27,600	£ 30,600
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 8 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 8 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 200
Coverage Ratio	0.00%	0.00%
Financial assets	£ 100	£ 200
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 8 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 8 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 3,300	£ 2,400
Coverage Ratio	0.00%	0.00%
Financial assets	£ 3,300	£ 2,400
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 8 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 8 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 8 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 8 | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 3,400	£ 3,200
Coverage Ratio	0.00%	0.00%
Financial assets	£ 3,400	£ 3,200
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 8 | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 7 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 67,700	£ 71,700
Coverage Ratio	0.00%	0.00%
Financial assets	£ 67,700	£ 71,700
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 7 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 7 | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 7 | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 7 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 74,000	£ 75,000
Coverage Ratio	0.00%	0.00%
Financial assets	£ 74,000	£ 75,000
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 7 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 7 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 400	£ 400
Coverage Ratio	0.00%	0.00%
Financial assets	£ 400	£ 400
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 7 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 7 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,500	£ 4,200
Coverage Ratio	0.00%	0.00%
Financial assets	£ 1,500	£ 4,200
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 7 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 7 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 7 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 7 | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 200	£ 400
Coverage Ratio	0.00%	0.00%
Financial assets	£ 200	£ 400
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 7 | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 6 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 35,500	£ 42,000
Coverage Ratio	0.00%	0.00%
Financial assets	£ 35,500	£ 42,000
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 6 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 6 | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 6 | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 6 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 42,600	£ 50,900
Coverage Ratio	0.20%	0.00%
Financial assets	£ 42,600	£ 50,900
Coverage	0.20%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 6 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 0
Financial assets	100	0
Stage 1 | Balance sheet exposures [member] | Internal grade 6 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 6 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 6 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 2,400	£ 1,500
Coverage Ratio	0.00%	0.00%
Financial assets	£ 2,400	£ 1,500
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 6 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 6 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 6 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 6 | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 100	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 6 | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 5 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 3,900	£ 5,700
Coverage Ratio	0.00%	0.00%
Financial assets	£ 3,900	£ 5,700
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 5 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 5 | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 5 | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 5 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 10,800	£ 16,100
Coverage Ratio	0.00%	0.00%
Financial assets	£ 10,800	£ 16,100
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 5 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 5 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 5 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 5 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 5 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 5 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 5 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 5 | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 5 | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 4 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 14,700	£ 1,100
Coverage Ratio	0.00%	0.00%
Financial assets	£ 14,700	£ 1,100
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 4 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 4 | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 4 | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 4 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 19,000	£ 6,200
Coverage Ratio	0.00%	1.60%
Financial assets	£ 19,000	£ 6,200
Coverage	0.00%	1.60%
Stage 1 | Balance sheet exposures [member] | Internal grade 4 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 100
Financial assets	0	100
Stage 1 | Balance sheet exposures [member] | Internal grade 4 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 4 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 4 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 4 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 4 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 4 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 4 | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 4 | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 3 to 1 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 600	£ 200
Coverage Ratio	0.00%	0.00%
Financial assets	£ 600	£ 200
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 3 to 1 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 3 to 1 | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 3 to 1 | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 3 to 1 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,100	£ 1,200
Coverage Ratio	9.10%	0.00%
Financial assets	£ 1,100	£ 1,200
Coverage	9.10%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 3 to 1 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 0
Financial assets	100	0
Stage 1 | Balance sheet exposures [member] | Internal grade 3 to 1 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 3 to 1 | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 3 to 1 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 3 to 1 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 3 to 1 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 3 to 1 | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Internal grade 3 to 1 | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Internal grade 3 to 1 | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Other | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Other | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Other | Cash and balances at central banks | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Other | Cash and balances at central banks | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Other | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 7,000	£ 2,800
Coverage Ratio	0.00%	0.00%
Financial assets	£ 7,000	£ 2,800
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Other | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Other | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,100	£ 1,300
Coverage Ratio	0.00%	0.00%
Financial assets	£ 1,100	£ 1,300
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Other | Loans and advances to banks [member] | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Other | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 200	£ 200
Coverage Ratio	0.00%	0.00%
Financial assets	£ 200	£ 200
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Other | Reverse repurchase agreements - non-trading | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Other | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Other | Other financial assets at amortised cost | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Balance sheet exposures [member] | Other | Financial assets at fair value through other comprehensive income | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 1 | Balance sheet exposures [member] | Other | Financial assets at fair value through other comprehensive income | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 1 | Off-Balance Sheet | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		40,900
Impairment loss allowances		£ 0
Coverage Ratio		0.00%
Loss allowance		£ 0
Financial assets		£ 40,900
Coverage		0.00%
Stage 1 | Off-Balance Sheet | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 1 | Off-Balance Sheet | Internal grade 9 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 900
Coverage Ratio		0.00%
Financial assets		£ 900
Coverage		0.00%
Stage 1 | Off-Balance Sheet | Internal grade 9 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 1 | Off-Balance Sheet | Internal grade 8 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 8,500
Coverage Ratio		0.00%
Financial assets		£ 8,500
Coverage		0.00%
Stage 1 | Off-Balance Sheet | Internal grade 8 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 1 | Off-Balance Sheet | Internal grade 7 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 8,400
Coverage Ratio		0.00%
Financial assets		£ 8,400
Coverage		0.00%
Stage 1 | Off-Balance Sheet | Internal grade 7 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 1 | Off-Balance Sheet | Internal grade 6 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 8,000
Coverage Ratio		0.00%
Financial assets		£ 8,000
Coverage		0.00%
Stage 1 | Off-Balance Sheet | Internal grade 6 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 1 | Off-Balance Sheet | Internal grade 5 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 4,700
Coverage Ratio		0.00%
Financial assets		£ 4,700
Coverage		0.00%
Stage 1 | Off-Balance Sheet | Internal grade 5 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 1 | Off-Balance Sheet | Internal grade 4 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 1,000
Coverage Ratio		0.00%
Financial assets		£ 1,000
Coverage		0.00%
Stage 1 | Off-Balance Sheet | Internal grade 4 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 1 | Off-Balance Sheet | Internal grade 3 to 1 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 300
Coverage Ratio		0.00%
Financial assets		£ 300
Coverage		0.00%
Stage 1 | Off-Balance Sheet | Internal grade 3 to 1 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 1 | Off-Balance Sheet | Other | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 9,100
Coverage Ratio		0.00%
Financial assets		£ 9,100
Coverage		0.00%
Stage 1 | Off-Balance Sheet | Other | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 2 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	1,200
Impairment loss allowances	£ 100
Coverage Ratio	8.30%
Loss allowance	£ (100)
Financial assets	£ 1,200
Coverage	8.30%
Stage 2 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 100
Financial assets	100
Stage 2 | Internal grade 9 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Coverage Ratio	0.00%
Financial assets	£ 0
Coverage	0.00%
Stage 2 | Internal grade 9 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 2 | Internal grade 8 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Coverage Ratio	0.00%
Financial assets	£ 0
Coverage	0.00%
Stage 2 | Internal grade 8 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 2 | Internal grade 7 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Coverage Ratio	0.00%
Financial assets	£ 0
Coverage	0.00%
Stage 2 | Internal grade 7 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 2 | Internal grade 6 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 200
Coverage Ratio	0.00%
Financial assets	£ 200
Coverage	0.00%
Stage 2 | Internal grade 6 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 2 | Internal grade 5 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 400
Coverage Ratio	0.00%
Financial assets	£ 400
Coverage	0.00%
Stage 2 | Internal grade 5 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 2 | Internal grade 4 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 500
Coverage Ratio	0.00%
Financial assets	£ 500
Coverage	0.00%
Stage 2 | Internal grade 4 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 2 | Internal grade 3 to 1 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 200
Coverage Ratio	50.00%
Financial assets	£ 200
Coverage	50.00%
Stage 2 | Internal grade 3 to 1 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 100
Financial assets	100
Stage 2 | Other | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Coverage Ratio	0.00%
Financial assets	£ 0
Coverage	0.00%
Stage 2 | Other | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 2 | Balance sheet exposures [member] | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	10,100
Financial assets	10,100
Stage 2 | Balance sheet exposures [member] | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets		8,100
Impairment loss allowances	£ 200	£ 100
Coverage Ratio	2.00%	1.20%
Loss allowance	£ (200)	£ (100)
Financial assets		£ 8,100
Coverage	2.00%	1.20%
Stage 2 | Balance sheet exposures [member] | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 200	£ 100
Financial assets	200	100
Stage 2 | Balance sheet exposures [member] | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	16,600	11,400
Impairment loss allowances	£ 500	£ 300
Coverage Ratio	3.00%	2.60%
Loss allowance	£ (500)	£ (300)
Financial assets	£ 16,600	£ 11,400
Coverage	3.00%	2.60%
Stage 2 | Balance sheet exposures [member] | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 500	£ 300
Financial assets	500	300
Stage 2 | Balance sheet exposures [member] | Internal grade 9 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 2 | Balance sheet exposures [member] | Internal grade 9 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 2 | Balance sheet exposures [member] | Internal grade 9 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 2 | Balance sheet exposures [member] | Internal grade 9 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 2 | Balance sheet exposures [member] | Internal grade 8 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 2 | Balance sheet exposures [member] | Internal grade 8 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 2 | Balance sheet exposures [member] | Internal grade 8 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 100	£ 0
Coverage	0.00%	0.00%
Stage 2 | Balance sheet exposures [member] | Internal grade 8 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 2 | Balance sheet exposures [member] | Internal grade 7 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 300	£ 200
Coverage Ratio	0.00%	0.00%
Financial assets	£ 300	£ 200
Coverage	0.00%	0.00%
Stage 2 | Balance sheet exposures [member] | Internal grade 7 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 2 | Balance sheet exposures [member] | Internal grade 7 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 400	£ 400
Coverage Ratio	0.00%	0.00%
Financial assets	£ 400	£ 400
Coverage	0.00%	0.00%
Stage 2 | Balance sheet exposures [member] | Internal grade 7 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 2 | Balance sheet exposures [member] | Internal grade 6 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,300	£ 900
Coverage Ratio	0.00%	0.00%
Financial assets	£ 1,300	£ 900
Coverage	0.00%	0.00%
Stage 2 | Balance sheet exposures [member] | Internal grade 6 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 2 | Balance sheet exposures [member] | Internal grade 6 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,500	£ 1,200
Coverage Ratio	0.00%	0.00%
Financial assets	£ 1,500	£ 1,200
Coverage	0.00%	0.00%
Stage 2 | Balance sheet exposures [member] | Internal grade 6 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 2 | Balance sheet exposures [member] | Internal grade 5 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,700	£ 2,000
Coverage Ratio	0.00%	0.00%
Financial assets	£ 1,700	£ 2,000
Coverage	0.00%	0.00%
Stage 2 | Balance sheet exposures [member] | Internal grade 5 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 2 | Balance sheet exposures [member] | Internal grade 5 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 2,700	£ 2,700
Coverage Ratio	3.70%	3.70%
Financial assets	£ 2,700	£ 2,700
Coverage	3.70%	3.70%
Stage 2 | Balance sheet exposures [member] | Internal grade 5 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 100
Financial assets	100	100
Stage 2 | Balance sheet exposures [member] | Internal grade 4 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 4,900	£ 3,100
Coverage Ratio	2.00%	3.20%
Financial assets	£ 4,900	£ 3,100
Coverage	2.00%	3.20%
Stage 2 | Balance sheet exposures [member] | Internal grade 4 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 100
Financial assets	100	100
Stage 2 | Balance sheet exposures [member] | Internal grade 4 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 8,000	£ 4,700
Coverage Ratio	2.50%	2.10%
Financial assets	£ 8,000	£ 4,700
Coverage	2.50%	2.10%
Stage 2 | Balance sheet exposures [member] | Internal grade 4 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 200	£ 100
Financial assets	200	100
Stage 2 | Balance sheet exposures [member] | Internal grade 3 to 1 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 2,100	£ 2,000
Coverage Ratio	4.80%	0.00%
Financial assets	£ 2,100	£ 2,000
Coverage	4.80%	0.00%
Stage 2 | Balance sheet exposures [member] | Internal grade 3 to 1 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 0
Financial assets	100	0
Stage 2 | Balance sheet exposures [member] | Internal grade 3 to 1 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 4,200	£ 2,700
Coverage Ratio	4.80%	3.70%
Financial assets	£ 4,200	£ 2,700
Coverage	4.80%	3.70%
Stage 2 | Balance sheet exposures [member] | Internal grade 3 to 1 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 200	£ 100
Financial assets	200	100
Stage 2 | Balance sheet exposures [member] | Other | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 2 | Balance sheet exposures [member] | Other | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 2 | Balance sheet exposures [member] | Other | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 200	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 200	£ 0
Coverage	0.00%	0.00%
Stage 2 | Balance sheet exposures [member] | Other | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 2 | Off-Balance Sheet | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		600
Impairment loss allowances		£ 100
Coverage Ratio		16.70%
Loss allowance		£ (100)
Financial assets		£ 600
Coverage		16.70%
Stage 2 | Off-Balance Sheet | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 100
Financial assets		100
Stage 2 | Off-Balance Sheet | Internal grade 9 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Coverage Ratio		0.00%
Financial assets		£ 0
Coverage		0.00%
Stage 2 | Off-Balance Sheet | Internal grade 9 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 2 | Off-Balance Sheet | Internal grade 8 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Coverage Ratio		0.00%
Financial assets		£ 0
Coverage		0.00%
Stage 2 | Off-Balance Sheet | Internal grade 8 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 2 | Off-Balance Sheet | Internal grade 7 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Coverage Ratio		0.00%
Financial assets		£ 0
Coverage		0.00%
Stage 2 | Off-Balance Sheet | Internal grade 7 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 2 | Off-Balance Sheet | Internal grade 6 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 100
Coverage Ratio		0.00%
Financial assets		£ 100
Coverage		0.00%
Stage 2 | Off-Balance Sheet | Internal grade 6 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 2 | Off-Balance Sheet | Internal grade 5 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 300
Coverage Ratio		0.00%
Financial assets		£ 300
Coverage		0.00%
Stage 2 | Off-Balance Sheet | Internal grade 5 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 2 | Off-Balance Sheet | Internal grade 4 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 100
Coverage Ratio		0.00%
Financial assets		£ 100
Coverage		0.00%
Stage 2 | Off-Balance Sheet | Internal grade 4 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 2 | Off-Balance Sheet | Internal grade 3 to 1 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 200
Coverage Ratio		50.00%
Financial assets		£ 200
Coverage		50.00%
Stage 2 | Off-Balance Sheet | Internal grade 3 to 1 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 100
Financial assets		100
Stage 2 | Off-Balance Sheet | Other | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Coverage Ratio		0.00%
Financial assets		£ 0
Coverage		0.00%
Stage 2 | Off-Balance Sheet | Other | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 3 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	100
Impairment loss allowances	£ 0
Coverage Ratio	0.00%
Loss allowance	£ 0
Financial assets	£ 100
Coverage	0.00%
Stage 3 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 3 | Internal grade 9 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Coverage Ratio	0.00%
Financial assets	£ 0
Coverage	0.00%
Stage 3 | Internal grade 9 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 3 | Internal grade 8 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Coverage Ratio	0.00%
Financial assets	£ 0
Coverage	0.00%
Stage 3 | Internal grade 8 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 3 | Internal grade 7 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Coverage Ratio	0.00%
Financial assets	£ 0
Coverage	0.00%
Stage 3 | Internal grade 7 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 3 | Internal grade 6 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Coverage Ratio	0.00%
Financial assets	£ 0
Coverage	0.00%
Stage 3 | Internal grade 6 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 3 | Internal grade 5 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Coverage Ratio	0.00%
Financial assets	£ 0
Coverage	0.00%
Stage 3 | Internal grade 5 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 3 | Internal grade 4 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Coverage Ratio	0.00%
Financial assets	£ 0
Coverage	0.00%
Stage 3 | Internal grade 4 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 3 | Internal grade 3 to 1 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 100
Coverage Ratio	0.00%
Financial assets	£ 100
Coverage	0.00%
Stage 3 | Internal grade 3 to 1 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 3 | Other | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Coverage Ratio	0.00%
Financial assets	£ 0
Coverage	0.00%
Stage 3 | Other | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0
Financial assets	0
Stage 3 | Balance sheet exposures [member] | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	1,700	1,600
Impairment loss allowances	£ 100	£ 100
Coverage Ratio	5.90%	6.30%
Loss allowance	£ (100)	£ (100)
Financial assets	£ 1,700	£ 1,600
Coverage	5.90%	6.30%
Stage 3 | Balance sheet exposures [member] | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 100
Financial assets	100	100
Stage 3 | Balance sheet exposures [member] | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	2,300	1,900
Impairment loss allowances	£ 600	£ 400
Coverage Ratio	26.10%	21.10%
Loss allowance	£ (600)	£ (400)
Financial assets	£ 2,300	£ 1,900
Coverage	26.10%	21.10%
Stage 3 | Balance sheet exposures [member] | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 600	£ 400
Financial assets	600	400
Stage 3 | Balance sheet exposures [member] | Internal grade 9 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 3 | Balance sheet exposures [member] | Internal grade 9 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 3 | Balance sheet exposures [member] | Internal grade 9 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 3 | Balance sheet exposures [member] | Internal grade 9 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 3 | Balance sheet exposures [member] | Internal grade 8 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 3 | Balance sheet exposures [member] | Internal grade 8 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 3 | Balance sheet exposures [member] | Internal grade 8 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 3 | Balance sheet exposures [member] | Internal grade 8 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 3 | Balance sheet exposures [member] | Internal grade 7 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 3 | Balance sheet exposures [member] | Internal grade 7 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 3 | Balance sheet exposures [member] | Internal grade 7 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 3 | Balance sheet exposures [member] | Internal grade 7 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 3 | Balance sheet exposures [member] | Internal grade 6 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 3 | Balance sheet exposures [member] | Internal grade 6 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 3 | Balance sheet exposures [member] | Internal grade 6 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 3 | Balance sheet exposures [member] | Internal grade 6 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 3 | Balance sheet exposures [member] | Internal grade 5 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 3 | Balance sheet exposures [member] | Internal grade 5 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 3 | Balance sheet exposures [member] | Internal grade 5 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 3 | Balance sheet exposures [member] | Internal grade 5 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 3 | Balance sheet exposures [member] | Internal grade 4 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 3 | Balance sheet exposures [member] | Internal grade 4 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 3 | Balance sheet exposures [member] | Internal grade 4 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 3 | Balance sheet exposures [member] | Internal grade 4 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 3 | Balance sheet exposures [member] | Internal grade 3 to 1 | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 1,800	£ 1,700
Coverage Ratio	5.60%	5.90%
Financial assets	£ 1,800	£ 1,700
Coverage	5.60%	5.90%
Stage 3 | Balance sheet exposures [member] | Internal grade 3 to 1 | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 100	£ 100
Financial assets	100	100
Stage 3 | Balance sheet exposures [member] | Internal grade 3 to 1 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 2,900	£ 2,300
Coverage Ratio	20.70%	17.40%
Financial assets	£ 2,900	£ 2,300
Coverage	20.70%	17.40%
Stage 3 | Balance sheet exposures [member] | Internal grade 3 to 1 | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 600	£ 400
Financial assets	600	400
Stage 3 | Balance sheet exposures [member] | Other | Financial assets at amortised cost | Credit risk | Total exposure | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 3 | Balance sheet exposures [member] | Other | Financial assets at amortised cost | Credit risk | Expected credit loss | Mortgages
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	0	0
Stage 3 | Balance sheet exposures [member] | Other | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Coverage Ratio	0.00%	0.00%
Financial assets	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 3 | Balance sheet exposures [member] | Other | Loans and advances to customers | Financial assets at amortised cost | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets	£ 0	£ 0
Financial assets	£ 0	0
Stage 3 | Off-Balance Sheet | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		0
Impairment loss allowances		£ 0
Coverage Ratio		0.00%
Loss allowance		£ 0
Financial assets		£ 0
Coverage		0.00%
Stage 3 | Off-Balance Sheet | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 3 | Off-Balance Sheet | Internal grade 9 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Coverage Ratio		0.00%
Financial assets		£ 0
Coverage		0.00%
Stage 3 | Off-Balance Sheet | Internal grade 8 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Coverage Ratio		0.00%
Financial assets		£ 0
Coverage		0.00%
Stage 3 | Off-Balance Sheet | Internal grade 7 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Coverage Ratio		0.00%
Financial assets		£ 0
Coverage		0.00%
Stage 3 | Off-Balance Sheet | Internal grade 6 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Coverage Ratio		0.00%
Financial assets		£ 0
Coverage		0.00%
Stage 3 | Off-Balance Sheet | Internal grade 5 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Coverage Ratio		0.00%
Financial assets		£ 0
Coverage		0.00%
Stage 3 | Off-Balance Sheet | Internal grade 4 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Coverage Ratio		0.00%
Financial assets		£ 0
Coverage		0.00%
Stage 3 | Off-Balance Sheet | Internal grade 3 to 1 | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Coverage Ratio		0.00%
Financial assets		£ 0
Coverage		0.00%
Stage 3 | Off-Balance Sheet | Internal grade 3 to 1 | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		0
Stage 3 | Off-Balance Sheet | Other | Credit risk | Total exposure
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Coverage Ratio		0.00%
Financial assets		£ 0
Coverage		0.00%
Stage 3 | Off-Balance Sheet | Other | Credit risk | Expected credit loss
Disclosure of internal credit grades [line items]
Derivative assets		£ 0
Financial assets		£ 0

Credit Risk - Summary of Cred_2

Credit Risk - Summary of Credit Performance (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Customer loans	£ 208,750	£ 207,287
Exposure	322,745	310,155
Impairment loss allowances	1,303	785
Drawn exposures
Disclosure of credit risk exposure [line items]
Exposure	279,585	268,560
Impairment loss allowances	1,302	785
Loans to Customers
Disclosure of credit risk exposure [line items]
Customer loans	207,000	205,000
Gross write-offs	253	249
Impairment loss allowances	1,377	863
Loans to Customers | Stage 1
Disclosure of credit risk exposure [line items]
Exposure	187,100	191,000
Loans to Customers | Stage 1 | Undrawn Exposures
Disclosure of credit risk exposure [line items]
Exposure	41,800	40,300
Loans to Customers | Stage 2
Disclosure of credit risk exposure [line items]
Exposure	17,000	11,700
Loans to Customers | Stage 2 | Undrawn Exposures
Disclosure of credit risk exposure [line items]
Exposure	1,300	700
Loans to Customers | Stage 3
Disclosure of credit risk exposure [line items]
Exposure	2,900	2,300
Loans to Customers | Stage 3 | Undrawn Exposures
Disclosure of credit risk exposure [line items]
Exposure	100	100
Retail banking | Drawn exposures
Disclosure of credit risk exposure [line items]
Exposure	183,404	178,762
Impairment loss allowances	668	522
Retail banking | Loans to Customers
Disclosure of credit risk exposure [line items]
Customer loans	183,400	178,800
Gross write-offs	180	196
Impairment loss allowances	706	549
Retail banking | Loans to Customers | Stage 1
Disclosure of credit risk exposure [line items]
Exposure	170,200	167,600
Retail banking | Loans to Customers | Stage 2
Disclosure of credit risk exposure [line items]
Exposure	11,300	9,300
Retail banking | Loans to Customers | Stage 3
Disclosure of credit risk exposure [line items]
Exposure	1,900	1,900
Corporate and commercial banking | Drawn exposures
Disclosure of credit risk exposure [line items]
Exposure	17,626	18,391
Impairment loss allowances	577	247
Corporate and commercial banking | Loans to Customers
Disclosure of credit risk exposure [line items]
Customer loans	17,600	18,400
Gross write-offs	51	53
Impairment loss allowances	603	262
Corporate and commercial banking | Loans to Customers | Stage 1
Disclosure of credit risk exposure [line items]
Exposure	11,100	15,900
Corporate and commercial banking | Loans to Customers | Stage 2
Disclosure of credit risk exposure [line items]
Exposure	5,500	2,100
Corporate and commercial banking | Loans to Customers | Stage 3
Disclosure of credit risk exposure [line items]
Exposure	1,000	400
Corporate and investment banking | Drawn exposures
Disclosure of credit risk exposure [line items]
Exposure	2,785	4,041
Impairment loss allowances	22	14
Corporate and investment banking | Loans to Customers
Disclosure of credit risk exposure [line items]
Customer loans	2,800	4,000
Gross write-offs	22	0
Impairment loss allowances	33	50
Corporate and investment banking | Loans to Customers | Stage 1
Disclosure of credit risk exposure [line items]
Exposure	2,600	3,800
Corporate and investment banking | Loans to Customers | Stage 2
Disclosure of credit risk exposure [line items]
Exposure	200	200
Corporate and investment banking | Loans to Customers | Stage 3
Disclosure of credit risk exposure [line items]
Exposure	0	0
Corporate centre | Drawn exposures
Disclosure of credit risk exposure [line items]
Exposure	75,770	67,366
Impairment loss allowances	35	2
Corporate centre | Loans to Customers
Disclosure of credit risk exposure [line items]
Customer loans	3,200	3,800
Gross write-offs	0	0
Impairment loss allowances	35	2
Corporate centre | Loans to Customers | Stage 1
Disclosure of credit risk exposure [line items]
Exposure	3,200	3,700
Corporate centre | Loans to Customers | Stage 2
Disclosure of credit risk exposure [line items]
Exposure	0	100
Corporate centre | Loans to Customers | Stage 3
Disclosure of credit risk exposure [line items]
Exposure	0	0
Business Banking | Loans to Customers
Disclosure of credit risk exposure [line items]
Customer loans	3,900	200
Gross write-offs	12	14
Impairment loss allowances	9	9
Business Banking | Loans to Customers | Stage 1
Disclosure of credit risk exposure [line items]
Exposure	3,900	200
Business Banking | Loans to Customers | Stage 2
Disclosure of credit risk exposure [line items]
Exposure	0	0
Business Banking | Loans to Customers | Stage 3
Disclosure of credit risk exposure [line items]
Exposure	0	0
Consumer (auto) finance | Loans to Customers
Disclosure of credit risk exposure [line items]
Customer loans	8,000	7,700
Gross write-offs	25	34
Impairment loss allowances	118	88
Consumer (auto) finance | Loans to Customers | Stage 1
Disclosure of credit risk exposure [line items]
Exposure	7,600	7,000
Consumer (auto) finance | Loans to Customers | Stage 2
Disclosure of credit risk exposure [line items]
Exposure	400	600
Consumer (auto) finance | Loans to Customers | Stage 3
Disclosure of credit risk exposure [line items]
Exposure	0	100
Unsecured Lending | Loans to Customers
Disclosure of credit risk exposure [line items]
Customer loans	4,800	5,500
Gross write-offs	129	134
Impairment loss allowances	299	234
Unsecured Lending | Loans to Customers | Stage 1
Disclosure of credit risk exposure [line items]
Exposure	4,100	4,900
Unsecured Lending | Loans to Customers | Stage 2
Disclosure of credit risk exposure [line items]
Exposure	600	500
Unsecured Lending | Loans to Customers | Stage 3
Disclosure of credit risk exposure [line items]
Exposure	100	100
Mortgages
Disclosure of credit risk exposure [line items]
Customer loans	166,730	165,356
Mortgages | Loans to Customers
Disclosure of credit risk exposure [line items]
Customer loans	166,700	165,400
Gross write-offs	14	14
Impairment loss allowances	280	218
Mortgages | Loans to Customers | Stage 1
Disclosure of credit risk exposure [line items]
Exposure	154,600	155,500
Mortgages | Loans to Customers | Stage 2
Disclosure of credit risk exposure [line items]
Exposure	10,300	8,200
Mortgages | Loans to Customers | Stage 3
Disclosure of credit risk exposure [line items]
Exposure	£ 1,800	£ 1,700
Corporate loans | Loans to Customers | Stage 1
Disclosure of credit risk exposure [line items]
Exposure ratio	90.34%	93.23%
Corporate loans | Loans to Customers | Stage 2
Disclosure of credit risk exposure [line items]
Exposure ratio	8.26%	5.70%
Corporate loans | Loans to Customers | Stage 3
Disclosure of credit risk exposure [line items]
Exposure ratio	1.45%	1.15%

Credit risk - COVID-19 (Details

Credit risk - COVID-19 (Details) £ in Billions	Dec. 31, 2020GBP (£)Complaints
Bounce Back Loan Scheme
Disclosure of credit risk exposure [line items]
Number Of Customers Affected | Complaints	148,000
Loans and receivables | £	£ 4
Percentage Of Relevant Loan Book	19.00%
Coronavirus Business Interruption Loan Scheme
Disclosure of credit risk exposure [line items]
Number Of Customers Affected | Complaints	2,000
Loans and receivables | £	£ 0.4
Percentage Of Relevant Loan Book	2.00%
Coronavirus Large Business Interruption Loan Scheme
Disclosure of credit risk exposure [line items]
Number Of Customers Affected | Complaints	30
Loans and receivables | £	£ 0.2
Percentage Of Relevant Loan Book	3.00%
Mortgages
Disclosure of credit risk exposure [line items]
Number Of Customers Affected | Complaints	251,000
Loans and receivables | £	£ 37.1
Unsecured Personal Loans
Disclosure of credit risk exposure [line items]
Number Of Customers Affected | Complaints	34,000
Loans and receivables | £	£ 0.2
Credit cards
Disclosure of credit risk exposure [line items]
Number Of Customers Affected | Complaints	32,000
Loans and receivables | £	£ 0.1
Consumer Auto Finance
Disclosure of credit risk exposure [line items]
Number Of Customers Affected | Complaints	54,000
Loans and receivables | £	£ 0.5
Business And Corporates
Disclosure of credit risk exposure [line items]
Number Of Customers Affected | Complaints	2,500
Loans and receivables | £	£ 2.4

Credit risk - Support Measures

Credit risk - Support Measures and Corporate Customer Sector Split (Details) £ in Billions	Dec. 31, 2020GBP (£)Complaints	Dec. 31, 2019GBP (£)
Mortgages
Payment Holiday [Line Items]
Number Of Customers Affected | Complaints	251,000
Loans and receivables	£ 37.1
Outstanding PH	£ 2.5
Mortgages | Up to date after PH
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	88.00%
Mortgages | Ongoing PH
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	8.00%
Mortgages | New to arrears after PH ends
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	2.00%
Mortgages | In arrears before PH
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	2.00%
Consumer Auto Finance
Payment Holiday [Line Items]
Number Of Customers Affected | Complaints	54,000
Loans and receivables	£ 0.5
Outstanding PH	£ 0.1
Consumer Auto Finance | Up to date after PH
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	77.00%
Consumer Auto Finance | Ongoing PH
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	11.00%
Consumer Auto Finance | New to arrears after PH ends
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	8.00%
Consumer Auto Finance | In arrears before PH
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	4.00%
Unsecured Personal Loans
Payment Holiday [Line Items]
Number Of Customers Affected | Complaints	34,000
Loans and receivables	£ 0.2
Unsecured Personal Loans | Up to date after PH
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	77.00%
Unsecured Personal Loans | Ongoing PH
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	13.00%
Unsecured Personal Loans | New to arrears after PH ends
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	4.00%
Unsecured Personal Loans | In arrears before PH
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	6.00%
Credit cards
Payment Holiday [Line Items]
Number Of Customers Affected | Complaints	32,000
Loans and receivables	£ 0.1
Credit cards | Up to date after PH
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	76.00%
Credit cards | Ongoing PH
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	12.00%
Credit cards | New to arrears after PH ends
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	8.00%
Credit cards | In arrears before PH
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	4.00%
Business And Corporates
Payment Holiday [Line Items]
Number Of Customers Affected | Complaints	2,500
Loans and receivables	£ 2.4
Outstanding PH	£ 0.1
Business And Corporates | Up to date after PH
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	98.00%
Business And Corporates | Ongoing PH
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	2.00%
Business And Corporates | New to arrears after PH ends
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	0.00%
Business And Corporates | In arrears before PH
Payment Holiday [Line Items]
Percentage Of Relevant Loan Book	0.00%
Social Housing
Payment Holiday [Line Items]
Loans and receivables	£ 5.8	£ 6.4
Other Real Estate
Payment Holiday [Line Items]
Loans and receivables	4.9	5.5
Wholesale & Retail Trade
Payment Holiday [Line Items]
Loans and receivables	3.7	3.8
Accommodation & Food
Payment Holiday [Line Items]
Loans and receivables	2.1	1.8
Construction
Payment Holiday [Line Items]
Loans and receivables	1.6	1.2
Human Health & Social Work
Payment Holiday [Line Items]
Loans and receivables	1.5	1.4
Professional, Scientific And Technical
Payment Holiday [Line Items]
Loans and receivables	1.3	1
Manufacturing
Payment Holiday [Line Items]
Loans and receivables	1	1.2
Other Segments
Payment Holiday [Line Items]
Loans and receivables	2	1
Administrative & Support Services
Payment Holiday [Line Items]
Loans and receivables	1.3	1.3
Information & Communication
Payment Holiday [Line Items]
Loans and receivables	0.8	0.5
Transport & Storage
Payment Holiday [Line Items]
Loans and receivables	0.5	0.5
Electricity & Gas
Payment Holiday [Line Items]
Loans and receivables	0.6	0.5
Arts, Entertainment & Recreation
Payment Holiday [Line Items]
Loans and receivables	£ 0.4	£ 0.3

Credit Risk - Summary of IFRS 9

Credit Risk - Summary of IFRS 9 Credit Quality (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Exposure	£ 322,745	£ 310,155
Impairment loss allowances	1,303	785
Provisions	464	572
Total ECL	£ 1,377	£ 863
Coverage	0.40%	0.30%
Retail banking mortgages
Disclosure of credit risk exposure [line items]
Impairment loss allowances	£ 132
Drawn exposures
Disclosure of credit risk exposure [line items]
Exposure	279,585	£ 268,560
Impairment loss allowances	£ 1,302	£ 785
Coverage	0.50%	0.30%
Drawn exposures | Retail banking mortgages
Disclosure of credit risk exposure [line items]
Exposure	£ 166,730	£ 165,356
Impairment loss allowances	£ 277	£ 214
Coverage	0.20%	0.10%
Drawn exposures | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Exposure	£ 17,626	£ 18,391
Impairment loss allowances	£ 577	£ 247
Coverage	3.30%	1.30%
Drawn exposures | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Exposure	£ 2,785	£ 4,041
Impairment loss allowances	£ 22	£ 14
Coverage	0.80%	0.30%
Drawn exposures | Corporate centre
Disclosure of credit risk exposure [line items]
Exposure	£ 75,770	£ 67,366
Impairment loss allowances	£ 35	£ 2
Coverage	0.00%	0.00%
Drawn exposures | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	£ 183,404	£ 178,762
Impairment loss allowances	£ 668	£ 522
Coverage	0.40%	0.30%
Off-Balance Sheet Exposures
Disclosure of credit risk exposure [line items]
Exposure	£ 43,160	£ 41,595
Provisions	£ 75	£ 78
Coverage	0.20%	0.20%
Off-Balance Sheet Exposures | Retail banking mortgages
Disclosure of credit risk exposure [line items]
Exposure	£ 13,276	£ 13,432
Provisions	£ 3	£ 4
Coverage	0.00%	0.00%
Off-Balance Sheet Exposures | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Exposure	£ 6,877	£ 5,727
Provisions	£ 26	£ 15
Coverage	0.40%	0.30%
Off-Balance Sheet Exposures | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Exposure	£ 8,861	£ 9,343
Provisions	£ 11	£ 36
Coverage	0.10%	0.40%
Off-Balance Sheet Exposures | Corporate centre
Disclosure of credit risk exposure [line items]
Exposure	£ 575	£ 507
Off-Balance Sheet Exposures | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	26,847	26,018
Provisions	£ 38	£ 27
Coverage	0.10%	0.10%
Stage 1
Disclosure of credit risk exposure [line items]
Exposure	£ 301,413	£ 295,436
Total ECL	£ 216	£ 147
Coverage	0.10%	0.00%
Stage 1 | Drawn exposures
Disclosure of credit risk exposure [line items]
Exposure	£ 259,625	£ 254,595
Impairment loss allowances	£ 186	£ 124
Coverage	0.10%	0.00%
Stage 1 | Drawn exposures | Retail banking mortgages
Disclosure of credit risk exposure [line items]
Exposure	£ 154,586	£ 155,477
Impairment loss allowances	£ 15	£ 11
Coverage	0.00%	0.00%
Stage 1 | Drawn exposures | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Exposure	£ 11,167	£ 15,871
Impairment loss allowances	£ 46	£ 46
Coverage	0.40%	0.30%
Stage 1 | Drawn exposures | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Exposure	£ 2,587	£ 3,843
Impairment loss allowances	£ 5	£ 1
Coverage	0.20%	0.00%
Stage 1 | Drawn exposures | Corporate centre
Disclosure of credit risk exposure [line items]
Exposure	£ 75,743	£ 67,272
Impairment loss allowances	£ 35	£ 1
Coverage	0.00%	0.00%
Stage 1 | Drawn exposures | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	£ 170,128	£ 167,609
Impairment loss allowances	£ 100	£ 76
Coverage	0.10%	0.00%
Stage 1 | Off-Balance Sheet Exposures
Disclosure of credit risk exposure [line items]
Exposure	£ 41,788	£ 40,841
Provisions	£ 30	£ 23
Coverage	0.10%	0.10%
Stage 1 | Off-Balance Sheet Exposures | Retail banking mortgages
Disclosure of credit risk exposure [line items]
Exposure	£ 13,180	£ 13,353
Provisions	£ 2	£ 3
Coverage	0.00%	0.00%
Stage 1 | Off-Balance Sheet Exposures | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Exposure	£ 6,050	£ 5,410
Provisions	£ 8	£ 7
Coverage	0.10%	0.10%
Stage 1 | Off-Balance Sheet Exposures | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Exposure	£ 8,630	£ 9,129
Provisions	£ 4	£ 3
Coverage	0.00%	0.00%
Stage 1 | Off-Balance Sheet Exposures | Corporate centre
Disclosure of credit risk exposure [line items]
Exposure	£ 558	£ 507
Stage 1 | Off-Balance Sheet Exposures | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	26,550	25,795
Provisions	£ 18	£ 13
Coverage	0.10%	0.10%
Stage 2
Disclosure of credit risk exposure [line items]
Exposure	£ 18,336	£ 12,351
Total ECL	£ 592	£ 348
Coverage	3.20%	2.80%
Stage 2 | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	£ 11,597	£ 9,475
Coverage	3.20%	2.70%
Stage 2 | Drawn exposures
Disclosure of credit risk exposure [line items]
Exposure	£ 17,064	£ 11,677
Impairment loss allowances	£ 556	£ 305
Coverage	3.30%	2.60%
Stage 2 | Drawn exposures | Retail banking mortgages
Disclosure of credit risk exposure [line items]
Exposure	£ 10,345	£ 8,157
Impairment loss allowances	£ 130	£ 100
Coverage	1.30%	1.20%
Stage 2 | Drawn exposures | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Exposure	£ 5,498	£ 2,096
Impairment loss allowances	£ 189	£ 45
Coverage	3.40%	2.10%
Stage 2 | Drawn exposures | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Exposure	£ 198	£ 198
Impairment loss allowances	£ 17	£ 13
Coverage	8.60%	6.60%
Stage 2 | Drawn exposures | Corporate centre
Disclosure of credit risk exposure [line items]
Exposure	£ 27	£ 94
Impairment loss allowances	£ 0	£ 1
Coverage	0.00%	1.10%
Stage 2 | Drawn exposures | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	£ 11,341	£ 9,289
Impairment loss allowances	£ 350	£ 246
Coverage	3.10%	2.60%
Stage 2 | Off-Balance Sheet Exposures
Disclosure of credit risk exposure [line items]
Exposure	£ 1,272	£ 674
Provisions	£ 36	£ 43
Coverage	2.80%	6.40%
Stage 2 | Off-Balance Sheet Exposures | Retail banking mortgages
Disclosure of credit risk exposure [line items]
Exposure	£ 82	£ 67
Provisions	£ 1	£ 1
Coverage	1.20%	1.50%
Stage 2 | Off-Balance Sheet Exposures | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Exposure	£ 768	£ 289
Provisions	£ 10	£ 6
Coverage	1.30%	2.10%
Stage 2 | Off-Balance Sheet Exposures | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Exposure	£ 231	£ 199
Provisions	£ 7	£ 24
Coverage	3.00%	12.10%
Stage 2 | Off-Balance Sheet Exposures | Corporate centre
Disclosure of credit risk exposure [line items]
Exposure	£ 17	£ 0
Stage 2 | Off-Balance Sheet Exposures | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	256	186
Provisions	£ 19	£ 13
Coverage	7.40%	7.00%
Stage 3
Disclosure of credit risk exposure [line items]
Exposure	£ 2,996	£ 2,368
Total ECL	£ 569	£ 368
Coverage	19.00%	15.50%
Stage 3 | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Exposure	£ 1,020	£ 452
Stage 3 | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Exposure	0	15
Stage 3 | Drawn exposures
Disclosure of credit risk exposure [line items]
Exposure	2,896	2,288
Impairment loss allowances	£ 560	£ 356
Coverage	19.30%	15.60%
Stage 3 | Drawn exposures | Retail banking mortgages
Disclosure of credit risk exposure [line items]
Exposure	£ 1,799	£ 1,722
Impairment loss allowances		£ 103
Coverage	7.30%	6.00%
Stage 3 | Drawn exposures | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Exposure	£ 961	£ 424
Impairment loss allowances	£ 342	£ 156
Coverage	35.60%	36.80%
Stage 3 | Drawn exposures | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Exposure	£ 0	£ 0
Impairment loss allowances	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 3 | Drawn exposures | Corporate centre
Disclosure of credit risk exposure [line items]
Exposure	£ 0	£ 0
Impairment loss allowances	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 3 | Drawn exposures | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	£ 1,935	£ 1,864
Impairment loss allowances	£ 218	£ 200
Coverage	11.30%	10.70%
Stage 3 | Off-Balance Sheet Exposures
Disclosure of credit risk exposure [line items]
Exposure	£ 100	£ 80
Provisions	£ 9	£ 12
Coverage	9.00%	15.00%
Stage 3 | Off-Balance Sheet Exposures | Retail banking mortgages
Disclosure of credit risk exposure [line items]
Exposure	£ 14	£ 12
Provisions	£ 0	£ 0
Coverage	0.00%	0.00%
Stage 3 | Off-Balance Sheet Exposures | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Exposure	£ 59	£ 28
Provisions	£ 8	£ 2
Coverage	13.60%	7.10%
Stage 3 | Off-Balance Sheet Exposures | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Exposure	£ 0	£ 15
Provisions	£ 0	£ 9
Coverage	0.00%	60.00%
Stage 3 | Off-Balance Sheet Exposures | Corporate centre
Disclosure of credit risk exposure [line items]
Exposure	£ 0	£ 0
Stage 3 | Off-Balance Sheet Exposures | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	41	37
Provisions	£ 1	£ 1
Coverage	2.40%	2.70%

Credit Risk - Summary of IFRS_2

Credit Risk - Summary of IFRS 9 Credit Quality (Parenthetical) (Detail) - GBP (£) £ in Billions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [abstract]
Retail mortgage offers in pipeline	£ 7.7	£ 7.6

Credit Risk - Summary of Stage

Credit Risk - Summary of Stage 2 Decomposition (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Exposure	£ 322,745	£ 310,155
Coverage	0.40%	0.30%
Stage 2
Disclosure of credit risk exposure [line items]
Exposure	£ 18,336	£ 12,351
ECL	£ 592	£ 348
Coverage	3.20%	2.80%
Stage 2 | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	£ 11,597	£ 9,475
ECL	£ 369	£ 259
Coverage	3.20%	2.70%
Stage 2 | Retail banking | Mortgages
Disclosure of credit risk exposure [line items]
Exposure	£ 10,426	£ 8,223
ECL	£ 131	£ 102
Coverage	1.30%	1.20%
Stage 2 | Retail banking | Business Banking
Disclosure of credit risk exposure [line items]
Exposure	£ 10	£ 6
ECL	£ 2	£ 2
Coverage	20.00%	33.30%
Stage 2 | Retail banking | Consumer Auto Finance
Disclosure of credit risk exposure [line items]
Exposure	£ 378	£ 604
ECL	£ 37	£ 28
Coverage	9.80%	4.60%
Stage 2 | Retail banking | Other Unsecured Lending
Disclosure of credit risk exposure [line items]
Exposure	£ 783	£ 642
ECL	£ 199	£ 127
Coverage	25.40%	19.80%
Stage 2 | Other business segments
Disclosure of credit risk exposure [line items]
Exposure	£ 6,739	£ 2,876
ECL	£ 223	£ 89
Coverage	3.30%	3.10%
Deterioration in probability of default | Stage 2
Disclosure of credit risk exposure [line items]
Exposure	£ 10,000	£ 8,842
ECL	£ 299	£ 219
Coverage	3.00%	2.50%
Deterioration in probability of default | Stage 2 | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	£ 7,840	£ 6,749
ECL	£ 260	£ 190
Coverage	3.30%	2.80%
Deterioration in probability of default | Stage 2 | Retail banking | Mortgages
Disclosure of credit risk exposure [line items]
Exposure	£ 7,102	£ 6,216
ECL	£ 92	£ 78
Coverage	1.30%	1.30%
Deterioration in probability of default | Stage 2 | Retail banking | Business Banking
Disclosure of credit risk exposure [line items]
Exposure	£ 0	£ 0
ECL	£ 0	£ 0
Coverage	0.00%	0.00%
Deterioration in probability of default | Stage 2 | Retail banking | Consumer Auto Finance
Disclosure of credit risk exposure [line items]
Exposure	£ 88	£ 91
ECL	£ 13	£ 13
Coverage	14.80%	14.30%
Deterioration in probability of default | Stage 2 | Retail banking | Other Unsecured Lending
Disclosure of credit risk exposure [line items]
Exposure	£ 650	£ 442
ECL	£ 155	£ 99
Coverage	23.80%	22.40%
Deterioration in probability of default | Stage 2 | Other business segments
Disclosure of credit risk exposure [line items]
Exposure	£ 2,160	£ 2,093
ECL	£ 39	£ 29
Coverage	1.80%	1.40%
Forbearance support provided | Stage 2
Disclosure of credit risk exposure [line items]
Exposure	£ 767	£ 561
ECL	£ 9	£ 5
Coverage	1.20%	0.90%
Forbearance support provided | Stage 2 | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	£ 612	£ 504
ECL	£ 3	£ 3
Coverage	0.50%	0.60%
Forbearance support provided | Stage 2 | Retail banking | Mortgages
Disclosure of credit risk exposure [line items]
Exposure	£ 602	£ 504
ECL	£ 3	£ 3
Coverage	0.50%	0.60%
Forbearance support provided | Stage 2 | Retail banking | Business Banking
Disclosure of credit risk exposure [line items]
Exposure	£ 0	£ 0
ECL	£ 0	£ 0
Coverage	0.00%	0.00%
Forbearance support provided | Stage 2 | Retail banking | Consumer Auto Finance
Disclosure of credit risk exposure [line items]
Exposure	£ 0	£ 0
ECL	£ 0	£ 0
Coverage	0.00%	0.00%
Forbearance support provided | Stage 2 | Retail banking | Other Unsecured Lending
Disclosure of credit risk exposure [line items]
Exposure	£ 10	£ 0
ECL	£ 0	£ 0
Coverage	0.00%	0.00%
Forbearance support provided | Stage 2 | Other business segments
Disclosure of credit risk exposure [line items]
Exposure	£ 155	£ 57
ECL	£ 6	£ 2
Coverage	3.90%	3.50%
Other | Stage 2
Disclosure of credit risk exposure [line items]
Exposure	£ 2,450	£ 1,735
ECL	£ 111	£ 76
Coverage	4.50%	4.40%
Other | Stage 2 | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	£ 1,404	£ 1,174
ECL	£ 21	£ 20
Coverage	1.50%	1.70%
Other | Stage 2 | Retail banking | Mortgages
Disclosure of credit risk exposure [line items]
Exposure	£ 1,122	£ 520
ECL	£ 7	£ 6
Coverage	0.60%	1.20%
Other | Stage 2 | Retail banking | Business Banking
Disclosure of credit risk exposure [line items]
Exposure	£ 0	£ 0
ECL	£ 0	£ 0
Coverage	0.00%	0.00%
Other | Stage 2 | Retail banking | Consumer Auto Finance
Disclosure of credit risk exposure [line items]
Exposure	£ 248	£ 478
ECL	£ 11	£ 4
Coverage	4.40%	0.80%
Other | Stage 2 | Retail banking | Other Unsecured Lending
Disclosure of credit risk exposure [line items]
Exposure	£ 34	£ 176
ECL	£ 3	£ 10
Coverage	8.80%	5.70%
Other | Stage 2 | Other business segments
Disclosure of credit risk exposure [line items]
Exposure	£ 1,046	£ 561
ECL	£ 90	£ 56
Coverage	8.60%	10.00%
30 DPD | Stage 2
Disclosure of credit risk exposure [line items]
Exposure	£ 1,150	£ 1,213
ECL	£ 63	£ 48
Coverage	5.50%	4.00%
30 DPD | Stage 2 | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	£ 897	£ 1,048
ECL	£ 58	£ 46
Coverage	6.50%	4.40%
30 DPD | Stage 2 | Retail banking | Mortgages
Disclosure of credit risk exposure [line items]
Exposure	£ 804	£ 983
ECL	£ 14	£ 15
Coverage	1.70%	1.50%
30 DPD | Stage 2 | Retail banking | Business Banking
Disclosure of credit risk exposure [line items]
Exposure	£ 10	£ 6
ECL	£ 2	£ 2
Coverage	20.00%	33.30%
30 DPD | Stage 2 | Retail banking | Consumer Auto Finance
Disclosure of credit risk exposure [line items]
Exposure	£ 27	£ 35
ECL	£ 12	£ 11
Coverage	44.40%	31.40%
30 DPD | Stage 2 | Retail banking | Other Unsecured Lending
Disclosure of credit risk exposure [line items]
Exposure	£ 56	£ 24
ECL	£ 30	£ 18
Coverage	53.60%	75.00%
30 DPD | Stage 2 | Other business segments
Disclosure of credit risk exposure [line items]
Exposure	£ 253	£ 165
ECL	£ 5	£ 2
Coverage	2.00%	1.20%
Payment Holiday | Stage 2
Disclosure of credit risk exposure [line items]
Exposure	£ 844
ECL	£ 27
Coverage	3.20%
Payment Holiday | Stage 2 | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	£ 844
ECL	£ 27
Coverage	3.20%
Payment Holiday | Stage 2 | Retail banking | Mortgages
Disclosure of credit risk exposure [line items]
Exposure	£ 796
ECL	£ 15
Coverage	1.90%
Payment Holiday | Stage 2 | Retail banking | Business Banking
Disclosure of credit risk exposure [line items]
Exposure	£ 0
ECL	£ 0
Coverage	0.00%
Payment Holiday | Stage 2 | Retail banking | Consumer Auto Finance
Disclosure of credit risk exposure [line items]
Exposure	£ 15
ECL	£ 1
Coverage	6.70%
Payment Holiday | Stage 2 | Retail banking | Other Unsecured Lending
Disclosure of credit risk exposure [line items]
Exposure	£ 33
ECL	£ 11
Coverage	33.30%
Payment Holiday | Stage 2 | Other business segments
Disclosure of credit risk exposure [line items]
Exposure	£ 0
ECL	£ 0
Coverage	0.00%
High Risk Corporate | Stage 2
Disclosure of credit risk exposure [line items]
Exposure	£ 3,125
ECL	£ 83
Coverage	2.70%
High Risk Corporate | Stage 2 | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	£ 0
ECL	£ 0
Coverage	0.00%
High Risk Corporate | Stage 2 | Other business segments
Disclosure of credit risk exposure [line items]
Exposure	£ 3,125
ECL	£ 83
Coverage	2.70%

Credit Risk - Disclosure of Sta

Credit Risk - Disclosure of Stage 2 Decomposition Cure and Not Cure Explanatory (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Exposure	£ 322,745	£ 310,155
Coverage	0.40%	0.30%
Stage 2
Disclosure of credit risk exposure [line items]
Exposure	£ 18,336	£ 12,351
ECL	£ 592	£ 348
Coverage	3.20%	2.80%
Stage 2 not in cure period [member] | Stage 2
Disclosure of credit risk exposure [line items]
Exposure	£ 16,992	£ 12,017
ECL	£ 554	£ 342
Coverage	3.30%	2.80%
Stage 2 in cure period (for transfer to Stage 1) [member] | Stage 2
Disclosure of credit risk exposure [line items]
Exposure	£ 1,344	£ 334
ECL	£ 38	£ 6
Coverage	2.80%	1.80%

Credit Risk - Summary of Reconc

Credit Risk - Summary of Reconciliation of Exposures, ECL and Net Carrying Amounts (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of credit risk exposure [line items]
Exposure	£ 322,745	£ 310,155
Impairment loss allowances	1,303	785
Provisions	464	572
Assets classified at FVTPL	195	374
Total assets	292,332	281,702	£ 283,372
Drawn exposures
Disclosure of credit risk exposure [line items]
Exposure	279,585	268,560
Impairment loss allowances	1,302	785
On-balance sheet net carrying amount	278,283	267,775
Other items	3,111	2,986
Adjusted net carrying amount	281,394	270,761
Assets classified at FVTPL	3,614	3,702
Non-financial assets	7,324	7,239
Total assets	292,332	281,702
Drawn exposures | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	183,404	178,762
Impairment loss allowances	668	522
On-balance sheet net carrying amount	182,736	178,240
Drawn exposures | Mortgages
Disclosure of credit risk exposure [line items]
Exposure	166,730	165,356
Impairment loss allowances	277	214
On-balance sheet net carrying amount	166,453	165,142
Drawn exposures | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Exposure	17,626	18,391
Impairment loss allowances	577	247
On-balance sheet net carrying amount	17,049	18,144
Drawn exposures | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Exposure	2,785	4,041
Impairment loss allowances	22	14
On-balance sheet net carrying amount	2,763	4,027
Drawn exposures | Corporate centre
Disclosure of credit risk exposure [line items]
Exposure	75,770	67,366
Impairment loss allowances	35	2
On-balance sheet net carrying amount	75,735	67,364
Off-Balance Sheet Exposures
Disclosure of credit risk exposure [line items]
Exposure	43,160	41,595
Provisions	75	78
Off-Balance Sheet Exposures | Retail banking
Disclosure of credit risk exposure [line items]
Exposure	26,847	26,018
Provisions	38	27
Off-Balance Sheet Exposures | Mortgages
Disclosure of credit risk exposure [line items]
Exposure	13,276	13,432
Provisions	3	4
Off-Balance Sheet Exposures | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Exposure	6,877	5,727
Provisions	26	15
Off-Balance Sheet Exposures | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Exposure	8,861	9,343
Provisions	11	36
Off-Balance Sheet Exposures | Corporate centre
Disclosure of credit risk exposure [line items]
Exposure	575	507
Provisions	£ 0	£ 0

Credit Risk - Summary of ECL Re

Credit Risk - Summary of ECL Reconciliation (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	£ 863
Ending balance	1,377	£ 863
Recoveries net of collection costs	(24)	(40)	£ (42)
Total credit impairment charge/(release)	645	221	153
Expected credit loss
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	863	807
Change in economic scenarios	164	(19)
Changes to model	25	13
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfer of financial instruments	0	0
Net remeasurement of ECL on stage transfer	379	160
New lending and assets purchased	156	70
Assets derecognised - Redemptions and repayments	(90)	(102)
Other	133	183
Assets derecognised - written off	(253)	(249)
Ending balance	1,377	863	807
Net movement in the year	514
Income Statement ECL charge/(release)	767	305
Less: ECL relating to derecognised income	(14)	(13)
Recoveries net of collection costs	(108)	(71)
Total credit impairment charge/(release)	645	221
Stage 1
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	147
Ending balance	216	147
Stage 2
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	348
Ending balance	592	348
Stage 3
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	368
Ending balance	569	368
Total exposure
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	310,155	305,464
Change in economic scenarios	0	0
Changes to model	0	0
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfer of financial instruments	0	0
Net remeasurement of ECL on stage transfer	0	0
New lending and assets purchased	57,021	43,257
Assets derecognised - Redemptions and repayments	(53,434)	(42,183)
Other	9,398	3,980
Assets derecognised - written off	(395)	(363)
Ending balance	322,745	310,155	305,464
Net movement in the year	12,590	4,691
Financial instruments not credit-impaired | Subject to 12-month ECL | Stage 1 ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	147	143
Change in economic scenarios	15	5
Changes to model	0	0
Transfers from Stage 1 to Stage 2	(47)	(11)
Transfers from Stage 2 to Stage 1	110	74
Transfers to Stage 3	(8)	(2)
Transfers from Stage 3	2	1
Transfer of financial instruments	57	62
Net remeasurement of ECL on stage transfer	(101)	(66)
New lending and assets purchased	40	29
Assets derecognised - Redemptions and repayments	(30)	(32)
Other	88	6
Assets derecognised - written off	0	0
Ending balance	216	147	143
Net movement in the year	69
Income Statement ECL charge/(release)	69	4
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	0	(10)
Total credit impairment charge/(release)	69	(6)
Financial instruments not credit-impaired | Subject to 12-month ECL | Stage 1
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	295,436	290,882
Change in economic scenarios	0	0
Changes to model	0	0
Transfers from Stage 1 to Stage 2	(9,815)	(4,101)
Transfers from Stage 2 to Stage 1	3,178	3,458
Transfers to Stage 3	(385)	(361)
Transfers from Stage 3	12	10
Transfer of financial instruments	(7,010)	(994)
Net remeasurement of ECL on stage transfer	0	0
New lending and assets purchased	55,546	42,415
Assets derecognised - Redemptions and repayments	(50,698)	(40,380)
Other	8,141	3,514
Assets derecognised - written off	(2)	(1)
Ending balance	301,413	295,436	290,882
Net movement in the year	5,977	4,554
Financial instruments not credit-impaired | Subject to Lifetime ECL | Stage 2 ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	348	307
Change in economic scenarios	139	(15)
Changes to model	0	0
Transfers from Stage 1 to Stage 2	47	11
Transfers from Stage 2 to Stage 1	(110)	(74)
Transfers to Stage 3	(61)	(24)
Transfers from Stage 3	21	23
Transfer of financial instruments	(103)	(64)
Net remeasurement of ECL on stage transfer	239	130
New lending and assets purchased	64	32
Assets derecognised - Redemptions and repayments	(42)	(28)
Other	(53)	(14)
Assets derecognised - written off	0	0
Ending balance	592	348	307
Net movement in the year	244
Income Statement ECL charge/(release)	244	41
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	0	(15)
Total credit impairment charge/(release)	244	26
Financial instruments not credit-impaired | Subject to Lifetime ECL | Stage 2
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	12,351	12,011
Change in economic scenarios	0	0
Changes to model	0	0
Transfers from Stage 1 to Stage 2	9,815	4,101
Transfers from Stage 2 to Stage 1	(3,178)	(3,458)
Transfers to Stage 3	(1,126)	(595)
Transfers from Stage 3	326	516
Transfer of financial instruments	5,837	564
Net remeasurement of ECL on stage transfer	0	0
New lending and assets purchased	1,371	827
Assets derecognised - Redemptions and repayments	(2,295)	(1,344)
Other	1,072	294
Assets derecognised - written off	0	(1)
Ending balance	18,336	12,351	12,011
Net movement in the year	5,985	340
Financial instruments credit-impaired | Subject to Lifetime ECL | Stage 3 ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	368	357
Change in economic scenarios	10	(9)
Changes to model	25	13
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	69	26
Transfers from Stage 3	(23)	(24)
Transfer of financial instruments	46	2
Net remeasurement of ECL on stage transfer	241	96
New lending and assets purchased	52	9
Assets derecognised - Redemptions and repayments	(18)	(42)
Other	98	191
Assets derecognised - written off	(253)	(249)
Ending balance	569	368	357
Net movement in the year	201
Income Statement ECL charge/(release)	454	260
Less: ECL relating to derecognised income	(14)	(13)
Recoveries net of collection costs	(108)	(46)
Total credit impairment charge/(release)	332	201
Financial instruments credit-impaired | Subject to Lifetime ECL | Stage 3
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	2,368	2,571
Change in economic scenarios	0	0
Changes to model	0	0
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	1,511	956
Transfers from Stage 3	(338)	(526)
Transfer of financial instruments	1,173	430
Net remeasurement of ECL on stage transfer	0	0
New lending and assets purchased	104	15
Assets derecognised - Redemptions and repayments	(441)	(459)
Other	185	172
Assets derecognised - written off	(393)	(361)
Ending balance	2,996	2,368	£ 2,571
Net movement in the year	£ 628	£ (203)

Credit Risk - Summary of Countr

Credit Risk - Summary of Country Risk Exposure (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Exposure	£ 322,745	£ 310,155
Governments
Disclosure of credit risk exposure [line items]
Exposure	46,000	32,100
Government Guaranteed
Disclosure of credit risk exposure [line items]
Exposure	0	0
Banks
Disclosure of credit risk exposure [line items]
Exposure	7,600	9,300
Other Financial Institutions
Disclosure of credit risk exposure [line items]
Exposure	22,600	26,000
Retail
Disclosure of credit risk exposure [line items]
Exposure	206,100	204,500
Corporate counterparty
Disclosure of credit risk exposure [line items]
Exposure	42,200	40,000
Total For Counterparties Excluding Other Banco Santander Group Members
Disclosure of credit risk exposure [line items]
Exposure	324,500	311,900
Eurozone Countries [member] | Governments
Disclosure of credit risk exposure [line items]
Exposure	500	400
Eurozone Countries [member] | Governments | Ireland
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Governments | Italy
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Governments | Spain (excl. Banco Santander)
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Governments | France
Disclosure of credit risk exposure [line items]
Exposure	100	100
Eurozone Countries [member] | Governments | Germany
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Governments | Luxembourg
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Governments | Other
Disclosure of credit risk exposure [line items]
Exposure	400	300
Eurozone Countries [member] | Government Guaranteed
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Government Guaranteed | Ireland
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Government Guaranteed | Italy
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Government Guaranteed | Spain (excl. Banco Santander)
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Government Guaranteed | France
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Government Guaranteed | Germany
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Government Guaranteed | Luxembourg
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Government Guaranteed | Other
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Banks
Disclosure of credit risk exposure [line items]
Exposure	2,400	3,000
Eurozone Countries [member] | Banks | Ireland
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Banks | Italy
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Banks | Spain (excl. Banco Santander)
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Banks | France
Disclosure of credit risk exposure [line items]
Exposure	500	600
Eurozone Countries [member] | Banks | Germany
Disclosure of credit risk exposure [line items]
Exposure	700	1,300
Eurozone Countries [member] | Banks | Luxembourg
Disclosure of credit risk exposure [line items]
Exposure	100	100
Eurozone Countries [member] | Banks | Other
Disclosure of credit risk exposure [line items]
Exposure	1,100	1,000
Eurozone Countries [member] | Other Financial Institutions
Disclosure of credit risk exposure [line items]
Exposure	7,600	10,900
Eurozone Countries [member] | Other Financial Institutions | Ireland
Disclosure of credit risk exposure [line items]
Exposure	6,000	7,500
Eurozone Countries [member] | Other Financial Institutions | Italy
Disclosure of credit risk exposure [line items]
Exposure	0	100
Eurozone Countries [member] | Other Financial Institutions | Spain (excl. Banco Santander)
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Other Financial Institutions | France
Disclosure of credit risk exposure [line items]
Exposure	200	500
Eurozone Countries [member] | Other Financial Institutions | Germany
Disclosure of credit risk exposure [line items]
Exposure	100	0
Eurozone Countries [member] | Other Financial Institutions | Luxembourg
Disclosure of credit risk exposure [line items]
Exposure	1,300	2,700
Eurozone Countries [member] | Other Financial Institutions | Other
Disclosure of credit risk exposure [line items]
Exposure	0	100
Eurozone Countries [member] | Retail
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Retail | Ireland
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Retail | Italy
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Retail | Spain (excl. Banco Santander)
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Retail | France
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Retail | Germany
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Retail | Luxembourg
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Retail | Other
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Corporate counterparty
Disclosure of credit risk exposure [line items]
Exposure	800	300
Eurozone Countries [member] | Corporate counterparty | Ireland
Disclosure of credit risk exposure [line items]
Exposure	100	100
Eurozone Countries [member] | Corporate counterparty | Italy
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Corporate counterparty | Spain (excl. Banco Santander)
Disclosure of credit risk exposure [line items]
Exposure	100	0
Eurozone Countries [member] | Corporate counterparty | France
Disclosure of credit risk exposure [line items]
Exposure	0	0
Eurozone Countries [member] | Corporate counterparty | Germany
Disclosure of credit risk exposure [line items]
Exposure	100	100
Eurozone Countries [member] | Corporate counterparty | Luxembourg
Disclosure of credit risk exposure [line items]
Exposure	100	100
Eurozone Countries [member] | Corporate counterparty | Other
Disclosure of credit risk exposure [line items]
Exposure	400	0
Eurozone Countries [member] | Total For Counterparties Excluding Other Banco Santander Group Members
Disclosure of credit risk exposure [line items]
Exposure	11,300	14,600
Eurozone Countries [member] | Total For Counterparties Excluding Other Banco Santander Group Members | Ireland
Disclosure of credit risk exposure [line items]
Exposure	6,100	7,600
Eurozone Countries [member] | Total For Counterparties Excluding Other Banco Santander Group Members | Italy
Disclosure of credit risk exposure [line items]
Exposure	0	100
Eurozone Countries [member] | Total For Counterparties Excluding Other Banco Santander Group Members | Spain (excl. Banco Santander)
Disclosure of credit risk exposure [line items]
Exposure	100	0
Eurozone Countries [member] | Total For Counterparties Excluding Other Banco Santander Group Members | France
Disclosure of credit risk exposure [line items]
Exposure	800	1,200
Eurozone Countries [member] | Total For Counterparties Excluding Other Banco Santander Group Members | Germany
Disclosure of credit risk exposure [line items]
Exposure	900	1,400
Eurozone Countries [member] | Total For Counterparties Excluding Other Banco Santander Group Members | Luxembourg
Disclosure of credit risk exposure [line items]
Exposure	1,500	2,900
Eurozone Countries [member] | Total For Counterparties Excluding Other Banco Santander Group Members | Other
Disclosure of credit risk exposure [line items]
Exposure	1,900	1,400
All Other Countries [member] | Governments | Other
Disclosure of credit risk exposure [line items]
Exposure	300	200
All Other Countries [member] | Governments | UK
Disclosure of credit risk exposure [line items]
Exposure	42,000	28,300
All Other Countries [member] | Governments | US
Disclosure of credit risk exposure [line items]
Exposure	800	1,000
All Other Countries [member] | Governments | Japan
Disclosure of credit risk exposure [line items]
Exposure	2,400	2,200
All Other Countries [member] | Governments | Denmark
Disclosure of credit risk exposure [line items]
Exposure	0	0
All Other Countries [member] | Government Guaranteed | Other
Disclosure of credit risk exposure [line items]
Exposure	0	0
All Other Countries [member] | Government Guaranteed | UK
Disclosure of credit risk exposure [line items]
Exposure	0	0
All Other Countries [member] | Government Guaranteed | US
Disclosure of credit risk exposure [line items]
Exposure	0	0
All Other Countries [member] | Government Guaranteed | Japan
Disclosure of credit risk exposure [line items]
Exposure	0	0
All Other Countries [member] | Government Guaranteed | Denmark
Disclosure of credit risk exposure [line items]
Exposure	0	0
All Other Countries [member] | Banks | Other
Disclosure of credit risk exposure [line items]
Exposure	500	500
All Other Countries [member] | Banks | UK
Disclosure of credit risk exposure [line items]
Exposure	2,800	3,100
All Other Countries [member] | Banks | US
Disclosure of credit risk exposure [line items]
Exposure	900	1,100
All Other Countries [member] | Banks | Japan
Disclosure of credit risk exposure [line items]
Exposure	1,000	1,600
All Other Countries [member] | Banks | Denmark
Disclosure of credit risk exposure [line items]
Exposure	0	0
All Other Countries [member] | Other Financial Institutions | Other
Disclosure of credit risk exposure [line items]
Exposure	100	100
All Other Countries [member] | Other Financial Institutions | UK
Disclosure of credit risk exposure [line items]
Exposure	14,800	15,000
All Other Countries [member] | Other Financial Institutions | US
Disclosure of credit risk exposure [line items]
Exposure	100	0
All Other Countries [member] | Other Financial Institutions | Japan
Disclosure of credit risk exposure [line items]
Exposure	0	0
All Other Countries [member] | Other Financial Institutions | Denmark
Disclosure of credit risk exposure [line items]
Exposure	0	0
All Other Countries [member] | Retail | Other
Disclosure of credit risk exposure [line items]
Exposure	0	0
All Other Countries [member] | Retail | UK
Disclosure of credit risk exposure [line items]
Exposure	206,100	204,500
All Other Countries [member] | Retail | US
Disclosure of credit risk exposure [line items]
Exposure	0	0
All Other Countries [member] | Retail | Japan
Disclosure of credit risk exposure [line items]
Exposure	0	0
All Other Countries [member] | Retail | Denmark
Disclosure of credit risk exposure [line items]
Exposure	0	0
All Other Countries [member] | Corporate counterparty | Other
Disclosure of credit risk exposure [line items]
Exposure	700	800
All Other Countries [member] | Corporate counterparty | UK
Disclosure of credit risk exposure [line items]
Exposure	40,400	38,200
All Other Countries [member] | Corporate counterparty | US
Disclosure of credit risk exposure [line items]
Exposure	300	200
All Other Countries [member] | Corporate counterparty | Japan
Disclosure of credit risk exposure [line items]
Exposure	0	0
All Other Countries [member] | Corporate counterparty | Denmark
Disclosure of credit risk exposure [line items]
Exposure	0	500
All Other Countries [member] | Total For Counterparties Excluding Other Banco Santander Group Members | Other
Disclosure of credit risk exposure [line items]
Exposure	1,600	1,600
All Other Countries [member] | Total For Counterparties Excluding Other Banco Santander Group Members | UK
Disclosure of credit risk exposure [line items]
Exposure	306,100	289,100
All Other Countries [member] | Total For Counterparties Excluding Other Banco Santander Group Members | US
Disclosure of credit risk exposure [line items]
Exposure	2,100	2,300
All Other Countries [member] | Total For Counterparties Excluding Other Banco Santander Group Members | Japan
Disclosure of credit risk exposure [line items]
Exposure	3,400	3,800
All Other Countries [member] | Total For Counterparties Excluding Other Banco Santander Group Members | Denmark
Disclosure of credit risk exposure [line items]
Exposure	0	500
All Countries Excluding Eurozone Countries [member] | Governments
Disclosure of credit risk exposure [line items]
Exposure	45,500	31,700
All Countries Excluding Eurozone Countries [member] | Government Guaranteed
Disclosure of credit risk exposure [line items]
Exposure	0	0
All Countries Excluding Eurozone Countries [member] | Banks
Disclosure of credit risk exposure [line items]
Exposure	5,200	6,300
All Countries Excluding Eurozone Countries [member] | Other Financial Institutions
Disclosure of credit risk exposure [line items]
Exposure	15,000	15,100
All Countries Excluding Eurozone Countries [member] | Retail
Disclosure of credit risk exposure [line items]
Exposure	206,100	204,500
All Countries Excluding Eurozone Countries [member] | Corporate counterparty
Disclosure of credit risk exposure [line items]
Exposure	41,400	39,700
All Countries Excluding Eurozone Countries [member] | Total For Counterparties Excluding Other Banco Santander Group Members
Disclosure of credit risk exposure [line items]
Exposure	£ 313,200	£ 297,300

Credit Risk - Summary of Coun_2

Credit Risk - Summary of Country Risk Exposure (Parenthetical) (Detail) - GBP (£)	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Exposure	£ 322,745,000,000	£ 310,155,000,000
Eurozone Countries [member] | Netherlands
Disclosure of credit risk exposure [line items]
Exposure	600,000,000	200,000,000
Eurozone Countries [member] | Belgium
Disclosure of credit risk exposure [line items]
Exposure	£ 900,000,000	£ 600,000,000

Credit Risk - Summary of Balanc

Credit Risk - Summary of Balances with Other Banco Santander Companies (Detail) - Santander UK Group Holdings plc - GBP (£) £ in Billions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of detailed information about financial instruments [line items]
Financial assets	£ 4	£ 4
Financial liabilities	13.9	13.6
Spain (excl. Banco Santander)
Disclosure of detailed information about financial instruments [line items]
Financial assets	1.6	1.6
Financial liabilities	2.3	2.2
UK
Disclosure of detailed information about financial instruments [line items]
Financial assets	2.4	2.4
Financial liabilities	11.5	11.2
Uruguay
Disclosure of detailed information about financial instruments [line items]
Financial liabilities	0.1	0.2
Banks
Disclosure of detailed information about financial instruments [line items]
Financial assets	1.6	1.6
Financial liabilities	2.3	2.3
Banks | Spain (excl. Banco Santander)
Disclosure of detailed information about financial instruments [line items]
Financial assets	1.6	1.6
Financial liabilities	2.2	2.1
Banks | UK
Disclosure of detailed information about financial instruments [line items]
Financial assets	0	0
Financial liabilities	0	0
Banks | Uruguay
Disclosure of detailed information about financial instruments [line items]
Financial liabilities	0.1	0.2
Other Financial Institutions
Disclosure of detailed information about financial instruments [line items]
Financial assets	2.4	2.4
Financial liabilities	11.6	11.3
Other Financial Institutions | Spain (excl. Banco Santander)
Disclosure of detailed information about financial instruments [line items]
Financial assets	0	0
Financial liabilities	0.1	0.1
Other Financial Institutions | UK
Disclosure of detailed information about financial instruments [line items]
Financial assets	2.4	2.4
Financial liabilities	11.5	11.2
Other Financial Institutions | Uruguay
Disclosure of detailed information about financial instruments [line items]
Financial liabilities	0	0
Corporate counterparty
Disclosure of detailed information about financial instruments [line items]
Financial assets	0	0
Financial liabilities	0	0
Corporate counterparty | Spain (excl. Banco Santander)
Disclosure of detailed information about financial instruments [line items]
Financial assets	0	0
Financial liabilities	0	0
Corporate counterparty | UK
Disclosure of detailed information about financial instruments [line items]
Financial assets	0	0
Financial liabilities	0	0
Corporate counterparty | Uruguay
Disclosure of detailed information about financial instruments [line items]
Financial liabilities	£ 0	£ 0

Credit risk - Covid-19 Support

Credit risk - Covid-19 Support Measures and Corporate Customer Sector Split (Details) £ in Billions	Dec. 31, 2020GBP (£)Complaints	Dec. 31, 2019GBP (£)
Social Housing
Disclosure of credit risk exposure [line items]
Loans and receivables	£ 5.8	£ 6.4
Other Real Estate
Disclosure of credit risk exposure [line items]
Loans and receivables	4.9	5.5
Wholesale & Retail Trade
Disclosure of credit risk exposure [line items]
Loans and receivables	3.7	3.8
Accommodation & Food
Disclosure of credit risk exposure [line items]
Loans and receivables	2.1	1.8
Construction
Disclosure of credit risk exposure [line items]
Loans and receivables	1.6	1.2
Human Health & Social Work
Disclosure of credit risk exposure [line items]
Loans and receivables	1.5	1.4
Professional, Scientific And Technical
Disclosure of credit risk exposure [line items]
Loans and receivables	1.3	1
Administrative & Support Services
Disclosure of credit risk exposure [line items]
Loans and receivables	1.3	1.3
Manufacturing
Disclosure of credit risk exposure [line items]
Loans and receivables	1	1.2
Information & Communication
Disclosure of credit risk exposure [line items]
Loans and receivables	0.8	0.5
Transport & Storage
Disclosure of credit risk exposure [line items]
Loans and receivables	0.5	0.5
Electricity & Gas
Disclosure of credit risk exposure [line items]
Loans and receivables	0.6	0.5
Arts, Entertainment & Recreation
Disclosure of credit risk exposure [line items]
Loans and receivables	0.4	0.3
Other Segments
Disclosure of credit risk exposure [line items]
Loans and receivables	2	1
Corporate loans
Disclosure of credit risk exposure [line items]
Loans and receivables	£ 27.5	26.4
Bounce Back Loan Scheme
Disclosure of credit risk exposure [line items]
Number Of Customers Affected | Complaints	148,000
Loans and receivables	£ 4
Percentage Of Relevant Loan Book	19.00%
Coronavirus Business Interruption Loan Scheme
Disclosure of credit risk exposure [line items]
Number Of Customers Affected | Complaints	2,000
Loans and receivables	£ 0.4
Percentage Of Relevant Loan Book	2.00%
Coronavirus Large Business Interruption Loan Scheme
Disclosure of credit risk exposure [line items]
Number Of Customers Affected | Complaints	30
Loans and receivables	£ 0.2
Percentage Of Relevant Loan Book	3.00%
Government Lending Schemes | Corporate loans
Disclosure of credit risk exposure [line items]
Loans and receivables	£ 4.6	£ 0
Mortgages
Disclosure of credit risk exposure [line items]
Number Of Customers Affected | Complaints	251,000
Loans and receivables	£ 37.1
Outstanding PH	£ 2.5
Mortgages | Up to date after PH
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	88.00%
Mortgages | Ongoing PH
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	8.00%
Mortgages | New to arrears after PH ends
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	2.00%
Mortgages | In arrears before PH
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	2.00%
Consumer Auto Finance
Disclosure of credit risk exposure [line items]
Number Of Customers Affected | Complaints	54,000
Loans and receivables	£ 0.5
Outstanding PH	£ 0.1
Consumer Auto Finance | Up to date after PH
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	77.00%
Consumer Auto Finance | Ongoing PH
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	11.00%
Consumer Auto Finance | New to arrears after PH ends
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	8.00%
Consumer Auto Finance | In arrears before PH
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	4.00%
Unsecured Personal Loans
Disclosure of credit risk exposure [line items]
Number Of Customers Affected | Complaints	34,000
Loans and receivables	£ 0.2
Unsecured Personal Loans | Up to date after PH
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	77.00%
Unsecured Personal Loans | Ongoing PH
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	13.00%
Unsecured Personal Loans | New to arrears after PH ends
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	4.00%
Unsecured Personal Loans | In arrears before PH
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	6.00%
Credit cards
Disclosure of credit risk exposure [line items]
Number Of Customers Affected | Complaints	32,000
Loans and receivables	£ 0.1
Credit cards | Up to date after PH
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	76.00%
Credit cards | Ongoing PH
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	12.00%
Credit cards | New to arrears after PH ends
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	8.00%
Credit cards | In arrears before PH
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	4.00%
Business And Corporates
Disclosure of credit risk exposure [line items]
Number Of Customers Affected | Complaints	2,500
Loans and receivables	£ 2.4
Outstanding PH	£ 0.1
Business And Corporates | Up to date after PH
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	98.00%
Business And Corporates | Ongoing PH
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	2.00%
Business And Corporates | New to arrears after PH ends
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	0.00%
Business And Corporates | In arrears before PH
Disclosure of credit risk exposure [line items]
Percentage Of Relevant Loan Book	0.00%

Credit Risk - Summary of ECL _2

Credit Risk - Summary of ECL Reconciliation for Retail Segment (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	£ 863
Ending balance	1,377	£ 863
Recoveries net of collection costs	(24)	(40)	£ (42)
Income statement charge/(release) for the year	645	221	153
Retail banking
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Income statement charge/(release) for the year	308	156	135
Stage 1
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	147
Ending balance	216	147
Stage 2
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	348
Ending balance	592	348
Stage 3
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	368
Ending balance	569	368
Total exposure
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	310,155	305,464
Change in economic scenarios	0	0
Changes to model	0	0
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	0	0
New lending and assets purchased	57,021	43,257
Assets derecognised - Redemptions and repayments	(53,434)	(42,183)
Other	9,398	3,980
Assets derecognised - written off	(395)	(363)
Ending balance	322,745	310,155	305,464
Net movement in the year	12,590	4,691
Total exposure | Retail banking
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	204,780	194,127
Change in economic scenarios	0	0
Changes to model	0	0
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	0	0
New lending and assets purchased	35,474	37,239
Assets derecognised - Redemptions and repayments	30,364	27,453
Other	591	1,119
Assets derecognised - written off	230	252
Ending balance	210,251	204,780	194,127
Net movement in the year	5,471	10,653
Expected credit loss
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	863	807
Change in economic scenarios	164	(19)
Changes to model	25	13
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	379	160
New lending and assets purchased	156	70
Assets derecognised - Redemptions and repayments	(90)	(102)
Other	133	183
Assets derecognised - written off	(253)	(249)
Ending balance	1,377	863	807
Increase decrease in total expected credit losses		56
Net movement in the year	514
Charge/(release) to the Income Statement	767	305
Less: ECL relating to derecognised income	(14)	(13)
Recoveries net of collection costs	(108)	(71)
Income statement charge/(release) for the year	645	221
Expected credit loss | Retail banking
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	549	550
Change in economic scenarios	99	(40)
Changes to model	25	13
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	107	71
New lending and assets purchased	71	56
Assets derecognised - Redemptions and repayments	30	69
Other	65	163
Assets derecognised - written off	180	195
Ending balance	706	549	550
Increase decrease in total expected credit losses	157	(1)
Charge/(release) to the Income Statement	337	194
Less: ECL relating to derecognised income	(9)	(8)
Recoveries net of collection costs	(19)	(31)
Income statement charge/(release) for the year	309	155
Financial instruments not credit-impaired | Subject to 12-month ECL | Stage 1
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	295,436	290,882
Change in economic scenarios	0	0
Changes to model	0	0
Transfers from Stage 1 to Stage 2	(9,815)	(4,101)
Transfers from Stage 2 to Stage 1	3,178	3,458
Transfers to Stage 3	(385)	(361)
Transfers from Stage 3	12	10
Transfers of financial instruments	(7,010)	(994)
Net remeasurement of ECL on stage transfer	0	0
New lending and assets purchased	55,546	42,415
Assets derecognised - Redemptions and repayments	(50,698)	(40,380)
Other	8,141	3,514
Assets derecognised - written off	(2)	(1)
Ending balance	301,413	295,436	290,882
Net movement in the year	5,977	4,554
Financial instruments not credit-impaired | Subject to 12-month ECL | Stage 1 | Retail banking
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	193,404	181,600
Change in economic scenarios	0	0
Changes to model	0	0
Transfers from Stage 1 to Stage 2	(5,242)	(3,078)
Transfers from Stage 2 to Stage 1	2,544	3,254
Transfers to Stage 3	(305)	(315)
Transfers from Stage 3	8	9
Transfers of financial instruments	(2,995)	(130)
Net remeasurement of ECL on stage transfer	0	0
New lending and assets purchased	34,985	36,779
Assets derecognised - Redemptions and repayments	28,864	25,809
Other	150	965
Assets derecognised - written off	2	1
Ending balance	196,678	193,404	181,600
Net movement in the year	3,274	11,804
Financial instruments not credit-impaired | Subject to Lifetime ECL | Stage 2
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	12,351	12,011
Change in economic scenarios	0	0
Changes to model	0	0
Transfers from Stage 1 to Stage 2	9,815	4,101
Transfers from Stage 2 to Stage 1	(3,178)	(3,458)
Transfers to Stage 3	(1,126)	(595)
Transfers from Stage 3	326	516
Transfers of financial instruments	5,837	564
Net remeasurement of ECL on stage transfer	0	0
New lending and assets purchased	1,371	827
Assets derecognised - Redemptions and repayments	(2,295)	(1,344)
Other	1,072	294
Assets derecognised - written off	0	(1)
Ending balance	18,336	12,351	12,011
Net movement in the year	5,985	340
Financial instruments not credit-impaired | Subject to Lifetime ECL | Stage 2 | Retail banking
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	9,475	10,357
Change in economic scenarios	0	0
Changes to model	0	0
Transfers from Stage 1 to Stage 2	5,242	3,078
Transfers from Stage 2 to Stage 1	(2,544)	(3,254)
Transfers to Stage 3	(540)	(482)
Transfers from Stage 3	324	500
Transfers of financial instruments	2,482	(158)
Net remeasurement of ECL on stage transfer	0	0
New lending and assets purchased	479	447
Assets derecognised - Redemptions and repayments	1,181	1,207
Other	342	37
Assets derecognised - written off	0	1
Ending balance	11,597	9,475	10,357
Net movement in the year	2,122	(882)
Financial instruments not credit-impaired | Stage 1 ECL | Subject to 12-month ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	147	143
Change in economic scenarios	15	5
Changes to model	0	0
Transfers from Stage 1 to Stage 2	(47)	(11)
Transfers from Stage 2 to Stage 1	110	74
Transfers to Stage 3	(8)	(2)
Transfers from Stage 3	2	1
Transfers of financial instruments	57	62
Net remeasurement of ECL on stage transfer	(101)	(66)
New lending and assets purchased	40	29
Assets derecognised - Redemptions and repayments	(30)	(32)
Other	88	6
Assets derecognised - written off	0	0
Ending balance	216	147	143
Increase decrease in total expected credit losses		4
Net movement in the year	69
Charge/(release) to the Income Statement	69	4
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	0	(10)
Income statement charge/(release) for the year	69	(6)
Financial instruments not credit-impaired | Stage 1 ECL | Subject to 12-month ECL | Retail banking
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	89	88
Change in economic scenarios	8	(5)
Changes to model	0	0
Transfers from Stage 1 to Stage 2	(9)	(8)
Transfers from Stage 2 to Stage 1	86	67
Transfers to Stage 3	(7)	(2)
Transfers from Stage 3	1	1
Transfers of financial instruments	71	58
Net remeasurement of ECL on stage transfer	(79)	(60)
New lending and assets purchased	28	23
Assets derecognised - Redemptions and repayments	16	20
Other	17	5
Assets derecognised - written off	0	0
Ending balance	118	89	88
Increase decrease in total expected credit losses	29	1
Charge/(release) to the Income Statement	29	1
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	0	0
Income statement charge/(release) for the year	29	1
Financial instruments not credit-impaired | Stage 2 ECL | Subject to Lifetime ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	348	307
Change in economic scenarios	139	(15)
Changes to model	0	0
Transfers from Stage 1 to Stage 2	47	11
Transfers from Stage 2 to Stage 1	(110)	(74)
Transfers to Stage 3	(61)	(24)
Transfers from Stage 3	21	23
Transfers of financial instruments	(103)	(64)
Net remeasurement of ECL on stage transfer	239	130
New lending and assets purchased	64	32
Assets derecognised - Redemptions and repayments	(42)	(28)
Other	(53)	(14)
Assets derecognised - written off	0	0
Ending balance	592	348	307
Increase decrease in total expected credit losses		41
Net movement in the year	244
Charge/(release) to the Income Statement	244	41
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	0	(15)
Income statement charge/(release) for the year	244	26
Financial instruments not credit-impaired | Stage 2 ECL | Subject to Lifetime ECL | Retail banking
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	259	259
Change in economic scenarios	81	(26)
Changes to model	0	0
Transfers from Stage 1 to Stage 2	9	8
Transfers from Stage 2 to Stage 1	(86)	(67)
Transfers to Stage 3	(27)	(19)
Transfers from Stage 3	20	22
Transfers of financial instruments	(84)	(56)
Net remeasurement of ECL on stage transfer	144	75
New lending and assets purchased	37	24
Assets derecognised - Redemptions and repayments	22	16
Other	(46)	(1)
Assets derecognised - written off	0	0
Ending balance	369	259	259
Increase decrease in total expected credit losses	110	0
Charge/(release) to the Income Statement	110	0
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	0	0
Income statement charge/(release) for the year	110	0
Financial instruments credit-impaired | Subject to Lifetime ECL | Stage 3
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	2,368	2,571
Change in economic scenarios	0	0
Changes to model	0	0
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	1,511	956
Transfers from Stage 3	(338)	(526)
Transfers of financial instruments	1,173	430
Net remeasurement of ECL on stage transfer	0	0
New lending and assets purchased	104	15
Assets derecognised - Redemptions and repayments	(441)	(459)
Other	185	172
Assets derecognised - written off	(393)	(361)
Ending balance	2,996	2,368	2,571
Net movement in the year	628	(203)
Financial instruments credit-impaired | Subject to Lifetime ECL | Stage 3 | Retail banking
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	1,901	2,170
Change in economic scenarios	0	0
Changes to model	0	0
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	845	797
Transfers from Stage 3	(332)	(509)
Transfers of financial instruments	513	288
Net remeasurement of ECL on stage transfer	0	0
New lending and assets purchased	10	13
Assets derecognised - Redemptions and repayments	319	437
Other	99	117
Assets derecognised - written off	228	250
Ending balance	1,976	1,901	2,170
Net movement in the year	75	(269)
Financial instruments credit-impaired | Stage 3 ECL | Subject to Lifetime ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	368	357
Change in economic scenarios	10	(9)
Changes to model	25	13
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	69	26
Transfers from Stage 3	(23)	(24)
Transfers of financial instruments	46	2
Net remeasurement of ECL on stage transfer	241	96
New lending and assets purchased	52	9
Assets derecognised - Redemptions and repayments	(18)	(42)
Other	98	191
Assets derecognised - written off	(253)	(249)
Ending balance	569	368	357
Increase decrease in total expected credit losses		11
Net movement in the year	201
Charge/(release) to the Income Statement	454	260
Less: ECL relating to derecognised income	(14)	(13)
Recoveries net of collection costs	(108)	(46)
Income statement charge/(release) for the year	332	201
Financial instruments credit-impaired | Stage 3 ECL | Subject to Lifetime ECL | Retail banking
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	201	203
Change in economic scenarios	10	(9)
Changes to model	25	13
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	34	21
Transfers from Stage 3	(21)	(23)
Transfers of financial instruments	13	(2)
Net remeasurement of ECL on stage transfer	42	56
New lending and assets purchased	6	9
Assets derecognised - Redemptions and repayments	(8)	33
Other	94	159
Assets derecognised - written off	180	195
Ending balance	219	201	£ 203
Increase decrease in total expected credit losses	18	(2)
Charge/(release) to the Income Statement	198	193
Less: ECL relating to derecognised income	(9)	(8)
Recoveries net of collection costs	(19)	(31)
Income statement charge/(release) for the year	£ 170	£ 154

Credit Risk - Summary of Reside

Credit Risk - Summary of Residential Mortgages by Borrower Profile (Detail) - Credit risk - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Stock
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 166,730	£ 165,356
Percentage of residential mortgage loans	100.00%	100.00%
New Business [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 24,953	£ 29,814
Percentage of residential mortgage loans	100.00%	100.00%
First-Time Buyers [member] | Stock
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 33,180	£ 32,112
Percentage of residential mortgage loans	20.00%	19.00%
First-Time Buyers [member] | New Business [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 5,354	£ 6,952
Percentage of residential mortgage loans	21.00%	23.00%
Home Movers [member] | Stock
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 71,008	£ 70,860
Percentage of residential mortgage loans	42.00%	43.00%
Home Movers [member] | New Business [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 10,116	£ 11,192
Percentage of residential mortgage loans	41.00%	38.00%
Remortgagers [member] | Stock
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 50,934	£ 52,480
Percentage of residential mortgage loans	31.00%	32.00%
Remortgagers [member] | New Business [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 6,861	£ 9,197
Percentage of residential mortgage loans	27.00%	31.00%
Buy-To-Let | Stock
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 11,608	£ 9,904
Percentage of residential mortgage loans	7.00%	6.00%
Buy-To-Let | New Business [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 2,622	£ 2,473
Percentage of residential mortgage loans	11.00%	8.00%

Credit Risk - Additional Inform

Credit Risk - Additional Information (Detail) £ in Millions	Dec. 31, 2020GBP (£)MortgageLoan	Dec. 31, 2019GBP (£)MortgageLoan
Disclosure of internal credit grades [line items]
Advances and flexible mortgage drawdowns	£ 1,200	£ 1,300
Maximum exposure to balance sheet	95,156	80,918
Loans and advances to customers	208,750	207,287
Interest-only maturity default risk
Disclosure of internal credit grades [line items]
Post model adjustments	49	51
Buy-To-Let
Disclosure of internal credit grades [line items]
Post model adjustments	24	21
Long-term indeterminate arrears
Disclosure of internal credit grades [line items]
Post model adjustments	29	19
Long-term indeterminate arrears | No Delay In Repossession
Disclosure of internal credit grades [line items]
Post model adjustments under reasonably possible judgements	18
Long-term indeterminate arrears | Three-Year Repossession Delay
Disclosure of internal credit grades [line items]
Post model adjustments under reasonably possible judgements	31
12+ months in arrears
Disclosure of internal credit grades [line items]
Post model adjustments	30	23
12+ months in arrears | No Delay In Repossession
Disclosure of internal credit grades [line items]
Post model adjustments under reasonably possible judgements	19
12+ months in arrears | Three-Year Repossession Delay
Disclosure of internal credit grades [line items]
Post model adjustments under reasonably possible judgements	35
Corporate: Tall Tree
Disclosure of internal credit grades [line items]
Post model adjustments	35	0
Corporate: Tall Tree | Two Average Losses
Disclosure of internal credit grades [line items]
Post model adjustments under reasonably possible judgements	23
Corporate Covid-19 Affected Segments
Disclosure of internal credit grades [line items]
Post model adjustments	193	0
Corporate Covid-19 Affected Segments | Lowest Observed PD Stress Factor
Disclosure of internal credit grades [line items]
Post model adjustments under reasonably possible judgements	35
Corporate Covid-19 Affected Segments | Highest Observed PD Stress Factor
Disclosure of internal credit grades [line items]
Post model adjustments under reasonably possible judgements	311
Payment Holidays
Disclosure of internal credit grades [line items]
Post model adjustments	27	0
Payment Holidays | Higher New to Arrears Flow Rate
Disclosure of internal credit grades [line items]
Post model adjustments under reasonably possible judgements	11
Payment Holidays | Lower New to Arrears Flow Rate
Disclosure of internal credit grades [line items]
Post model adjustments under reasonably possible judgements	65
Credit risk
Disclosure of internal credit grades [line items]
Maximum exposure to balance sheet	281,600	270,900
Credit risk | Accumulated impairment
Disclosure of internal credit grades [line items]
Maximum exposure to balance sheet	(1,300)	(800)
Credit risk | Off-Balance Sheet
Disclosure of internal credit grades [line items]
Maximum exposure to balance sheet	43,100	41,500
Credit risk | Off-Balance Sheet | Accumulated impairment
Disclosure of internal credit grades [line items]
Maximum exposure to balance sheet	(100)	(100)
Exposures Transferred From Stage 1 To Stage 2 | Corporate Covid-19 Affected Segments
Disclosure of internal credit grades [line items]
Post model adjustments	68
Loans and advances to customers	3,100
Stage 2 Exposures Transferred To Stage 3 | Corporate Covid-19 Affected Segments
Disclosure of internal credit grades [line items]
Post model adjustments	122
Loans and advances to customers	400
High Risk Stage 2 Exposures | Corporate Covid-19 Affected Segments
Disclosure of internal credit grades [line items]
Loans and advances to customers	900
Internal Remortgages [member]
Disclosure of internal credit grades [line items]
Internal remortgages	£ 31,600	£ 31,600
Greater Than 2.0m | Credit risk
Disclosure of internal credit grades [line items]
Number of individual mortgage loans | MortgageLoan	98	76
New business number of individual mortgage loans | MortgageLoan	38	32
Bottom of range | Interest-only maturity default risk | Expected Proportion of Customers who Cannot Repay at Maturity
Disclosure of internal credit grades [line items]
Post model adjustments under reasonably possible judgements	£ 24
Top of range | Interest-only maturity default risk | Expected Proportion of Customers who Cannot Repay at Maturity
Disclosure of internal credit grades [line items]
Post model adjustments under reasonably possible judgements	£ 56

Credit Risk - Summary of Resi_2

Credit Risk - Summary of Residential Mortgages by Interest Rate Profile (Detail) - Stock - Credit risk - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 166,730	£ 165,356
Percentage of residential mortgage loans	100.00%	100.00%
Fixed Rate
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 133,231	£ 128,798
Percentage of residential mortgage loans	80.00%	78.00%
Variable Rate
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 20,986	£ 22,116
Percentage of residential mortgage loans	13.00%	13.00%
Standard Variable Rate
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 10,627	£ 14,124
Percentage of residential mortgage loans	6.00%	9.00%
Follow On Rate
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 1,886	£ 318
Percentage of residential mortgage loans	1.00%	0.00%

Credit Risk - Summary of Resi_3

Credit Risk - Summary of Residential Mortgages by Geographical Distribution (Detail) - Credit risk - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Stock
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 166,730	£ 165,356
New Business [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	24,953	29,814
London [member] | Stock
Disclosure of credit risk exposure [line items]
Residential mortgage loans	41,800	41,400
London [member] | New Business [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	6,100	7,500
Midlands and East Anglia [member] | Stock
Disclosure of credit risk exposure [line items]
Residential mortgage loans	22,500	22,100
Midlands and East Anglia [member] | New Business [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	3,700	4,300
North UK [member] | Stock
Disclosure of credit risk exposure [line items]
Residential mortgage loans	22,600	22,700
North UK [member] | New Business [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	3,300	3,800
Northern Ireland [member] | Stock
Disclosure of credit risk exposure [line items]
Residential mortgage loans	3,100	3,300
Northern Ireland [member] | New Business [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	200	300
Scotland [member] | Stock
Disclosure of credit risk exposure [line items]
Residential mortgage loans	6,700	6,800
Scotland [member] | New Business [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	800	1,200
South East Excluding London [member] | Stock
Disclosure of credit risk exposure [line items]
Residential mortgage loans	52,500	51,700
South East Excluding London [member] | New Business [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	8,300	9,700
South West and Wales and Other [member] | Stock
Disclosure of credit risk exposure [line items]
Residential mortgage loans	17,500	17,400
South West and Wales and Other [member] | New Business [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 2,600	£ 3,000

Credit Risk - Summary of Resi_4

Credit Risk - Summary of Residential Mortgages by Average Loan Size for New Business (Detail) - GBP (£) £ in Thousands	Dec. 31, 2020	Dec. 31, 2019
South East Including London [member]
Disclosure of credit risk exposure [line items]
New business residential mortgage loans average size	£ 284	£ 277
Rest of UK [member]
Disclosure of credit risk exposure [line items]
New business residential mortgage loans average size	166	154
UK
Disclosure of credit risk exposure [line items]
New business residential mortgage loans average size	£ 218	£ 207

Credit Risk - Summary of Resi_5

Credit Risk - Summary of Residential Mortgages by Loan Size (Detail) - Credit risk - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
South East Including London [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 94,330	£ 93,050
UK
Disclosure of credit risk exposure [line items]
Residential mortgage loans	166,730	165,356
Less Than 0.25m [member] | South East Including London [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	44,790	45,828
Less Than 0.25m [member] | UK
Disclosure of credit risk exposure [line items]
Residential mortgage loans	103,785	105,855
0.25m-0.50m[member] | South East Including London [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	35,487	34,027
0.25m-0.50m[member] | UK
Disclosure of credit risk exposure [line items]
Residential mortgage loans	46,914	44,549
0.50m-1.0m [member] | South East Including London [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	11,942	11,471
0.50m-1.0m [member] | UK
Disclosure of credit risk exposure [line items]
Residential mortgage loans	13,763	13,114
1.0m-2.0m [member] | South East Including London [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	1,875	1,538
1.0m-2.0m [member] | UK
Disclosure of credit risk exposure [line items]
Residential mortgage loans	2,024	1,644
Greater Than 2.0m | South East Including London [member]
Disclosure of credit risk exposure [line items]
Residential mortgage loans	236	186
Greater Than 2.0m | UK
Disclosure of credit risk exposure [line items]
Residential mortgage loans	£ 244	£ 194

Credit Risk - Summary of Resi_6

Credit Risk - Summary of Residential Mortgages by Loan to Value (Detail) - Credit risk - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Loan-to-value, Stock	£ 166,730	£ 165,356
Collateral value of residential properties, Stock	166,623	165,229
Loan-to-value, Stage 3	1,799	1,722
Collateral value of residential properties, Stage 3	1,783	1,702
Loan-to-value, New business	24,953	29,814
Collateral value of residential properties, New business	24,953	29,813
ECL
Disclosure of credit risk exposure [line items]
Loan-to-value, Stock	280	218
Loan-to-value, Stage 3	132	103
Up to 50%
Disclosure of credit risk exposure [line items]
Loan-to-value, Stock	73,489	70,714
Loan-to-value, Stage 3	858	743
Loan-to-value, New business	4,180	5,113
Up to 50% | ECL
Disclosure of credit risk exposure [line items]
Loan-to-value, Stock	28	24
Loan-to-value, Stage 3	11	11
50 - 75%
Disclosure of credit risk exposure [line items]
Loan-to-value, Stock	68,324	67,311
Loan-to-value, Stage 3	633	626
Loan-to-value, New business	10,088	11,876
50 - 75% | ECL
Disclosure of credit risk exposure [line items]
Loan-to-value, Stock	89	65
Loan-to-value, Stage 3	36	24
75 - 85%
Disclosure of credit risk exposure [line items]
Loan-to-value, Stock	18,113	17,436
Loan-to-value, Stage 3	125	136
Loan-to-value, New business	5,858	6,130
75 - 85% | ECL
Disclosure of credit risk exposure [line items]
Loan-to-value, Stock	41	31
Loan-to-value, Stage 3	19	13
85 - 100%
Disclosure of credit risk exposure [line items]
Loan-to-value, Stock	6,070	9,011
Loan-to-value, Stage 3	93	110
Loan-to-value, New business	4,781	6,650
85 - 100% | ECL
Disclosure of credit risk exposure [line items]
Loan-to-value, Stock	44	34
Loan-to-value, Stage 3	22	17
Greater Than 100%
Disclosure of credit risk exposure [line items]
Loan-to-value, Stock	734	884
Loan-to-value, Stage 3	90	107
Loan-to-value, New business	46	45
Greater Than 100% | ECL
Disclosure of credit risk exposure [line items]
Loan-to-value, Stock	78	64
Loan-to-value, Stage 3	£ 44	£ 38
Simple Average
Disclosure of credit risk exposure [line items]
Percentage of loan to value, Stock	42.00%	43.00%
Percentage of loan to value, Stage 3	41.00%	42.00%
Percentage of loan to value, New business	64.00%	65.00%

Credit Risk - Summary of Resi_7

Credit Risk - Summary of Residential Mortgages by Loan to Value (Parenthetical) (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Credit risk
Disclosure of credit risk exposure [line items]
Collateral against loans in negative equity	£ 629	£ 757

Credit Risk - Summary of Resi_8

Credit Risk - Summary of Residential Mortgages by Credit Performance (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 208,750	£ 207,287
Impairment loss allowances	1,303	785
Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	166,730	165,356
Credit risk | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	166,730	165,356
Impairment loss allowances	£ 280	£ 218
Stage 1 ratio	92.72%	94.03%
Stage 2 ratio	6.20%	4.93%
Stage 3 ratio	1.09%	1.05%
Stage 1 | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 154,586	£ 155,477
Stage 1 | Credit risk | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	154,586	155,477
Stage 2 | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	10,345	8,157
Stage 2 | Credit risk | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	10,345	8,157
Stage 3 | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 1,799	£ 1,722
Stage 3 ratio	1.09%	1.05%
Stage 3 | Credit risk | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 1,799	£ 1,722

Credit Risk - Summary of ECL _3

Credit Risk - Summary of ECL Reconciliation (Residential Mortgages) (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	£ 863
Ending balance	1,377	£ 863
Recoveries net of collection costs	(24)	(40)	£ (42)
Income statement charge/(release) for the year	645	221	153
Stage 1
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	147
Ending balance	216	147
Stage 1 | Subject to 12-month ECL | Financial instruments not credit-impaired
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	295,436	290,882
Transfers from Stage 1 to Stage 2	(9,815)	(4,101)
Transfers from Stage 2 to Stage 1	3,178	3,458
Transfers to Stage 3	(385)	(361)
Transfers from Stage 3	12	10
Transfers of financial instruments	(7,010)	(994)
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
Changes to model	0	0
New lending and assets purchased	55,546	42,415
Assets derecognised - Redemptions and repayments	(50,698)	(40,380)
Other	8,141	3,514
Assets derecognised - written off	(2)	(1)
Ending balance	301,413	295,436	290,882
Net movement in the year	5,977	4,554
Stage 1 | Subject to 12-month ECL | Financial instruments not credit-impaired | Residential mortgages
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	168,830	157,739
Transfers from Stage 1 to Stage 2	(4,686)	(2,345)
Transfers from Stage 2 to Stage 1	1,911	2,921
Transfers to Stage 3	(229)	(231)
Transfers from Stage 3	4	5
Transfers of financial instruments	(3,000)	350
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
Changes to model	0	0
New lending and assets purchased	26,102	31,090
Assets derecognised - Redemptions and repayments	(23,707)	(22,439)
Other	(459)	2,090
Assets derecognised - written off	0	0
Ending balance	167,766	168,830	157,739
Net movement in the year	(1,064)	11,091
Stage 2
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	348
Ending balance	592	348
Stage 2 | Subject to Lifetime ECL | Financial instruments not credit-impaired
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	12,351	12,011
Transfers from Stage 1 to Stage 2	9,815	4,101
Transfers from Stage 2 to Stage 1	(3,178)	(3,458)
Transfers to Stage 3	(1,126)	(595)
Transfers from Stage 3	326	516
Transfers of financial instruments	5,837	564
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
Changes to model	0	0
New lending and assets purchased	1,371	827
Assets derecognised - Redemptions and repayments	(2,295)	(1,344)
Other	1,072	294
Assets derecognised - written off	0	(1)
Ending balance	18,336	12,351	12,011
Net movement in the year	5,985	340
Stage 2 | Subject to Lifetime ECL | Financial instruments not credit-impaired | Residential mortgages
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	8,224	9,432
Transfers from Stage 1 to Stage 2	4,686	2,345
Transfers from Stage 2 to Stage 1	(1,911)	(2,921)
Transfers to Stage 3	(491)	(429)
Transfers from Stage 3	311	485
Transfers of financial instruments	2,595	(520)
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
Changes to model	0	0
New lending and assets purchased	237	198
Assets derecognised - Redemptions and repayments	(899)	(1,039)
Other	270	153
Assets derecognised - written off	0	0
Ending balance	10,427	8,224	9,432
Net movement in the year	2,203	(1,208)
Stage 3
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	368
Ending balance	569	368
Stage 3 | Subject to Lifetime ECL | Financial instruments credit-impaired
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	2,368	2,571
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	1,511	956
Transfers from Stage 3	(338)	(526)
Transfers of financial instruments	1,173	430
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
Changes to model	0	0
New lending and assets purchased	104	15
Assets derecognised - Redemptions and repayments	(441)	(459)
Other	185	172
Assets derecognised - written off	(393)	(361)
Ending balance	2,996	2,368	2,571
Net movement in the year	628	(203)
Stage 3 | Subject to Lifetime ECL | Financial instruments credit-impaired | Residential mortgages
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	1,734	1,999
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	720	660
Transfers from Stage 3	(315)	(490)
Transfers of financial instruments	405	170
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
Changes to model	0	0
New lending and assets purchased	1	2
Assets derecognised - Redemptions and repayments	(297)	(396)
Other	24	31
Assets derecognised - written off	(54)	(72)
Ending balance	1,813	1,734	1,999
Net movement in the year	79	(265)
Total exposure
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	310,155	305,464
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
Changes to model	0	0
New lending and assets purchased	57,021	43,257
Assets derecognised - Redemptions and repayments	(53,434)	(42,183)
Other	9,398	3,980
Assets derecognised - written off	(395)	(363)
Ending balance	322,745	310,155	305,464
Net movement in the year	12,590	4,691
Total exposure | Residential mortgages
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	178,788	169,170
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
Changes to model	0	0
New lending and assets purchased	26,340	31,290
Assets derecognised - Redemptions and repayments	(24,903)	(23,874)
Other	(165)	2,274
Assets derecognised - written off	(54)	(72)
Ending balance	180,006	178,788	169,170
Net movement in the year	1,218	9,618
Stage 1 ECL | Subject to 12-month ECL | Financial instruments not credit-impaired
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	147	143
Transfers from Stage 1 to Stage 2	(47)	(11)
Transfers from Stage 2 to Stage 1	110	74
Transfers to Stage 3	(8)	(2)
Transfers from Stage 3	2	1
Transfers of financial instruments	57	62
Net remeasurement of ECL on stage transfer	(101)	(66)
Change in economic scenarios	15	5
Changes to model	0	0
New lending and assets purchased	40	29
Assets derecognised - Redemptions and repayments	(30)	(32)
Other	88	6
Assets derecognised - written off	0	0
Ending balance	216	147	143
Net movement in the year		4
Net movement in the year	69
Charge/(release) to the Income Statement	69	4
Recoveries net of collection costs	0	(10)
Income statement charge/(release) for the year	69	(6)
Stage 1 ECL | Subject to 12-month ECL | Financial instruments not credit-impaired | Residential mortgages
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	14	12
Transfers from Stage 1 to Stage 2	(2)	(1)
Transfers from Stage 2 to Stage 1	17	24
Transfers to Stage 3	(3)	0
Transfers from Stage 3	0	(1)
Transfers of financial instruments	12	22
Net remeasurement of ECL on stage transfer	(15)	(23)
Change in economic scenarios	7	(5)
Changes to model	0	0
New lending and assets purchased	4	5
Assets derecognised - Redemptions and repayments	(4)	(1)
Other	(1)	4
Assets derecognised - written off	0	0
Ending balance	17	14	12
Net movement in the year	3	2
Charge/(release) to the Income Statement	3	2
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	0	0
Income statement charge/(release) for the year	3	2
Stage 2 ECL | Subject to Lifetime ECL | Financial instruments not credit-impaired
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	348	307
Transfers from Stage 1 to Stage 2	47	11
Transfers from Stage 2 to Stage 1	(110)	(74)
Transfers to Stage 3	(61)	(24)
Transfers from Stage 3	21	23
Transfers of financial instruments	(103)	(64)
Net remeasurement of ECL on stage transfer	239	130
Change in economic scenarios	139	(15)
Changes to model	0	0
New lending and assets purchased	64	32
Assets derecognised - Redemptions and repayments	(42)	(28)
Other	(53)	(14)
Assets derecognised - written off	0	0
Ending balance	592	348	307
Net movement in the year		41
Net movement in the year	244
Charge/(release) to the Income Statement	244	41
Recoveries net of collection costs	0	(15)
Income statement charge/(release) for the year	244	26
Stage 2 ECL | Subject to Lifetime ECL | Financial instruments not credit-impaired | Residential mortgages
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	101	119
Transfers from Stage 1 to Stage 2	2	1
Transfers from Stage 2 to Stage 1	(17)	(24)
Transfers to Stage 3	(11)	(8)
Transfers from Stage 3	15	16
Transfers of financial instruments	(11)	(15)
Net remeasurement of ECL on stage transfer	49	16
Change in economic scenarios	13	(22)
Changes to model	0	0
New lending and assets purchased	5	3
Assets derecognised - Redemptions and repayments	(7)	(6)
Other	(19)	6
Assets derecognised - written off	0	0
Ending balance	131	101	119
Net movement in the year	30	(18)
Charge/(release) to the Income Statement	29	(17)
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	0	0
Income statement charge/(release) for the year	29	(17)
Stage 3 ECL | Subject to Lifetime ECL | Financial instruments credit-impaired
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	368	357
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	69	26
Transfers from Stage 3	(23)	(24)
Transfers of financial instruments	46	2
Net remeasurement of ECL on stage transfer	241	96
Change in economic scenarios	10	(9)
Changes to model	25	13
New lending and assets purchased	52	9
Assets derecognised - Redemptions and repayments	(18)	(42)
Other	98	191
Assets derecognised - written off	(253)	(249)
Ending balance	569	368	357
Net movement in the year		11
Net movement in the year	201
Charge/(release) to the Income Statement	454	260
Recoveries net of collection costs	(108)	(46)
Income statement charge/(release) for the year	332	201
Stage 3 ECL | Subject to Lifetime ECL | Financial instruments credit-impaired | Residential mortgages
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	103	106
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	14	8
Transfers from Stage 3	(15)	(15)
Transfers of financial instruments	(1)	(7)
Net remeasurement of ECL on stage transfer	17	12
Change in economic scenarios	10	(9)
Changes to model	25	13
New lending and assets purchased	0	0
Assets derecognised - Redemptions and repayments	(15)	(12)
Other	7	14
Assets derecognised - written off	(14)	(14)
Ending balance	132	103	106
Net movement in the year	29	(3)
Charge/(release) to the Income Statement	43	11
Less: ECL relating to derecognised income	(2)	(4)
Recoveries net of collection costs	(1)	0
Income statement charge/(release) for the year	40	7
Expected credit loss
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	863	807
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	379	160
Change in economic scenarios	164	(19)
Changes to model	25	13
New lending and assets purchased	156	70
Assets derecognised - Redemptions and repayments	(90)	(102)
Other	133	183
Assets derecognised - written off	(253)	(249)
Ending balance	1,377	863	807
Net movement in the year		56
Net movement in the year	514
Charge/(release) to the Income Statement	767	305
Recoveries net of collection costs	(108)	(71)
Income statement charge/(release) for the year	645	221
Expected credit loss | Residential mortgages
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	218	237
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	51	5
Change in economic scenarios	30	(36)
Changes to model	25	13
New lending and assets purchased	9	8
Assets derecognised - Redemptions and repayments	(26)	(19)
Other	(13)	24
Assets derecognised - written off	(14)	(14)
Ending balance	280	218	£ 237
Net movement in the year	62	(19)
Charge/(release) to the Income Statement	75	(4)
Less: ECL relating to derecognised income	(2)	(4)
Recoveries net of collection costs	(1)	0
Income statement charge/(release) for the year	£ 72	£ (8)

Credit Risk - Summary of Modifi

Credit Risk - Summary of Modification of Loan Terms (Residential Mortgages) (Detail) - Mortgages - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Amortised cost before modification	£ 305	£ 384
Net modification loss	7	7
Gross carrying amount of financial assets for which the ECL allowance has changed to 12-month measurement during the year	£ 114	£ 89

Credit Risk - Summary of Resi_9

Credit Risk - Summary of Residential Mortgages by Forbearance Applied (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Capitalisation	£ 628	£ 602
Term extension	476	429
Interest-only	396	439
Concessionary interest rate	28	11
Forbearance	1,528	1,481
Impairment loss allowances	£ 42	£ 36
Loans capitalisation forbearance Percentage	0.40%	0.40%
Loans term extension forbearance percentage	0.30%	0.30%
Loans interest only forbearance percentage	0.20%	0.30%
Loans Concessionary interest rate forbearance percentage	0.00%	0.00%
Loans forbearance percentage	0.90%	0.90%
Stage 2 | Mortgages
Disclosure of credit risk exposure [line items]
Capitalisation	£ 409	£ 405
Term extension	393	338
Interest-only	310	342
Concessionary interest rate	0	1
Forbearance	1,112	1,086
Impairment loss allowances	13	12
Stage 3 | Mortgages
Disclosure of credit risk exposure [line items]
Capitalisation	219	197
Term extension	83	91
Interest-only	86	97
Concessionary interest rate	28	10
Forbearance	416	395
Impairment loss allowances	£ 29	£ 24

Credit Risk - Summary of Res_10

Credit Risk - Summary of Residential Mortgages Portfolios of Particular Interest by Credit Performance (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 208,750	£ 207,287
PIPs	£ 10	£ 32
Percenatge of simple average loan to value	42.00%	43.00%
Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 166,730	£ 165,356
Interest-only
Disclosure of credit risk exposure [line items]
PIPs	£ 5	£ 14
Percenatge of simple average loan to value	44.00%	45.00%
Interest-only | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 38,441	£ 38,062
Part Interest Only Part Repayment
Disclosure of credit risk exposure [line items]
PIPs	£ 2	£ 9
Percenatge of simple average loan to value	44.00%	45.00%
Part Interest Only Part Repayment | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 13,234	£ 13,247
Flexible
Disclosure of credit risk exposure [line items]
PIPs	£ 1	£ 2
Percenatge of simple average loan to value	26.00%	28.00%
Flexible | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 9,953	£ 11,273
LTV >100%
Disclosure of credit risk exposure [line items]
PIPs	£ 4	£ 13
Percenatge of simple average loan to value	117.00%	117.00%
LTV >100% | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 734	£ 884
Buy-to-Let
Disclosure of credit risk exposure [line items]
PIPs	£ 0	£ 1
Percenatge of simple average loan to value	59.00%	60.00%
Buy-to-Let | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 11,608	£ 9,904
Other
Disclosure of credit risk exposure [line items]
PIPs	£ 2	£ 8
Percenatge of simple average loan to value	43.00%	44.00%
Other | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 110,854	£ 109,234
Stage 1 | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	154,586	155,477
Stage 1 | Interest-only | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	33,330	33,739
Stage 1 | Part Interest Only Part Repayment | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	11,860	12,112
Stage 1 | Flexible | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	8,731	10,183
Stage 1 | LTV >100% | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	423	594
Stage 1 | Buy-to-Let | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	11,180	9,593
Stage 1 | Other | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	105,514	105,114
Stage 2 | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	10,345	8,157
Stage 2 | Interest-only | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	4,228	3,502
Stage 2 | Part Interest Only Part Repayment | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	1,126	888
Stage 2 | Flexible | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	989	873
Stage 2 | LTV >100% | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	221	183
Stage 2 | Buy-to-Let | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	393	285
Stage 2 | Other | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	4,728	3,526
Stage 3 | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 1,799	£ 1,722
Stage 3 ratio	1.09%	1.05%
Stage 3 | Interest-only | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 883	£ 821
Stage 3 ratio	2.31%	2.17%
Stage 3 | Part Interest Only Part Repayment | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 248	£ 247
Stage 3 ratio	1.88%	1.87%
Stage 3 | Flexible | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 233	£ 217
Stage 3 ratio	2.48%	2.03%
Stage 3 | LTV >100% | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 90	£ 107
Stage 3 ratio	12.30%	12.11%
Stage 3 | Buy-to-Let | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 35	£ 26
Stage 3 ratio	0.30%	0.26%
Stage 3 | Other | Mortgages
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 612	£ 594
Stage 3 ratio	0.55%	0.54%

Credit Risk - Summary of Res_11

Credit Risk - Summary of Residential Mortgages Portfolios of Particular Interest by Credit Performance (Parenthetical) (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Part Interest Only Part Repayment
Disclosure of credit risk exposure [line items]
Mortgage loan including interest-only part	£ 9,847	£ 9,823

Credit Risk - Summary of Res_12

Credit Risk - Summary of Residential Mortgages Portfolios of Particular Interest by Forbearance Applied (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Forbearance	£ 1,528	£ 1,481
Stage 2 | Mortgages
Disclosure of credit risk exposure [line items]
Forbearance	1,112	1,086
Stage 3 | Mortgages
Disclosure of credit risk exposure [line items]
Forbearance	416	395
Interest-only | Mortgages
Disclosure of credit risk exposure [line items]
Forbearance	285	392
Interest-only | Stage 2 | Mortgages
Disclosure of credit risk exposure [line items]
Forbearance	184	285
Interest-only | Stage 3 | Mortgages
Disclosure of credit risk exposure [line items]
Forbearance	101	107
Flexible | Mortgages
Disclosure of credit risk exposure [line items]
Forbearance	48	73
Flexible | Stage 2 | Mortgages
Disclosure of credit risk exposure [line items]
Forbearance	34	56
Flexible | Stage 3 | Mortgages
Disclosure of credit risk exposure [line items]
Forbearance	14	17
LTV >100% | Mortgages
Disclosure of credit risk exposure [line items]
Forbearance	10	17
LTV >100% | Stage 2 | Mortgages
Disclosure of credit risk exposure [line items]
Forbearance	3	8
LTV >100% | Stage 3 | Mortgages
Disclosure of credit risk exposure [line items]
Forbearance	7	9
Buy-to-Let | Mortgages
Disclosure of credit risk exposure [line items]
Forbearance	9	10
Buy-to-Let | Stage 2 | Mortgages
Disclosure of credit risk exposure [line items]
Forbearance	6	8
Buy-to-Let | Stage 3 | Mortgages
Disclosure of credit risk exposure [line items]
Forbearance	£ 3	£ 2

Credit Risk - Summary of Consum

Credit Risk - Summary of Consumer (Auto) Finance and Other Unsecured Lending by Credit Performance (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 208,750	£ 207,287
Impairment loss allowances	1,303	785
Consumer finance and unsecured lending
Disclosure of credit risk exposure [line items]
Loans and advances to customers	12,819	13,197
Impairment loss allowances	417	321
Gross write-offs	154	168
Consumer finance and unsecured lending | Undrawn Exposures
Disclosure of credit risk exposure [line items]
Stage 3 undrawn exposures	27	25
Consumer finance and unsecured lending | Stage 1
Disclosure of credit risk exposure [line items]
Loans and advances to customers	11,696	11,935
Consumer finance and unsecured lending | Stage 2
Disclosure of credit risk exposure [line items]
Loans and advances to customers	991	1,126
Consumer finance and unsecured lending | Stage 3
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 132	£ 136
Stage 3 ratio	1.24%	1.21%
Consumer (auto) finance | Consumer finance and unsecured lending
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 8,024	£ 7,684
Impairment loss allowances	£ 118	£ 88
Stage 3 ratio	0.72%	0.55%
Gross write-offs	£ 25	£ 34
Consumer (auto) finance | Consumer finance and unsecured lending | Undrawn Exposures
Disclosure of credit risk exposure [line items]
Stage 3 undrawn exposures	0	0
Consumer (auto) finance | Consumer finance and unsecured lending | Stage 1
Disclosure of credit risk exposure [line items]
Loans and advances to customers	7,587	7,038
Consumer (auto) finance | Consumer finance and unsecured lending | Stage 2
Disclosure of credit risk exposure [line items]
Loans and advances to customers	379	604
Consumer (auto) finance | Consumer finance and unsecured lending | Stage 3
Disclosure of credit risk exposure [line items]
Loans and advances to customers	58	42
Personal loans | Consumer finance and unsecured lending
Disclosure of credit risk exposure [line items]
Loans and advances to customers	2,038	2,135
Impairment loss allowances	80	51
Personal loans | Consumer finance and unsecured lending | Stage 1
Disclosure of credit risk exposure [line items]
Loans and advances to customers	1,881	2,020
Personal loans | Consumer finance and unsecured lending | Stage 2
Disclosure of credit risk exposure [line items]
Loans and advances to customers	139	95
Personal loans | Consumer finance and unsecured lending | Stage 3
Disclosure of credit risk exposure [line items]
Loans and advances to customers	18	20
Credit Card [member] | Consumer finance and unsecured lending
Disclosure of credit risk exposure [line items]
Loans and advances to customers	2,349	2,788
Impairment loss allowances	158	120
Credit Card [member] | Consumer finance and unsecured lending | Stage 1
Disclosure of credit risk exposure [line items]
Loans and advances to customers	1,975	2,473
Credit Card [member] | Consumer finance and unsecured lending | Stage 2
Disclosure of credit risk exposure [line items]
Loans and advances to customers	335	267
Credit Card [member] | Consumer finance and unsecured lending | Stage 3
Disclosure of credit risk exposure [line items]
Loans and advances to customers	39	48
Overdrafts | Consumer finance and unsecured lending
Disclosure of credit risk exposure [line items]
Loans and advances to customers	408	590
Impairment loss allowances	61	62
Overdrafts | Consumer finance and unsecured lending | Stage 1
Disclosure of credit risk exposure [line items]
Loans and advances to customers	253	404
Overdrafts | Consumer finance and unsecured lending | Stage 2
Disclosure of credit risk exposure [line items]
Loans and advances to customers	138	160
Overdrafts | Consumer finance and unsecured lending | Stage 3
Disclosure of credit risk exposure [line items]
Loans and advances to customers	17	26
Other unsecured advances [member] | Consumer finance and unsecured lending
Disclosure of credit risk exposure [line items]
Loans and advances to customers	4,795	5,513
Impairment loss allowances	£ 299	£ 233
Stage 3 ratio	2.09%	2.15%
Gross write-offs	£ 129	£ 134
Other unsecured advances [member] | Consumer finance and unsecured lending | Undrawn Exposures
Disclosure of credit risk exposure [line items]
Stage 3 undrawn exposures	27	25
Other unsecured advances [member] | Consumer finance and unsecured lending | Stage 1
Disclosure of credit risk exposure [line items]
Loans and advances to customers	4,109	4,897
Other unsecured advances [member] | Consumer finance and unsecured lending | Stage 2
Disclosure of credit risk exposure [line items]
Loans and advances to customers	612	522
Other unsecured advances [member] | Consumer finance and unsecured lending | Stage 3
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 74	£ 94

Credit Risk - Summary of Modi_2

Credit Risk - Summary of Modification of Loan Terms ( Consumer (Auto) Finance and Other Unsecured Lending) (Detail) - Consumer finance and unsecured lending - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Amortised cost before modification	£ 26	£ 38
Net modification loss	12	20
Gross carrying amount of financial assets for which the ECL allowance has changed to 12-month measurement during the year	4	5
Credit cards
Disclosure of credit risk exposure [line items]
Amortised cost before modification	18	23
Net modification loss	8	12
Gross carrying amount of financial assets for which the ECL allowance has changed to 12-month measurement during the year	2	3
Overdrafts
Disclosure of credit risk exposure [line items]
Amortised cost before modification	8	15
Net modification loss	4	8
Gross carrying amount of financial assets for which the ECL allowance has changed to 12-month measurement during the year	£ 2	£ 2

Credit Risk - Summary of Cons_2

Credit Risk - Summary of Consumer (Auto) Finance and Other Unsecured Lending by Forbearance (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Forbearance	£ 1,528	£ 1,481
Consumer finance and unsecured lending
Disclosure of credit risk exposure [line items]
Forbearance	68	83
Consumer finance and unsecured lending | Stage 2
Disclosure of credit risk exposure [line items]
Forbearance	19	22
Consumer finance and unsecured lending | Stage 3
Disclosure of credit risk exposure [line items]
Forbearance	49	61
Consumer Auto Finance | Consumer finance and unsecured lending
Disclosure of credit risk exposure [line items]
Forbearance	9	7
Consumer Auto Finance | Consumer finance and unsecured lending | Stage 2
Disclosure of credit risk exposure [line items]
Forbearance	4	5
Consumer Auto Finance | Consumer finance and unsecured lending | Stage 3
Disclosure of credit risk exposure [line items]
Forbearance	5	2
Personal loans | Consumer finance and unsecured lending
Disclosure of credit risk exposure [line items]
Forbearance	0	0
Personal loans | Consumer finance and unsecured lending | Stage 2
Disclosure of credit risk exposure [line items]
Forbearance	0	0
Personal loans | Consumer finance and unsecured lending | Stage 3
Disclosure of credit risk exposure [line items]
Forbearance	0	0
Credit cards | Consumer finance and unsecured lending
Disclosure of credit risk exposure [line items]
Forbearance	43	51
Credit cards | Consumer finance and unsecured lending | Stage 2
Disclosure of credit risk exposure [line items]
Forbearance	10	10
Credit cards | Consumer finance and unsecured lending | Stage 3
Disclosure of credit risk exposure [line items]
Forbearance	33	41
Overdrafts | Consumer finance and unsecured lending
Disclosure of credit risk exposure [line items]
Forbearance	16	25
Overdrafts | Consumer finance and unsecured lending | Stage 2
Disclosure of credit risk exposure [line items]
Forbearance	5	7
Overdrafts | Consumer finance and unsecured lending | Stage 3
Disclosure of credit risk exposure [line items]
Forbearance	11	18
Total other unsecured [member] | Consumer finance and unsecured lending
Disclosure of credit risk exposure [line items]
Forbearance	59	76
Total other unsecured [member] | Consumer finance and unsecured lending | Stage 2
Disclosure of credit risk exposure [line items]
Forbearance	15	17
Total other unsecured [member] | Consumer finance and unsecured lending | Stage 3
Disclosure of credit risk exposure [line items]
Forbearance	£ 44	£ 59

Credit Risk - Summary of Full I

Credit Risk - Summary of Full Interest-Only New Business in the Year (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Information Related To Full Interest Only Loans [Abstract]
Full interest-only loans	£ 4,267	£ 4,000

Credit Risk - Summary of Detail

Credit Risk - Summary of Detailed Information of Interest only Loan Maturity Analysis (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Interest Only Loans [Line Items]
carrying amount of interest-only loans	£ 38,441	£ 38,062
Term expired [member]
Interest Only Loans [Line Items]
carrying amount of interest-only loans	374	338
Within 2 years [member]
Interest Only Loans [Line Items]
carrying amount of interest-only loans	1,726	1,541
Between 2-5 years [member]
Interest Only Loans [Line Items]
carrying amount of interest-only loans	3,697	3,706
Between 5-15 years [member]
Interest Only Loans [Line Items]
carrying amount of interest-only loans	20,318	20,984
Greater than 15 years [member]
Interest Only Loans [Line Items]
carrying amount of interest-only loans	12,326	11,493
Weighted average LTV [member]
Interest Only Loans [Line Items]
carrying amount of interest-only loans	2,712	3,185
Weighted average LTV [member] | Term expired [member]
Interest Only Loans [Line Items]
carrying amount of interest-only loans	13	11
Weighted average LTV [member] | Within 2 years [member]
Interest Only Loans [Line Items]
carrying amount of interest-only loans	131	111
Weighted average LTV [member] | Between 2-5 years [member]
Interest Only Loans [Line Items]
carrying amount of interest-only loans	154	219
Weighted average LTV [member] | Between 5-15 years [member]
Interest Only Loans [Line Items]
carrying amount of interest-only loans	1,397	1,793
Weighted average LTV [member] | Greater than 15 years [member]
Interest Only Loans [Line Items]
carrying amount of interest-only loans	£ 1,017	£ 1,051

Credit Risk - Summary of Busine

Credit Risk - Summary of Business Banking by Credit Performance (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 208,750	£ 207,287
Impairment loss allowances	1,303	785
Exposure	322,745	310,155
Stage 1
Disclosure of credit risk exposure [line items]
Exposure	301,413	295,436
Stage 2
Disclosure of credit risk exposure [line items]
Exposure	18,336	12,351
Stage 3
Disclosure of credit risk exposure [line items]
Exposure	2,996	2,368
Credit risk
Disclosure of credit risk exposure [line items]
Exposure	108,700	94,200
Credit risk | Business Banking
Disclosure of credit risk exposure [line items]
Loans and advances to customers	3,855	209
Impairment loss allowances	9	9
Gross write offs	12	14
Credit risk | Business Banking | Stage 1
Disclosure of credit risk exposure [line items]
Loans and advances to customers	3,845	197
Credit risk | Business Banking | Stage 2
Disclosure of credit risk exposure [line items]
Loans and advances to customers	6	6
Credit risk | Business Banking | Stage 3
Disclosure of credit risk exposure [line items]
Loans and advances to customers	£ 4	£ 6
Stage 3 ratio	0.10%	2.87%
Credit risk | Business Banking | Stage 3 | Undrawn Exposures
Disclosure of credit risk exposure [line items]
Exposure	£ 0	£ 0

Credit Risk - Summary of Modi_3

Credit Risk - Summary of Modification of Loan Terms (Business Banking) (Detail) - Business Banking - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Amortised cost before modification	£ 0	£ 0
Net modification loss	0	0
Gross carrying amount of financial assets for which the ECL allowance has changed to 12-month measurement during the year	£ 0	£ 0

Credit Risk - Summary of Busi_2

Credit Risk - Summary of Business Banking by Forbearance (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Forbearance	£ 1,528	£ 1,481
Business Banking
Disclosure of credit risk exposure [line items]
Forbearance	4	4
Business Banking | Stage 2
Disclosure of credit risk exposure [line items]
Forbearance	0	0
Business Banking | Stage 3
Disclosure of credit risk exposure [line items]
Forbearance	£ 4	£ 4

Credit Risk - Summary of ECL _4

Credit Risk - Summary of ECL Reconciliation For Other Business Segments (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	£ 863
Ending balance	1,377	£ 863
Recoveries net of collection costs	(24)	(40)	£ (42)
Income statement charge/(release) for the year	645	221	153
Stage 1
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	147
Ending balance	216	147
Stage 2
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	348
Ending balance	592	348
Stage 3
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	368
Ending balance	569	368
Total exposure
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	310,155	305,464
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
Changes to model	0	0
New lending and assets purchased	57,021	43,257
Redemptions and repayments	(53,434)	(42,183)
Other	9,398	3,980
Assets written off	395	363
Ending balance	322,745	310,155	305,464
Net movement in the year	12,590	4,691
Expected credit loss
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	863	807
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	379	160
Change in economic scenarios	164	(19)
Changes to model	25	13
New lending and assets purchased	156	70
Redemptions and repayments	(90)	(102)
Other	133	183
Assets written off	253	249
Ending balance	1,377	863	807
Net movement in the year	514
Increase decrease in total expected credit losses		56
Charge/(release) to the Income Statement	767	305
Less: ECL relating to derecognised income	(14)	(13)
Recoveries net of collection costs	(108)	(71)
Income statement charge/(release) for the year	645	221
Corporate and commercial banking
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Income statement charge/(release) for the year	294	45	8
Corporate and commercial banking | Total exposure
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	24,118	25,056
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
New lending and assets purchased	8,883	4,114
Redemptions and repayments	7,423	4,101
Other	(954)	(843)
Assets written off	(121)	(108)
Ending balance	24,503	24,118	25,056
Net movement in the year	385	(938)
Corporate and commercial banking | Expected credit loss
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	262	235
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	272	49
Change in economic scenarios	51	19
New lending and assets purchased	82	14
Redemptions and repayments	56	30
Other	43	29
Assets written off	(51)	(54)
Ending balance	603	262	235
Increase decrease in total expected credit losses	341	27
Charge/(release) to the Income Statement	392	81
Less: ECL relating to derecognised income	(5)	(6)
Recoveries net of collection costs	(93)	(30)
Income statement charge/(release) for the year	294	45
Corporate and investment banking
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Income statement charge/(release) for the year	7	22	14
Corporate and investment banking | Total exposure
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	13,384	17,549
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0
New lending and assets purchased	1,162	54
Redemptions and repayments	5,293	5,157
Other	2,435	938
Assets written off	(42)	0
Ending balance	11,646	13,384	17,549
Net movement in the year	(1,738)	(4,165)
Corporate and investment banking | Expected credit loss
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	50	18
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	0	36
Change in economic scenarios	8
New lending and assets purchased	1	0
Redemptions and repayments	2	2
Other	(2)	(2)
Assets written off	(22)	0
Ending balance	33	50	18
Increase decrease in total expected credit losses	(17)	32
Charge/(release) to the Income Statement	5	32
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	2	(10)
Income statement charge/(release) for the year	7	22
Corporate centre
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Income statement charge/(release) for the year	36	(2)	(4)
Corporate centre | Total exposure
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	67,873	68,731
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
New lending and assets purchased	11,503	1,849
Redemptions and repayments	10,354	5,469
Other	7,325	2,765
Assets written off	(2)	(3)
Ending balance	76,345	67,873	68,731
Net movement in the year	8,472	(858)
Corporate centre | Expected credit loss
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	2	5
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	1	1
Change in economic scenarios	6	1
New lending and assets purchased	1	0
Redemptions and repayments	3	0
Other	28	(5)
Assets written off	0	0
Ending balance	35	2	5
Net movement in the year	33
Increase decrease in total expected credit losses		(3)
Charge/(release) to the Income Statement	33	(3)
Less: ECL relating to derecognised income	0	1
Recoveries net of collection costs	4	(1)
Income statement charge/(release) for the year	37	(3)
Financial instruments not credit-impaired | Subject to 12-month ECL | Stage 1
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	295,436	290,882
Transfers from Stage 1 to Stage 2	(9,815)	(4,101)
Transfers from Stage 2 to Stage 1	3,178	3,458
Transfers to Stage 3	(385)	(361)
Transfers from Stage 3	12	10
Transfers of financial instruments	(7,010)	(994)
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
Changes to model	0	0
New lending and assets purchased	55,546	42,415
Redemptions and repayments	(50,698)	(40,380)
Other	8,141	3,514
Assets written off	2	1
Ending balance	301,413	295,436	290,882
Net movement in the year	5,977	4,554
Financial instruments not credit-impaired | Subject to Lifetime ECL | Stage 2
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	12,351	12,011
Transfers from Stage 1 to Stage 2	9,815	4,101
Transfers from Stage 2 to Stage 1	(3,178)	(3,458)
Transfers to Stage 3	(1,126)	(595)
Transfers from Stage 3	326	516
Transfers of financial instruments	5,837	564
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
Changes to model	0	0
New lending and assets purchased	1,371	827
Redemptions and repayments	(2,295)	(1,344)
Other	1,072	294
Assets written off	0	1
Ending balance	18,336	12,351	12,011
Net movement in the year	5,985	340
Financial instruments not credit-impaired | Stage 1 ECL | Subject to 12-month ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	147	143
Transfers from Stage 1 to Stage 2	(47)	(11)
Transfers from Stage 2 to Stage 1	110	74
Transfers to Stage 3	(8)	(2)
Transfers from Stage 3	2	1
Transfers of financial instruments	57	62
Net remeasurement of ECL on stage transfer	(101)	(66)
Change in economic scenarios	15	5
Changes to model	0	0
New lending and assets purchased	40	29
Redemptions and repayments	(30)	(32)
Other	88	6
Assets written off	0	0
Ending balance	216	147	143
Net movement in the year	69
Increase decrease in total expected credit losses		4
Charge/(release) to the Income Statement	69	4
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	0	(10)
Income statement charge/(release) for the year	69	(6)
Financial instruments not credit-impaired | Stage 2 ECL | Subject to Lifetime ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	348	307
Transfers from Stage 1 to Stage 2	47	11
Transfers from Stage 2 to Stage 1	(110)	(74)
Transfers to Stage 3	(61)	(24)
Transfers from Stage 3	21	23
Transfers of financial instruments	(103)	(64)
Net remeasurement of ECL on stage transfer	239	130
Change in economic scenarios	139	(15)
Changes to model	0	0
New lending and assets purchased	64	32
Redemptions and repayments	(42)	(28)
Other	(53)	(14)
Assets written off	0	0
Ending balance	592	348	307
Net movement in the year	244
Increase decrease in total expected credit losses		41
Charge/(release) to the Income Statement	244	41
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	0	(15)
Income statement charge/(release) for the year	244	26
Financial instruments not credit-impaired | Corporate and commercial banking | Subject to 12-month ECL | Stage 1
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	21,281	23,259
Transfers from Stage 1 to Stage 2	(4,443)	(801)
Transfers from Stage 2 to Stage 1	528	156
Transfers to Stage 3	(78)	(42)
Transfers from Stage 3	4	0
Transfers of financial instruments	(3,989)	(687)
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
New lending and assets purchased	7,984	3,733
Redemptions and repayments	6,487	4,062
Other	(1,572)	(962)
Assets written off	0	0
Ending balance	17,217	21,281	23,259
Net movement in the year	(4,064)	(1,978)
Financial instruments not credit-impaired | Corporate and commercial banking | Subject to Lifetime ECL | Stage 2
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	2,385	1,422
Transfers from Stage 1 to Stage 2	4,443	801
Transfers from Stage 2 to Stage 1	(528)	(156)
Transfers to Stage 3	(543)	(109)
Transfers from Stage 3	1	16
Transfers of financial instruments	3,373	552
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
New lending and assets purchased	805	379
Redemptions and repayments	818	32
Other	521	64
Assets written off	0	0
Ending balance	6,266	2,385	1,422
Net movement in the year	3,881	963
Financial instruments not credit-impaired | Corporate and commercial banking | Stage 1 ECL | Subject to 12-month ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	53	47
Transfers from Stage 1 to Stage 2	(37)	(3)
Transfers from Stage 2 to Stage 1	20	7
Transfers to Stage 3	(1)	0
Transfers from Stage 3	1	0
Transfers of financial instruments	(17)	4
Net remeasurement of ECL on stage transfer	(18)	(7)
Change in economic scenarios	7	8
New lending and assets purchased	10	6
Redemptions and repayments	13	10
Other	32	5
Assets written off	0	0
Ending balance	54	53	47
Increase decrease in total expected credit losses	1	6
Charge/(release) to the Income Statement	1	6
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	0	(10)
Income statement charge/(release) for the year	1	(4)
Financial instruments not credit-impaired | Corporate and commercial banking | Stage 2 ECL | Subject to Lifetime ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	51	45
Transfers from Stage 1 to Stage 2	37	3
Transfers from Stage 2 to Stage 1	(20)	(7)
Transfers to Stage 3	(21)	(5)
Transfers from Stage 3	0	1
Transfers of financial instruments	(4)	(8)
Net remeasurement of ECL on stage transfer	92	18
Change in economic scenarios	44	11
New lending and assets purchased	28	7
Redemptions and repayments	18	11
Other	6	(11)
Assets written off	0	0
Ending balance	199	51	45
Increase decrease in total expected credit losses	148	6
Charge/(release) to the Income Statement	148	6
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	0	(8)
Income statement charge/(release) for the year	148	(2)
Financial instruments not credit-impaired | Corporate and investment banking | Subject to 12-month ECL | Stage 1
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	12,972	17,389
Transfers from Stage 1 to Stage 2	(43)	(208)
Transfers from Stage 2 to Stage 1	0	41
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	(43)	(167)
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0
New lending and assets purchased	1,119	54
Redemptions and repayments	5,005	5,179
Other	2,174	875
Assets written off	0	0
Ending balance	11,217	12,972	17,389
Net movement in the year	(1,755)	(4,417)
Financial instruments not credit-impaired | Corporate and investment banking | Subject to Lifetime ECL | Stage 2
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	397	134
Transfers from Stage 1 to Stage 2	43	208
Transfers from Stage 2 to Stage 1	0	(41)
Transfers to Stage 3	(40)	0
Transfers from Stage 3	0	0
Transfers of financial instruments	3	167
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0
New lending and assets purchased	43	0
Redemptions and repayments	287	(50)
Other	273	46
Assets written off	0	0
Ending balance	429	397	134
Net movement in the year	32	263
Financial instruments not credit-impaired | Corporate and investment banking | Stage 1 ECL | Subject to 12-month ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	4	5
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	0	0
Transfers from Stage 3	0	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0
New lending and assets purchased	1	0
Redemptions and repayments	1	1
Other	5	0
Assets written off	0	0
Ending balance	9	4	5
Increase decrease in total expected credit losses	5	(1)
Charge/(release) to the Income Statement	5	(1)
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	0	0
Income statement charge/(release) for the year	5	(1)
Financial instruments not credit-impaired | Corporate and investment banking | Stage 2 ECL | Subject to Lifetime ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	37	3
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	(13)	0
Transfers from Stage 3	0	0
Transfers of financial instruments	(13)	0
Net remeasurement of ECL on stage transfer	0	36
Change in economic scenarios	8
New lending and assets purchased	0	0
Redemptions and repayments	1	1
Other	(7)	(1)
Assets written off	0	0
Ending balance	24	37	3
Increase decrease in total expected credit losses	(13)	34
Charge/(release) to the Income Statement	(13)	34
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	0	(8)
Income statement charge/(release) for the year	(13)	26
Financial instruments not credit-impaired | Corporate centre | Subject to 12-month ECL | Stage 1
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	67,779	68,635
Transfers from Stage 1 to Stage 2	(87)	(15)
Transfers from Stage 2 to Stage 1	106	7
Transfers to Stage 3	(2)	(4)
Transfers from Stage 3	1	0
Transfers of financial instruments	18	(12)
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
New lending and assets purchased	11,457	1,849
Redemptions and repayments	10,342	5,328
Other	7,389	2,635
Assets written off	0	0
Ending balance	76,301	67,779	68,635
Net movement in the year	8,522	(856)
Financial instruments not credit-impaired | Corporate centre | Subject to Lifetime ECL | Stage 2
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	94	97
Transfers from Stage 1 to Stage 2	87	15
Transfers from Stage 2 to Stage 1	(106)	(7)
Transfers to Stage 3	(3)	(4)
Transfers from Stage 3	1	0
Transfers of financial instruments	(21)	4
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
New lending and assets purchased	45	0
Redemptions and repayments	9	155
Other	(65)	148
Assets written off	0	0
Ending balance	44	94	97
Net movement in the year	(50)	(3)
Financial instruments not credit-impaired | Corporate centre | Stage 1 ECL | Subject to 12-month ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	1	3
Transfers from Stage 1 to Stage 2	(2)	0
Transfers from Stage 2 to Stage 1	4	0
Transfers to Stage 3	0	0
Transfers from Stage 3	1	0
Transfers of financial instruments	3	0
Net remeasurement of ECL on stage transfer	(3)	0
Change in economic scenarios	0	1
New lending and assets purchased	1	0
Redemptions and repayments	1	0
Other	34	(3)
Assets written off	0	0
Ending balance	35	1	3
Net movement in the year	34
Increase decrease in total expected credit losses		(2)
Charge/(release) to the Income Statement	34	(2)
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	0	0
Income statement charge/(release) for the year	34	(2)
Financial instruments not credit-impaired | Corporate centre | Stage 2 ECL | Subject to Lifetime ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	1	1
Transfers from Stage 1 to Stage 2	2	0
Transfers from Stage 2 to Stage 1	(4)	0
Transfers to Stage 3	(1)	0
Transfers from Stage 3	0	0
Transfers of financial instruments	(3)	0
Net remeasurement of ECL on stage transfer	3	0
Change in economic scenarios	6	0
New lending and assets purchased	0	0
Redemptions and repayments	1	0
Other	(6)	0
Assets written off	0	0
Ending balance	0	1	1
Net movement in the year	(1)
Increase decrease in total expected credit losses		0
Charge/(release) to the Income Statement	(1)	0
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	0	0
Income statement charge/(release) for the year	(1)	0
Financial instruments credit-impaired | Subject to Lifetime ECL | Stage 3
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	2,368	2,571
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	1,511	956
Transfers from Stage 3	(338)	(526)
Transfers of financial instruments	1,173	430
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
Changes to model	0	0
New lending and assets purchased	104	15
Redemptions and repayments	(441)	(459)
Other	185	172
Assets written off	393	361
Ending balance	2,996	2,368	2,571
Net movement in the year	628	(203)
Financial instruments credit-impaired | Stage 3 ECL | Subject to Lifetime ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	368	357
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	69	26
Transfers from Stage 3	(23)	(24)
Transfers of financial instruments	46	2
Net remeasurement of ECL on stage transfer	241	96
Change in economic scenarios	10	(9)
Changes to model	25	13
New lending and assets purchased	52	9
Redemptions and repayments	(18)	(42)
Other	98	191
Assets written off	253	249
Ending balance	569	368	357
Net movement in the year	201
Increase decrease in total expected credit losses		11
Charge/(release) to the Income Statement	454	260
Less: ECL relating to derecognised income	(14)	(13)
Recoveries net of collection costs	(108)	(46)
Income statement charge/(release) for the year	332	201
Financial instruments credit-impaired | Corporate and commercial banking | Subject to Lifetime ECL | Stage 3
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	452	375
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	621	151
Transfers from Stage 3	(5)	(16)
Transfers of financial instruments	616	135
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
New lending and assets purchased	94	2
Redemptions and repayments	118	7
Other	97	55
Assets written off	(121)	(108)
Ending balance	1,020	452	375
Net movement in the year	568	77
Financial instruments credit-impaired | Corporate and commercial banking | Stage 3 ECL | Subject to Lifetime ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	158	143
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	22	5
Transfers from Stage 3	(1)	(1)
Transfers of financial instruments	21	4
Net remeasurement of ECL on stage transfer	198	38
Change in economic scenarios	0	0
New lending and assets purchased	44	1
Redemptions and repayments	25	9
Other	5	35
Assets written off	(51)	(54)
Ending balance	350	158	143
Increase decrease in total expected credit losses	192	15
Charge/(release) to the Income Statement	243	69
Less: ECL relating to derecognised income	(5)	(6)
Recoveries net of collection costs	(93)	(12)
Income statement charge/(release) for the year	145	51
Financial instruments credit-impaired | Corporate and investment banking | Subject to Lifetime ECL | Stage 3
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	15	26
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	40	0
Transfers from Stage 3	0	0
Transfers of financial instruments	40	0
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0
New lending and assets purchased	0	0
Redemptions and repayments	1	28
Other	(12)	17
Assets written off	(42)	0
Ending balance	0	15	26
Net movement in the year	(15)	(11)
Financial instruments credit-impaired | Corporate and investment banking | Stage 3 ECL | Subject to Lifetime ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	9	10
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	13	0
Transfers from Stage 3	0	0
Transfers of financial instruments	13	0
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0
New lending and assets purchased	0	0
Redemptions and repayments	0	0
Other	0	(1)
Assets written off	(22)	0
Ending balance	0	9	10
Increase decrease in total expected credit losses	(9)	(1)
Charge/(release) to the Income Statement	13	(1)
Less: ECL relating to derecognised income	0	0
Recoveries net of collection costs	2	(2)
Income statement charge/(release) for the year	15	(3)
Financial instruments credit-impaired | Corporate centre | Subject to Lifetime ECL | Stage 3
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	0	(1)
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	5	8
Transfers from Stage 3	(2)	0
Transfers of financial instruments	3	8
Net remeasurement of ECL on stage transfer	0	0
Change in economic scenarios	0	0
New lending and assets purchased	1	0
Redemptions and repayments	3	(14)
Other	1	(18)
Assets written off	(2)	(3)
Ending balance	0	0	(1)
Net movement in the year	0	1
Financial instruments credit-impaired | Corporate centre | Stage 3 ECL | Subject to Lifetime ECL
Disclosure of reconciliation of changes in loss allowance and explanation of changes in gross carrying amount for financial instruments [line items]
Beginning balance	0	1
Transfers from Stage 1 to Stage 2	0	0
Transfers from Stage 2 to Stage 1	0	0
Transfers to Stage 3	1	0
Transfers from Stage 3	(1)	0
Transfers of financial instruments	0	0
Net remeasurement of ECL on stage transfer	1	1
Change in economic scenarios	0	0
New lending and assets purchased	0	0
Redemptions and repayments	1	0
Other	0	(2)
Assets written off	0	0
Ending balance	0	0	£ 1
Net movement in the year	0
Increase decrease in total expected credit losses		(1)
Charge/(release) to the Income Statement	0	(1)
Less: ECL relating to derecognised income	0	1
Recoveries net of collection costs	4	(1)
Income statement charge/(release) for the year	£ 4	£ (1)

Credit Risk - Summary of Other

Credit Risk - Summary of Other Segments Exposures by Credit Rating (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	£ 95,156	£ 80,918
Stage 1
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	87,397	77,575
Stage 2
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	6,739	2,879
Stage 3
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,020	464
Corporate and investment banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	12,347	14,565
Corporate and investment banking | Large Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	10,886	12,646
Corporate and investment banking | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,461	1,919
Corporate centre
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	58,129	42,129
Corporate centre | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,817	4,060
Corporate centre | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	712	857
Corporate centre | Sovereign And Supranational
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	50,962	34,400
Corporate centre | Structured Products
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2,482	2,572
Corporate centre | Legacy Portfolios in Run-Off
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	156	240
Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	24,680	24,224
Corporate and commercial banking | SME and Mid Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	14,446	13,254
Corporate and commercial banking | Commercial Real Estate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	5,152	6,057
Corporate and commercial banking | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	5,082	4,913
Internal grade 9
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	50,899	36,682
Internal grade 9 | Stage 1
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	50,899	36,682
Internal grade 9 | Stage 2
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 9 | Stage 3
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 9 | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	541	664
Internal grade 9 | Corporate and investment banking | Large Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	174	281
Internal grade 9 | Corporate and investment banking | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	367	383
Internal grade 9 | Corporate centre
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	50,246	34,787
Internal grade 9 | Corporate centre | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2	934
Internal grade 9 | Corporate centre | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	497	542
Internal grade 9 | Corporate centre | Sovereign And Supranational
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	48,579	32,145
Internal grade 9 | Corporate centre | Structured Products
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,168	1,166
Internal grade 9 | Corporate centre | Legacy Portfolios in Run-Off
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 9 | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	112	1,231
Internal grade 9 | Corporate and commercial banking | SME and Mid Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 9 | Corporate and commercial banking | Commercial Real Estate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 9 | Corporate and commercial banking | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	112	1,231
Internal grade 8
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	10,972	13,371
Internal grade 8 | Stage 1
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	10,889	13,371
Internal grade 8 | Stage 2
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	83	0
Internal grade 8 | Stage 3
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 8 | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2,245	3,178
Internal grade 8 | Corporate and investment banking | Large Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,802	2,356
Internal grade 8 | Corporate and investment banking | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	443	822
Internal grade 8 | Corporate centre
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	5,628	6,518
Internal grade 8 | Corporate centre | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2,001	3,036
Internal grade 8 | Corporate centre | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	200	246
Internal grade 8 | Corporate centre | Sovereign And Supranational
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2,383	2,255
Internal grade 8 | Corporate centre | Structured Products
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,044	981
Internal grade 8 | Corporate centre | Legacy Portfolios in Run-Off
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 8 | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,099	3,675
Internal grade 8 | Corporate and commercial banking | SME and Mid Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	468	25
Internal grade 8 | Corporate and commercial banking | Commercial Real Estate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 8 | Corporate and commercial banking | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2,631	3,650
Internal grade 7
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	9,628	6,475
Internal grade 7 | Stage 1
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	9,567	6,449
Internal grade 7 | Stage 2
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	61	26
Internal grade 7 | Stage 3
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 7 | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,889	5,122
Internal grade 7 | Corporate and investment banking | Large Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,267	4,419
Internal grade 7 | Corporate and investment banking | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	622	703
Internal grade 7 | Corporate centre
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2,052	529
Internal grade 7 | Corporate centre | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,814	90
Internal grade 7 | Corporate centre | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	9	43
Internal grade 7 | Corporate centre | Sovereign And Supranational
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 7 | Corporate centre | Structured Products
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	229	396
Internal grade 7 | Corporate centre | Legacy Portfolios in Run-Off
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 7 | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,687	824
Internal grade 7 | Corporate and commercial banking | SME and Mid Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,288	798
Internal grade 7 | Corporate and commercial banking | Commercial Real Estate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	65	0
Internal grade 7 | Corporate and commercial banking | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2,334	26
Internal grade 6
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	6,806	7,132
Internal grade 6 | Stage 1
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	6,541	6,940
Internal grade 6 | Stage 2
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	265	192
Internal grade 6 | Stage 3
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 6 | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,905	4,569
Internal grade 6 | Corporate and investment banking | Large Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,882	4,558
Internal grade 6 | Corporate and investment banking | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	23	11
Internal grade 6 | Corporate centre
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	47	184
Internal grade 6 | Corporate centre | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 6 | Corporate centre | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	6	25
Internal grade 6 | Corporate centre | Sovereign And Supranational
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 6 | Corporate centre | Structured Products
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	41	29
Internal grade 6 | Corporate centre | Legacy Portfolios in Run-Off
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	130
Internal grade 6 | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2,854	2,379
Internal grade 6 | Corporate and commercial banking | SME and Mid Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2,640	1,953
Internal grade 6 | Corporate and commercial banking | Commercial Real Estate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	214	426
Internal grade 6 | Corporate and commercial banking | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 5
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	5,045	9,463
Internal grade 5 | Stage 1
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	4,136	8,714
Internal grade 5 | Stage 2
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	909	749
Internal grade 5 | Stage 3
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 5 | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,529	842
Internal grade 5 | Corporate and investment banking | Large Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,523	842
Internal grade 5 | Corporate and investment banking | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	6	0
Internal grade 5 | Corporate centre
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	1
Internal grade 5 | Corporate centre | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 5 | Corporate centre | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	1
Internal grade 5 | Corporate centre | Sovereign And Supranational
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 5 | Corporate centre | Structured Products
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 5 | Corporate centre | Legacy Portfolios in Run-Off
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 5 | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,516	8,620
Internal grade 5 | Corporate and commercial banking | SME and Mid Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2,820	4,833
Internal grade 5 | Corporate and commercial banking | Commercial Real Estate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	696	3,787
Internal grade 5 | Corporate and commercial banking | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 4
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	7,361	5,547
Internal grade 4 | Stage 1
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	4,029	4,175
Internal grade 4 | Stage 2
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,332	1,372
Internal grade 4 | Stage 3
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 4 | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	157	75
Internal grade 4 | Corporate and investment banking | Large Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	157	75
Internal grade 4 | Corporate and investment banking | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 4 | Corporate centre
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 4 | Corporate centre | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 4 | Corporate centre | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 4 | Corporate centre | Sovereign And Supranational
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 4 | Corporate centre | Structured Products
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 4 | Corporate centre | Legacy Portfolios in Run-Off
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 4 | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	7,204	5,472
Internal grade 4 | Corporate and commercial banking | SME and Mid Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,518	3,896
Internal grade 4 | Corporate and commercial banking | Commercial Real Estate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,684	1,574
Internal grade 4 | Corporate and commercial banking | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2	2
Internal grade 3 to 1
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,355	1,800
Internal grade 3 to 1 | Stage 1
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	431	813
Internal grade 3 to 1 | Stage 2
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,904	523
Internal grade 3 to 1 | Stage 3
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,020	464
Internal grade 3 to 1 | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	81	115
Internal grade 3 to 1 | Corporate and investment banking | Large Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	81	115
Internal grade 3 to 1 | Corporate and investment banking | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 3 to 1 | Corporate centre
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	110
Internal grade 3 to 1 | Corporate centre | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 3 to 1 | Corporate centre | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 3 to 1 | Corporate centre | Sovereign And Supranational
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 3 to 1 | Corporate centre | Structured Products
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Internal grade 3 to 1 | Corporate centre | Legacy Portfolios in Run-Off
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	110
Internal grade 3 to 1 | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,274	1,575
Internal grade 3 to 1 | Corporate and commercial banking | SME and Mid Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2,790	1,450
Internal grade 3 to 1 | Corporate and commercial banking | Commercial Real Estate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	484	121
Internal grade 3 to 1 | Corporate and commercial banking | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	4
Other
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,090	448
Other | Stage 1
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	905	431
Other | Stage 2
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	185	17
Other | Stage 3
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Other | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Other | Corporate and investment banking | Large Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Other | Corporate and investment banking | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Other | Corporate centre
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	156	0
Other | Corporate centre | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Other | Corporate centre | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Other | Corporate centre | Sovereign And Supranational
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Other | Corporate centre | Structured Products
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Other | Corporate centre | Legacy Portfolios in Run-Off
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	156	0
Other | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	934	448
Other | Corporate and commercial banking | SME and Mid Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	922	299
Other | Corporate and commercial banking | Commercial Real Estate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	9	149
Other | Corporate and commercial banking | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	£ 3	£ 0

Credit Risk - Summary of Othe_2

Credit Risk - Summary of Other Segments Exposures by Geographical Distribution (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	£ 95,156	£ 80,918
Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	24,680	24,224
Corporate and commercial banking | SME and Mid Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	14,446	13,254
Corporate and commercial banking | Commercial Real Estate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	5,152	6,057
Corporate and commercial banking | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	5,082	4,913
Corporate and investment banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	12,347	14,565
Corporate and investment banking | Large Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	10,886	12,646
Corporate and investment banking | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,461	1,919
Corporate centre
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	58,129	42,129
Corporate centre | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,817	4,060
Corporate centre | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	712	857
Corporate centre | Sovereign And Supranational
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	50,962	34,400
Corporate centre | Structured Products
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2,482	2,572
Corporate centre | Legacy Portfolios in Run-Off
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	156	240
UK | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	24,632	24,139
UK | Corporate and commercial banking | SME and Mid Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	14,399	13,169
UK | Corporate and commercial banking | Commercial Real Estate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	5,151	6,057
UK | Corporate and commercial banking | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	5,082	4,913
UK | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	10,456	11,269
UK | Corporate and investment banking | Large Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	9,814	10,665
UK | Corporate and investment banking | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	642	604
UK | Corporate centre
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	50,484	34,216
UK | Corporate centre | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,817	4,060
UK | Corporate centre | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	301	329
UK | Corporate centre | Sovereign And Supranational
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	44,624	27,987
UK | Corporate centre | Structured Products
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,683	1,710
UK | Corporate centre | Legacy Portfolios in Run-Off
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	59	130
Europe [member] | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	47	84
Europe [member] | Corporate and commercial banking | SME and Mid Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	47	84
Europe [member] | Corporate and commercial banking | Commercial Real Estate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Europe [member] | Corporate and commercial banking | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Europe [member] | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,487	2,763
Europe [member] | Corporate and investment banking | Large Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,052	1,922
Europe [member] | Corporate and investment banking | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	435	841
Europe [member] | Corporate centre
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2,680	2,695
Europe [member] | Corporate centre | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Europe [member] | Corporate centre | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	365	335
Europe [member] | Corporate centre | Sovereign And Supranational
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,516	1,549
Europe [member] | Corporate centre | Structured Products
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	799	811
Europe [member] | Corporate centre | Legacy Portfolios in Run-Off
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
US | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
US | Corporate and commercial banking | SME and Mid Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
US | Corporate and commercial banking | Commercial Real Estate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
US | Corporate and commercial banking | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
US | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	130	171
US | Corporate and investment banking | Large Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	2
US | Corporate and investment banking | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	130	169
US | Corporate centre
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	855	1,013
US | Corporate centre | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
US | Corporate centre | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	6	157
US | Corporate centre | Sovereign And Supranational
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	849	856
US | Corporate centre | Structured Products
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
US | Corporate centre | Legacy Portfolios in Run-Off
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Rest Of World [member] | Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1	1
Rest Of World [member] | Corporate and commercial banking | SME and Mid Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	1
Rest Of World [member] | Corporate and commercial banking | Commercial Real Estate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1	0
Rest Of World [member] | Corporate and commercial banking | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Rest Of World [member] | Corporate and investment banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	274	362
Rest Of World [member] | Corporate and investment banking | Large Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	20	57
Rest Of World [member] | Corporate and investment banking | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	254	305
Rest Of World [member] | Corporate centre
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	4,110	4,205
Rest Of World [member] | Corporate centre | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Rest Of World [member] | Corporate centre | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	40	36
Rest Of World [member] | Corporate centre | Sovereign And Supranational
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,973	4,008
Rest Of World [member] | Corporate centre | Structured Products
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	51
Rest Of World [member] | Corporate centre | Legacy Portfolios in Run-Off
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	£ 97	£ 110

Credit Risk - Summary of Deta_2

Credit Risk - Summary of Detailed Information about Gross and Net Credit Exposure (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Statement [Line Items]
Exposure	£ 322,745	£ 310,155
Stage 3
Statement [Line Items]
Exposure	2,996	2,368
Stage 3 | Corporate and commercial banking
Statement [Line Items]
Exposure	1,020	452
Collateral	391	110
Net Exposure To Credit Risk	629	342
Stage 3 | Corporate and commercial banking | SME and Mid Corporate
Statement [Line Items]
Exposure	853	363
Collateral	286	52
Net Exposure To Credit Risk	567	311
Stage 3 | Corporate and commercial banking | Commercial Real Estate
Statement [Line Items]
Exposure	167	89
Collateral	105	58
Net Exposure To Credit Risk	62	31
Stage 3 | Corporate and investment banking
Statement [Line Items]
Exposure	0	15
Collateral	0	0
Net Exposure To Credit Risk	0	15
Stage 3 | Corporate and investment banking | Large Corporate
Statement [Line Items]
Exposure	0	15
Collateral	0	0
Net Exposure To Credit Risk	£ 0	£ 15

Credit Risk - Summary of Othe_3

Credit Risk - Summary of Other Segments Exposures by Credit Performance (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	£ 95,156	£ 80,918
Observed impairment loss allowances	671	314
Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	24,680	24,224
Observed impairment loss allowances	603	262
Corporate and commercial banking | Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	20,044	21,998
Corporate and commercial banking | Enhanced Monitoring [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	573	1,359
Corporate and commercial banking | Proactive Management [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,043	415
Corporate and commercial banking | Non-Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,020	452
Corporate and commercial banking | SME and Mid Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	14,446	13,254
Observed impairment loss allowances	478	213
Corporate and commercial banking | SME and Mid Corporate | Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	10,844	11,397
Corporate and commercial banking | SME and Mid Corporate | Enhanced Monitoring [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	340	1,162
Corporate and commercial banking | SME and Mid Corporate | Proactive Management [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2,409	332
Corporate and commercial banking | SME and Mid Corporate | Non-Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	853	363
Corporate and commercial banking | Commercial Real Estate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	5,152	6,057
Observed impairment loss allowances	125	49
Corporate and commercial banking | Commercial Real Estate | Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	4,191	5,778
Corporate and commercial banking | Commercial Real Estate | Enhanced Monitoring [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	233	107
Corporate and commercial banking | Commercial Real Estate | Proactive Management [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	561	83
Corporate and commercial banking | Commercial Real Estate | Non-Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	167	89
Corporate and commercial banking | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	5,082	4,913
Observed impairment loss allowances	0	0
Corporate and commercial banking | Social Housing | Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	5,009	4,823
Corporate and commercial banking | Social Housing | Enhanced Monitoring [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	90
Corporate and commercial banking | Social Housing | Proactive Management [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	73	0
Corporate and commercial banking | Social Housing | Non-Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Corporate and investment banking
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	12,347	14,565
Observed impairment loss allowances	33	50
Corporate and investment banking | Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	10,835	13,742
Corporate and investment banking | Enhanced Monitoring [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	252	252
Corporate and investment banking | Proactive Management [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,260	556
Corporate and investment banking | Non-Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	15
Corporate and investment banking | Large Corporate
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	10,886	12,646
Observed impairment loss allowances	33	50
Corporate and investment banking | Large Corporate | Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	9,374	11,833
Corporate and investment banking | Large Corporate | Enhanced Monitoring [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	252	252
Corporate and investment banking | Large Corporate | Proactive Management [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,260	546
Corporate and investment banking | Large Corporate | Non-Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	15
Corporate and investment banking | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,461	1,919
Observed impairment loss allowances	0	0
Corporate and investment banking | Financial Institutions | Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	1,461	1,909
Corporate and investment banking | Financial Institutions | Enhanced Monitoring [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Corporate and investment banking | Financial Institutions | Proactive Management [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	10
Corporate and investment banking | Financial Institutions | Non-Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Corporate centre
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	58,129	42,129
Observed impairment loss allowances	35	2
Corporate centre | Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	58,060	42,113
Corporate centre | Enhanced Monitoring [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	69	13
Corporate centre | Proactive Management [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	3
Corporate centre | Non-Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Corporate centre | Social Housing
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,817	4,060
Observed impairment loss allowances	0	0
Corporate centre | Social Housing | Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	3,748	4,047
Corporate centre | Social Housing | Enhanced Monitoring [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	69	13
Corporate centre | Social Housing | Proactive Management [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Corporate centre | Social Housing | Non-Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Corporate centre | Financial Institutions
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	712	857
Observed impairment loss allowances	0	0
Corporate centre | Financial Institutions | Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	712	854
Corporate centre | Financial Institutions | Enhanced Monitoring [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Corporate centre | Financial Institutions | Proactive Management [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	3
Corporate centre | Financial Institutions | Non-Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Corporate centre | Sovereign And Supranational
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	50,962	34,400
Observed impairment loss allowances	0	0
Corporate centre | Sovereign And Supranational | Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	50,962	34,400
Corporate centre | Sovereign And Supranational | Enhanced Monitoring [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Corporate centre | Sovereign And Supranational | Proactive Management [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Corporate centre | Sovereign And Supranational | Non-Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Corporate centre | Structured Products
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2,482	2,572
Observed impairment loss allowances	35	0
Corporate centre | Structured Products | Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	2,482	2,572
Corporate centre | Structured Products | Enhanced Monitoring [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Corporate centre | Structured Products | Proactive Management [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Corporate centre | Structured Products | Non-Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Corporate centre | Legacy Portfolios in Run-Off
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	156	240
Observed impairment loss allowances	0	2
Corporate centre | Legacy Portfolios in Run-Off | Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	156	240
Corporate centre | Legacy Portfolios in Run-Off | Enhanced Monitoring [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Corporate centre | Legacy Portfolios in Run-Off | Proactive Management [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	0	0
Corporate centre | Legacy Portfolios in Run-Off | Non-Performing Exposure [member]
Disclosure of credit risk exposure [line items]
Maximum exposure to credit risk	£ 0	£ 0

Credit Risk - Summary of Modi_4

Credit Risk - Summary of Modification of Loan Terms (Other Segments Exposures) (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Corporate and commercial banking
Disclosure of credit risk exposure [line items]
Amortised cost before modification	£ 201	£ 160
Net modification loss	(5)	1
Gross carrying amount of financial assets for which the ECL allowance has changed to 12-month measurement during the year	40	38
Corporate and investment banking
Disclosure of credit risk exposure [line items]
Amortised cost before modification	23	0
Net modification loss	1	0
Gross carrying amount of financial assets for which the ECL allowance has changed to 12-month measurement during the year	0	0
Corporate centre
Disclosure of credit risk exposure [line items]
Amortised cost before modification	0	0
Net modification loss	0	0
Gross carrying amount of financial assets for which the ECL allowance has changed to 12-month measurement during the year	£ 0	£ 0

Credit Risk - Summary of Othe_4

Credit Risk - Summary of Other Segments by Forbearance Applied (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure Of Forbearance [line items]
- Term extension	£ 476	£ 429
- Interest-only	396	439
- Other payment rescheduling	628	602
Loans forbearance	1,528	1,481
Corporate and commercial banking
Disclosure Of Forbearance [line items]
- Term extension	141	66
- Interest-only	175	153
- Other payment rescheduling	180	229
Loans forbearance	£ 496	£ 448
Proportion of portfolio	2.00%	1.80%
Corporate and commercial banking | Stage 1
Disclosure Of Forbearance [line items]
Loans forbearance	£ 13	£ 61
Corporate and commercial banking | Stage 2
Disclosure Of Forbearance [line items]
Loans forbearance	179	111
Corporate and commercial banking | Stage 3
Disclosure Of Forbearance [line items]
Loans forbearance	304	276
Corporate and investment banking
Disclosure Of Forbearance [line items]
- Term extension	23	42
- Interest-only	0	0
- Other payment rescheduling	0	15
Loans forbearance	£ 23	£ 57
Proportion of portfolio	0.20%	0.40%
Corporate and investment banking | Stage 1
Disclosure Of Forbearance [line items]
Loans forbearance	£ 0	£ 42
Corporate and investment banking | Stage 2
Disclosure Of Forbearance [line items]
Loans forbearance	23	0
Corporate and investment banking | Stage 3
Disclosure Of Forbearance [line items]
Loans forbearance	0	15
Corporate centre
Disclosure Of Forbearance [line items]
- Term extension	0	0
- Interest-only	0	0
- Other payment rescheduling	0	0
Loans forbearance	£ 0	£ 0
Proportion of portfolio	0.00%	0.00%
Corporate centre | Stage 1
Disclosure Of Forbearance [line items]
Loans forbearance	£ 0	£ 0
Corporate centre | Stage 2
Disclosure Of Forbearance [line items]
Loans forbearance	0	0
Corporate centre | Stage 3
Disclosure Of Forbearance [line items]
Loans forbearance	£ 0	£ 0

Credit Risk - Summary of Commer

Credit Risk - Summary of Commercial Real Estate by Credit Performance (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of performance obligations [line items]
Customer loans	£ 208,750	£ 207,287

Credit Risk - Summary of Comm_2

Credit Risk - Summary of Commercial Real Estate by Loan to Value (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure Of Loan To Value [line items]
Loans and advances to customers	£ 208,750	£ 207,287

Credit Risk - Summary of Comm_3

Credit Risk - Summary of Commercial Real Estate by Sector (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure Of Loans And Advances By Industry Sector [line items]
Loans and advances to customers	£ 208,750	£ 207,287

Credit Risk - Summary of Social

Credit Risk - Summary of Social Housing Exposure (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure Of Social Housing Exposure [line items]
Loans and advances to customers	£ 208,750	£ 207,287
Maximum exposure to balance sheet	95,156	80,918
Loss allowance	1,303	785
Corporate and commercial banking
Disclosure Of Social Housing Exposure [line items]
Maximum exposure to balance sheet	24,680	24,224
Corporate centre
Disclosure Of Social Housing Exposure [line items]
Maximum exposure to balance sheet	58,129	42,129
Social Housing | Corporate and commercial banking
Disclosure Of Social Housing Exposure [line items]
Maximum exposure to balance sheet	5,082	4,913
Social Housing | Corporate centre
Disclosure Of Social Housing Exposure [line items]
Maximum exposure to balance sheet	£ 3,817	£ 4,060

Market Risk - Summary of NIM an

Market Risk - Summary of NIM and EVE Sensitivity of Interest Rate Risk (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
+50bps
Disclosure Of Sensitivity Limit Of Risk [line items]
NIM sensitivity	£ 225	£ 99
EVE sensitivity (unaudited)	367	10
-50bps
Disclosure Of Sensitivity Limit Of Risk [line items]
NIM sensitivity	(15)	56
EVE sensitivity (unaudited)	£ (585)	£ (88)

Liquidity Risk - Summary of Rec

Liquidity Risk - Summary of Reconciliation of Wholesale Funding to Balance Sheet (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Deposits by banks	£ 20,958	£ 14,353
Deposits by customers	195,135	181,883
Repurchase agreements – non trading	15,848	18,286
Financial liabilities at fair value	1,434	1,713
Debt securities in issue	35,566	41,129
Subordinated liabilities	2,556	3,528
Liquidity risk
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Funding analysis	86,600	90,300
Deposits by banks	21,000	14,400
Deposits by customers	8,100	9,100
Repurchase agreements – non trading	15,800	18,300
Financial liabilities at fair value	1,400	1,700
Debt securities in issue	35,600	41,100
Subordinated liabilities	2,500	3,500
Other equity and non-controlling interests	2,200	2,200
Liquidity risk | Repurchase Agreement
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Funding analysis	15,800	16,900
Deposits by banks	0	0
Deposits by customers	0	0
Repurchase agreements – non trading	15,800	16,900
Financial liabilities at fair value	0	0
Debt securities in issue	0	0
Subordinated liabilities	0	0
Other equity and non-controlling interests	0	0
Liquidity risk | Foreign Exchange And Hedge Accounting
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Funding analysis	2,500	2,500
Deposits by banks	0	0
Deposits by customers	200	400
Repurchase agreements – non trading	0	0
Financial liabilities at fair value	0	0
Debt securities in issue	2,000	1,600
Subordinated liabilities	300	500
Other equity and non-controlling interests	0	0
Liquidity risk | Other Borrowing
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Funding analysis	3,000	3,500
Deposits by banks	3,000	3,300
Deposits by customers	0	0
Repurchase agreements – non trading	0	0
Financial liabilities at fair value	0	200
Debt securities in issue	0	0
Subordinated liabilities	0	0
Other equity and non-controlling interests	0	0
Liquidity risk | Wholesale Funding
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Funding analysis	65,300	67,400
Deposits by banks	18,000	11,100
Deposits by customers	7,900	8,700
Repurchase agreements – non trading	0	1,400
Financial liabilities at fair value	1,400	1,500
Debt securities in issue	33,600	39,500
Subordinated liabilities	2,200	3,000
Other equity and non-controlling interests	2,200	2,200
Liquidity risk | Wholesale Funding | Deposits by banks
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Funding analysis	0	300
Deposits by banks	0	300
Deposits by customers	0	0
Repurchase agreements – non trading	0	0
Financial liabilities at fair value	0	0
Debt securities in issue	0	0
Subordinated liabilities	0	0
Other equity and non-controlling interests	0	0
Liquidity risk | Wholesale Funding | Certificates of Deposit and Commercial Paper
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Funding analysis	5,700	5,800
Deposits by banks	0	0
Deposits by customers	0	0
Repurchase agreements – non trading	0	0
Financial liabilities at fair value	0	0
Debt securities in issue	5,700	5,800
Subordinated liabilities	0	0
Other equity and non-controlling interests	0	0
Liquidity risk | Wholesale Funding | Senior Unsecured - Public Benchmark
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Funding analysis	15,400	18,800
Deposits by banks	0	0
Deposits by customers	7,800	8,600
Repurchase agreements – non trading	0	0
Financial liabilities at fair value	0	0
Debt securities in issue	7,600	10,200
Subordinated liabilities	0
Other equity and non-controlling interests	0	0
Liquidity risk | Wholesale Funding | Senior Unsecured - Privately Placed
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Funding analysis	1,100	2,700
Deposits by banks	0	0
Deposits by customers	100	100
Repurchase agreements – non trading	0	0
Financial liabilities at fair value	900	1,000
Debt securities in issue	100	1,600
Subordinated liabilities	0	0
Other equity and non-controlling interests	0	0
Liquidity risk | Wholesale Funding | Covered Bonds
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Funding analysis	17,900	18,200
Deposits by banks	0	0
Deposits by customers	0	0
Repurchase agreements – non trading	0	0
Financial liabilities at fair value	0	0
Debt securities in issue	17,900	18,200
Subordinated liabilities	0	0
Other equity and non-controlling interests	0	0
Liquidity risk | Wholesale Funding | Securitisation and Structured Issuance
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Funding analysis	2,800	5,600
Deposits by banks	0	0
Deposits by customers	0	0
Repurchase agreements – non trading	0	1,400
Financial liabilities at fair value	500	500
Debt securities in issue	2,300	3,700
Subordinated liabilities	0	0
Other equity and non-controlling interests	0	0
Liquidity risk | Wholesale Funding | Term Funding Scheme
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Funding analysis	6,300	10,800
Deposits by banks	6,300	10,800
Deposits by customers	0	0
Repurchase agreements – non trading	0	0
Financial liabilities at fair value	0	0
Debt securities in issue	0	0
Subordinated liabilities	0	0
Other equity and non-controlling interests	0	0
Liquidity risk | Wholesale Funding | TFSME
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Funding analysis	11,700	0
Deposits by banks	11,700	0
Deposits by customers	0	0
Repurchase agreements – non trading	0	0
Financial liabilities at fair value	0	0
Debt securities in issue	0	0
Subordinated liabilities	0	0
Other equity and non-controlling interests	0	0
Liquidity risk | Wholesale Funding | Subordinated Liabilities and Equity
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Funding analysis	4,400	5,200
Deposits by banks	0	0
Deposits by customers	0	0
Repurchase agreements – non trading	0	0
Financial liabilities at fair value	0	0
Debt securities in issue	0	0
Subordinated liabilities	2,200	3,000
Other equity and non-controlling interests	£ 2,200	£ 2,200

Liquidity Risk - Summary of R_2

Liquidity Risk - Summary of Reconciliation of Wholesale Funding to Balance Sheet (Parenthetical) (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Deposit by customers	£ 195,135	£ 181,883
Other equity and non-controlling interests	2,191	2,191
Liquidity risk
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Deposit by customers	8,100	9,100
Fixed/Floating Rate Non-Cumulative Callable Preference Shares [member] | Liquidity risk
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Other equity and non-controlling interests	0	0
Step-Up Callable Perpetual Reserve Capital Instruments [member] | Liquidity risk
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Other equity and non-controlling interests	235	235
Perpetual Capital Securities [member] | Liquidity risk
Reconciliation Of Wholesale Funding To Balance Sheet [line items]
Other equity and non-controlling interests	£ 1,956	£ 1,956

Liquidity Risk - Summary of Sou

Liquidity Risk - Summary of Sources of Wholesale Funding by Maturity (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of Maturity of Wholesale Funding [Line Items]
Deposits by banks	£ 20,958	£ 14,353
Subordinated liabilities	2,556	3,528
Wholesale fund amount		67,400
Of which: - secured	38,700	34,600
Of which: - unsecured	26,600	32,800
Santander UK Group Holdings Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Senior unsecured - public benchmark	9,100
Senior unsecured - privately placed	100
Subordinated liabilities and equity (incl. AT1)	2,700
Wholesale fund amount	11,900
Other Santander UK plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Certificates of deposit and commercial paper	5,700
Senior unsecured - public benchmark	6,300
Senior unsecured - privately placed	1,000
Covered bonds	17,900
Subordinated liabilities and equity (incl. AT1)	1,600
Term Funding Scheme	6,300
Subordinated liabilities	1,700
Wholesale fund amount	52,200
Other Group Entities
Disclosure of Maturity of Wholesale Funding [Line Items]
Securitisation and structured issuance	1,200
Wholesale fund amount	65,300
Santander UK Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Term Funding Scheme With Additional Incentives For SMEs	11,700
≤ 1 month
Disclosure of Maturity of Wholesale Funding [Line Items]
Wholesale fund amount		1,700
Of which: - secured	400	200
Of which: - unsecured	1,800	1,500
≤ 1 month | Santander UK Group Holdings Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Senior unsecured - public benchmark	700
Senior unsecured - privately placed	0
Subordinated liabilities and equity (incl. AT1)	0
Wholesale fund amount	700
≤ 1 month | Other Santander UK plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Certificates of deposit and commercial paper	700
Senior unsecured - public benchmark	400
Senior unsecured - privately placed	0
Covered bonds	0
Subordinated liabilities and equity (incl. AT1)	200
Term Funding Scheme	0
Subordinated liabilities	0
Wholesale fund amount	1,300
≤ 1 month | Other Group Entities
Disclosure of Maturity of Wholesale Funding [Line Items]
Securitisation and structured issuance	200
Wholesale fund amount	2,200
≤ 1 month | Santander UK Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Term Funding Scheme With Additional Incentives For SMEs	0
>1 and ≤ 3 months
Disclosure of Maturity of Wholesale Funding [Line Items]
Wholesale fund amount		5,500
Of which: - secured	1,600	100
Of which: - unsecured	4,000	5,400
>1 and ≤ 3 months | Santander UK Group Holdings Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Senior unsecured - public benchmark	0
Senior unsecured - privately placed	0
Subordinated liabilities and equity (incl. AT1)	0
Wholesale fund amount	0
>1 and ≤ 3 months | Other Santander UK plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Certificates of deposit and commercial paper	3,700
Senior unsecured - public benchmark	0
Senior unsecured - privately placed	300
Covered bonds	0
Subordinated liabilities and equity (incl. AT1)	0
Term Funding Scheme	1,500
Subordinated liabilities	0
Wholesale fund amount	5,500
>1 and ≤ 3 months | Other Group Entities
Disclosure of Maturity of Wholesale Funding [Line Items]
Securitisation and structured issuance	100
Wholesale fund amount	5,600
>1 and ≤ 3 months | Santander UK Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Term Funding Scheme With Additional Incentives For SMEs	0
>3 and ≤ 6 months
Disclosure of Maturity of Wholesale Funding [Line Items]
Wholesale fund amount		4,700
Of which: - secured	5,200	2,700
Of which: - unsecured	2,800	2,000
>3 and ≤ 6 months | Santander UK Group Holdings Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Senior unsecured - public benchmark	0
Senior unsecured - privately placed	0
Subordinated liabilities and equity (incl. AT1)	0
Wholesale fund amount	0
>3 and ≤ 6 months | Other Santander UK plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Certificates of deposit and commercial paper	1,200
Senior unsecured - public benchmark	1,600
Senior unsecured - privately placed	0
Covered bonds	3,700
Subordinated liabilities and equity (incl. AT1)	0
Term Funding Scheme	1,500
Subordinated liabilities	0
Wholesale fund amount	8,000
>3 and ≤ 6 months | Other Group Entities
Disclosure of Maturity of Wholesale Funding [Line Items]
Securitisation and structured issuance	0
Wholesale fund amount	8,000
>3 and ≤ 6 months | Santander UK Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Term Funding Scheme With Additional Incentives For SMEs	0
>6 and ≤ 9 months
Disclosure of Maturity of Wholesale Funding [Line Items]
Wholesale fund amount		2,100
Of which: - secured	3,000	300
Of which: - unsecured	1,100	1,800
>6 and ≤ 9 months | Santander UK Group Holdings Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Senior unsecured - public benchmark	1,100
Senior unsecured - privately placed	0
Subordinated liabilities and equity (incl. AT1)	0
Wholesale fund amount	1,100
>6 and ≤ 9 months | Other Santander UK plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Certificates of deposit and commercial paper	0
Senior unsecured - public benchmark	0
Senior unsecured - privately placed	0
Covered bonds	1,700
Subordinated liabilities and equity (incl. AT1)	200
Term Funding Scheme	1,000
Subordinated liabilities	0
Wholesale fund amount	2,900
>6 and ≤ 9 months | Other Group Entities
Disclosure of Maturity of Wholesale Funding [Line Items]
Securitisation and structured issuance	100
Wholesale fund amount	4,100
>6 and ≤ 9 months | Santander UK Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Term Funding Scheme With Additional Incentives For SMEs	0
>9 and ≤ 12 months
Disclosure of Maturity of Wholesale Funding [Line Items]
Wholesale fund amount		8,500
Of which: - secured	200	5,800
Of which: - unsecured	1,000	2,700
>9 and ≤ 12 months | Santander UK Group Holdings Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Senior unsecured - public benchmark	0
Senior unsecured - privately placed	0
Subordinated liabilities and equity (incl. AT1)	0
Wholesale fund amount	0
>9 and ≤ 12 months | Other Santander UK plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Certificates of deposit and commercial paper	100
Senior unsecured - public benchmark	800
Senior unsecured - privately placed	100
Covered bonds	0
Subordinated liabilities and equity (incl. AT1)	0
Term Funding Scheme	0
Subordinated liabilities	0
Wholesale fund amount	1,000
>9 and ≤ 12 months | Other Group Entities
Disclosure of Maturity of Wholesale Funding [Line Items]
Securitisation and structured issuance	200
Wholesale fund amount	1,200
>9 and ≤ 12 months | Santander UK Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Term Funding Scheme With Additional Incentives For SMEs	0
Sub-total ≤ 1 year
Disclosure of Maturity of Wholesale Funding [Line Items]
Wholesale fund amount		22,500
Of which: - secured	10,400	9,100
Of which: - unsecured	10,700	13,400
Sub-total ≤ 1 year | Santander UK Group Holdings Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Senior unsecured - public benchmark	1,800
Senior unsecured - privately placed	0
Subordinated liabilities and equity (incl. AT1)	0
Wholesale fund amount	1,800
Sub-total ≤ 1 year | Other Santander UK plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Certificates of deposit and commercial paper	5,700
Senior unsecured - public benchmark	2,800
Senior unsecured - privately placed	400
Covered bonds	5,400
Subordinated liabilities and equity (incl. AT1)	400
Term Funding Scheme	4,000
Subordinated liabilities	0
Wholesale fund amount	18,700
Sub-total ≤ 1 year | Other Group Entities
Disclosure of Maturity of Wholesale Funding [Line Items]
Securitisation and structured issuance	600
Wholesale fund amount	21,100
Sub-total ≤ 1 year | Santander UK Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Term Funding Scheme With Additional Incentives For SMEs	0
>1 and ≤ 2 years
Disclosure of Maturity of Wholesale Funding [Line Items]
Wholesale fund amount		16,600
Of which: - secured	5,300	11,500
Of which: - unsecured	2,600	5,100
>1 and ≤ 2 years | Santander UK Group Holdings Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Senior unsecured - public benchmark	1,200
Senior unsecured - privately placed	0
Subordinated liabilities and equity (incl. AT1)	800
Wholesale fund amount	2,000
>1 and ≤ 2 years | Other Santander UK plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Certificates of deposit and commercial paper	0
Senior unsecured - public benchmark	600
Senior unsecured - privately placed	0
Covered bonds	1,700
Subordinated liabilities and equity (incl. AT1)	900
Term Funding Scheme	2,300
Subordinated liabilities	0
Wholesale fund amount	5,500
>1 and ≤ 2 years | Other Group Entities
Disclosure of Maturity of Wholesale Funding [Line Items]
Securitisation and structured issuance	400
Wholesale fund amount	7,900
>1 and ≤ 2 years | Santander UK Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Term Funding Scheme With Additional Incentives For SMEs	0
>2 and ≤ 5 years
Disclosure of Maturity of Wholesale Funding [Line Items]
Wholesale fund amount		18,500
Of which: - secured	18,600	10,400
Of which: - unsecured	9,600	8,100
>2 and ≤ 5 years | Santander UK Group Holdings Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Senior unsecured - public benchmark	4,800
Senior unsecured - privately placed	0
Subordinated liabilities and equity (incl. AT1)	1,500
Wholesale fund amount	6,300
>2 and ≤ 5 years | Other Santander UK plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Certificates of deposit and commercial paper	0
Senior unsecured - public benchmark	2,600
Senior unsecured - privately placed	200
Covered bonds	6,400
Subordinated liabilities and equity (incl. AT1)	300
Term Funding Scheme	0
Subordinated liabilities	500
Wholesale fund amount	21,700
>2 and ≤ 5 years | Other Group Entities
Disclosure of Maturity of Wholesale Funding [Line Items]
Securitisation and structured issuance	200
Wholesale fund amount	28,200
>2 and ≤ 5 years | Santander UK Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Term Funding Scheme With Additional Incentives For SMEs	11,700
>5 years
Disclosure of Maturity of Wholesale Funding [Line Items]
Wholesale fund amount		9,800
Of which: - secured	4,400	3,600
Of which: - unsecured	3,700	£ 6,200
>5 years | Santander UK Group Holdings Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Senior unsecured - public benchmark	1,300
Senior unsecured - privately placed	100
Subordinated liabilities and equity (incl. AT1)	400
Wholesale fund amount	1,800
>5 years | Other Santander UK plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Certificates of deposit and commercial paper	0
Senior unsecured - public benchmark	300
Senior unsecured - privately placed	400
Covered bonds	4,400
Subordinated liabilities and equity (incl. AT1)	0
Term Funding Scheme	0
Subordinated liabilities	1,200
Wholesale fund amount	6,300
>5 years | Other Group Entities
Disclosure of Maturity of Wholesale Funding [Line Items]
Securitisation and structured issuance	0
Wholesale fund amount	8,100
>5 years | Santander UK Plc
Disclosure of Maturity of Wholesale Funding [Line Items]
Term Funding Scheme With Additional Incentives For SMEs	£ 0

Liquidity Risk - Summary of S_2

Liquidity Risk - Summary of Sources of Wholesale Funding by Maturity (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2020
Disclosure of Maturity of Wholesale Funding [Line Items]
Unsecured Debt Issued	95.00%

Liquidity Risk - Summary of Who

Liquidity Risk - Summary of Wholesale Funding by Currency (Detail) - Liquidity risk	Dec. 31, 2020	Dec. 31, 2019
United Kingdom, Pounds | Santander UK Group Holdings Plc
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	24.00%	25.00%
United Kingdom, Pounds | Santander UK Group Holdings Plc | Senior Unsecured - Public Benchmark
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	10.00%	11.00%
United Kingdom, Pounds | Santander UK Group Holdings Plc | Senior Unsecured - Privately Placed
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
United Kingdom, Pounds | Santander UK Group Holdings Plc | Subordinated Liabilities and Equity
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	73.00%	67.00%
United Kingdom, Pounds | Other Santander UK plc
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	63.00%	54.00%
United Kingdom, Pounds | Other Santander UK plc | Senior Unsecured - Public Benchmark
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	10.00%	14.00%
United Kingdom, Pounds | Other Santander UK plc | Senior Unsecured - Privately Placed
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	41.00%	21.00%
United Kingdom, Pounds | Other Santander UK plc | Deposits by banks
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	3.00%
United Kingdom, Pounds | Other Santander UK plc | Certificates of Deposit and Commercial Paper
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	51.00%	45.00%
United Kingdom, Pounds | Other Santander UK plc | Covered Bonds
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	48.00%	54.00%
United Kingdom, Pounds | Other Santander UK plc | Securitisation and Structured Issuance
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	77.00%	72.00%
United Kingdom, Pounds | Other Santander UK plc | Term Funding Scheme
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	100.00%	100.00%
United Kingdom, Pounds | Other Santander UK plc | Subordinated liabilities
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	63.00%	49.00%
United Kingdom, Pounds | Other Group Entities
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	57.00%	50.00%
United Kingdom, Pounds | Other Group Entities | Securitisation and Structured Issuance
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	100.00%	95.00%
United Kingdom, Pounds | Santander UK Plc | TFSME
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	100.00%	0.00%
United States of America, Dollars | Santander UK Group Holdings Plc
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	53.00%	56.00%
United States of America, Dollars | Santander UK Group Holdings Plc | Senior Unsecured - Public Benchmark
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	62.00%	65.00%
United States of America, Dollars | Santander UK Group Holdings Plc | Senior Unsecured - Privately Placed
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
United States of America, Dollars | Santander UK Group Holdings Plc | Subordinated Liabilities and Equity
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	27.00%	33.00%
United States of America, Dollars | Other Santander UK plc
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	18.00%	22.00%
United States of America, Dollars | Other Santander UK plc | Senior Unsecured - Public Benchmark
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	73.00%	54.00%
United States of America, Dollars | Other Santander UK plc | Senior Unsecured - Privately Placed
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	37.00%	15.00%
United States of America, Dollars | Other Santander UK plc | Deposits by banks
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	97.00%
United States of America, Dollars | Other Santander UK plc | Certificates of Deposit and Commercial Paper
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	44.00%	54.00%
United States of America, Dollars | Other Santander UK plc | Covered Bonds
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	5.00%	0.00%
United States of America, Dollars | Other Santander UK plc | Securitisation and Structured Issuance
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	23.00%	28.00%
United States of America, Dollars | Other Santander UK plc | Term Funding Scheme
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
United States of America, Dollars | Other Santander UK plc | Subordinated liabilities
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	37.00%	51.00%
United States of America, Dollars | Other Group Entities
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	24.00%	27.00%
United States of America, Dollars | Other Group Entities | Securitisation and Structured Issuance
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	5.00%
United States of America, Dollars | Santander UK Plc | TFSME
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
Euro Member Countries, Euro | Santander UK Group Holdings Plc
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	22.00%	16.00%
Euro Member Countries, Euro | Santander UK Group Holdings Plc | Senior Unsecured - Public Benchmark
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	28.00%	22.00%
Euro Member Countries, Euro | Santander UK Group Holdings Plc | Senior Unsecured - Privately Placed
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
Euro Member Countries, Euro | Santander UK Group Holdings Plc | Subordinated Liabilities and Equity
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
Euro Member Countries, Euro | Other Santander UK plc
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	19.00%	24.00%
Euro Member Countries, Euro | Other Santander UK plc | Senior Unsecured - Public Benchmark
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	17.00%	32.00%
Euro Member Countries, Euro | Other Santander UK plc | Senior Unsecured - Privately Placed
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	10.00%	59.00%
Euro Member Countries, Euro | Other Santander UK plc | Deposits by banks
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
Euro Member Countries, Euro | Other Santander UK plc | Certificates of Deposit and Commercial Paper
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	4.00%	1.00%
Euro Member Countries, Euro | Other Santander UK plc | Covered Bonds
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	46.00%	45.00%
Euro Member Countries, Euro | Other Santander UK plc | Securitisation and Structured Issuance
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
Euro Member Countries, Euro | Other Santander UK plc | Term Funding Scheme
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
Euro Member Countries, Euro | Other Santander UK plc | Subordinated liabilities
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
Euro Member Countries, Euro | Other Group Entities
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	19.00%	22.00%
Euro Member Countries, Euro | Other Group Entities | Securitisation and Structured Issuance
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
Euro Member Countries, Euro | Santander UK Plc | TFSME
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
Other | Santander UK Group Holdings Plc
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	1.00%	3.00%
Other | Santander UK Group Holdings Plc | Senior Unsecured - Public Benchmark
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	2.00%
Other | Santander UK Group Holdings Plc | Senior Unsecured - Privately Placed
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	100.00%	100.00%
Other | Santander UK Group Holdings Plc | Subordinated Liabilities and Equity
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
Other | Other Santander UK plc
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
Other | Other Santander UK plc | Senior Unsecured - Public Benchmark
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
Other | Other Santander UK plc | Senior Unsecured - Privately Placed
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	12.00%	5.00%
Other | Other Santander UK plc | Deposits by banks
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
Other | Other Santander UK plc | Certificates of Deposit and Commercial Paper
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	1.00%	0.00%
Other | Other Santander UK plc | Covered Bonds
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	1.00%	1.00%
Other | Other Santander UK plc | Securitisation and Structured Issuance
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
Other | Other Santander UK plc | Term Funding Scheme
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
Other | Other Santander UK plc | Subordinated liabilities
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
Other | Other Group Entities
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	1.00%
Other | Other Group Entities | Securitisation and Structured Issuance
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%
Other | Santander UK Plc | TFSME
Currency Composition Of Wholesale Funds [line items]
Percentage of wholesale funding by major currencies	0.00%	0.00%

Liquidity Risk - Summary of Ext

Liquidity Risk - Summary of External Term Issuance (Sterling Equivalent) (Detail) - Liquidity risk - GBP (£) £ in Billions	Dec. 31, 2020	Dec. 31, 2019
Santander UK Group Holdings Plc
Disclosure of detailed information about borrowings [line items]
Borrowings	£ 1.4	£ 0.5
Santander UK Group Holdings Plc | Senior Unsecured - Public Benchmark
Disclosure of detailed information about borrowings [line items]
Borrowings	1.4	0
Santander UK Group Holdings Plc | Subordinated Liabilities and Equity
Disclosure of detailed information about borrowings [line items]
Borrowings	0	0.5
Other Santander UK plc
Disclosure of detailed information about borrowings [line items]
Borrowings	15.7	3.8
Other Santander UK plc | Senior Unsecured - Public Benchmark
Disclosure of detailed information about borrowings [line items]
Borrowings	1	0.9
Other Santander UK plc | Covered Bonds
Disclosure of detailed information about borrowings [line items]
Borrowings	3	2.9
Other Santander UK plc | TFSME
Disclosure of detailed information about borrowings [line items]
Borrowings	11.7	0
Other Group Entities
Disclosure of detailed information about borrowings [line items]
Borrowings	17.1	4.5
Other Group Entities | Securitisations
Disclosure of detailed information about borrowings [line items]
Borrowings	0	£ 0.2
United Kingdom, Pounds | Santander UK Group Holdings Plc
Disclosure of detailed information about borrowings [line items]
Borrowings	0
United Kingdom, Pounds | Santander UK Group Holdings Plc | Senior Unsecured - Public Benchmark
Disclosure of detailed information about borrowings [line items]
Borrowings	0
United Kingdom, Pounds | Santander UK Group Holdings Plc | Subordinated Liabilities and Equity
Disclosure of detailed information about borrowings [line items]
Borrowings	0
United Kingdom, Pounds | Other Santander UK plc
Disclosure of detailed information about borrowings [line items]
Borrowings	12.7
United Kingdom, Pounds | Other Santander UK plc | Senior Unsecured - Public Benchmark
Disclosure of detailed information about borrowings [line items]
Borrowings	0
United Kingdom, Pounds | Other Santander UK plc | Covered Bonds
Disclosure of detailed information about borrowings [line items]
Borrowings	1
United Kingdom, Pounds | Other Santander UK plc | TFSME
Disclosure of detailed information about borrowings [line items]
Borrowings	11.7
United Kingdom, Pounds | Other Group Entities
Disclosure of detailed information about borrowings [line items]
Borrowings	12.7
United Kingdom, Pounds | Other Group Entities | Securitisations
Disclosure of detailed information about borrowings [line items]
Borrowings	0
United States of America, Dollars | Santander UK Group Holdings Plc
Disclosure of detailed information about borrowings [line items]
Borrowings	0.8
United States of America, Dollars | Santander UK Group Holdings Plc | Senior Unsecured - Public Benchmark
Disclosure of detailed information about borrowings [line items]
Borrowings	0.8
United States of America, Dollars | Santander UK Group Holdings Plc | Subordinated Liabilities and Equity
Disclosure of detailed information about borrowings [line items]
Borrowings	0
United States of America, Dollars | Other Santander UK plc
Disclosure of detailed information about borrowings [line items]
Borrowings	1.9
United States of America, Dollars | Other Santander UK plc | Senior Unsecured - Public Benchmark
Disclosure of detailed information about borrowings [line items]
Borrowings	1
United States of America, Dollars | Other Santander UK plc | Covered Bonds
Disclosure of detailed information about borrowings [line items]
Borrowings	0.9
United States of America, Dollars | Other Santander UK plc | TFSME
Disclosure of detailed information about borrowings [line items]
Borrowings	0
United States of America, Dollars | Other Group Entities
Disclosure of detailed information about borrowings [line items]
Borrowings	2.7
United States of America, Dollars | Other Group Entities | Securitisations
Disclosure of detailed information about borrowings [line items]
Borrowings	0
Euro Member Countries, Euro | Santander UK Group Holdings Plc
Disclosure of detailed information about borrowings [line items]
Borrowings	0.6
Euro Member Countries, Euro | Santander UK Group Holdings Plc | Senior Unsecured - Public Benchmark
Disclosure of detailed information about borrowings [line items]
Borrowings	0.6
Euro Member Countries, Euro | Santander UK Group Holdings Plc | Subordinated Liabilities and Equity
Disclosure of detailed information about borrowings [line items]
Borrowings	0
Euro Member Countries, Euro | Other Santander UK plc
Disclosure of detailed information about borrowings [line items]
Borrowings	1.1
Euro Member Countries, Euro | Other Santander UK plc | Senior Unsecured - Public Benchmark
Disclosure of detailed information about borrowings [line items]
Borrowings	0
Euro Member Countries, Euro | Other Santander UK plc | Covered Bonds
Disclosure of detailed information about borrowings [line items]
Borrowings	1.1
Euro Member Countries, Euro | Other Santander UK plc | TFSME
Disclosure of detailed information about borrowings [line items]
Borrowings	0
Euro Member Countries, Euro | Other Group Entities
Disclosure of detailed information about borrowings [line items]
Borrowings	1.7
Euro Member Countries, Euro | Other Group Entities | Securitisations
Disclosure of detailed information about borrowings [line items]
Borrowings	£ 0

Accounting Policies - Additiona

Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2020
180 Days | Unsecured Debt
Disclosure of changes in accounting estimates [line items]
Written off period for past due	180 days

Accounting Policies - Narrative

Accounting Policies - Narrative (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of changes in accounting estimates [line items]
Other assets	£ 3,013	£ 2,520
Other liabilities	2,337	2,344
Provisions	464	£ 572
Additional provisions	£ 289
Unsecured Debt | 180 Days
Disclosure of changes in accounting estimates [line items]
Written off period for past due	180 days
Other intangible assets [member] | Bottom of range
Disclosure of changes in accounting estimates [line items]
Useful life measured as period of time, intangible assets other than goodwill	3 years
Other intangible assets [member] | Top of range
Disclosure of changes in accounting estimates [line items]
Useful life measured as period of time, intangible assets other than goodwill	7 years
Computer software | Bottom of range
Disclosure of changes in accounting estimates [line items]
Useful life measured as period of time, intangible assets other than goodwill	3 years
Computer software | Top of range
Disclosure of changes in accounting estimates [line items]
Useful life measured as period of time, intangible assets other than goodwill	7 years

Accounting Policies - Summary o

Accounting Policies - Summary of Useful Lives or Depreciation Rates Used for Property, Plant and Equipment (Detail)	12 Months Ended
	Dec. 31, 2020
Bottom of range | Office Fixtures and Equipment
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment	3 years
Bottom of range | Computer software
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment	3 years
Top of range | Owner-occupied properties
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment	50 years
Top of range | Office Fixtures and Equipment
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment	15 years
Top of range | Computer software
Disclosure of detailed information about property, plant and equipment [line items]
Useful lives or depreciation rates, property, plant and equipment	7 years

Accounting Policies - Summary_2

Accounting Policies - Summary of Increase (Decrease in Profit Before Tax (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of significant accounting policies [Abstract]
Increase in profit before tax due to 20 percentage increase in HPI rate	£ 63	£ 41
Increase in profit before tax due to 10 percentage increase in HPI rate	38	25
Decrease in profit before tax due to 10 percentage decrease in HPI rate	(66)	(39)
Decrease in profit before tax due to 20 percentage decrease in HPI rate	£ (183)	£ (105)

Segments - Additional Informati

Segments - Additional Information (Detail) £ in Millions	12 Months Ended
	Dec. 31, 2020GBP (£)segment	Dec. 31, 2019GBP (£)	Dec. 31, 2018GBP (£)
Disclosure of operating segments [line items]
Number of segments | segment	4
Assets	£ 292,332	£ 281,702	£ 283,372
Customer deposits	185,707	171,715	167,315
Corporate loans	24,474	27,043
Profit before tax	605	1,012	1,545
Retail banking
Disclosure of operating segments [line items]
Assets	192,070	185,920	177,959
Customer deposits	152,167	142,735	139,976
Profit before tax	1,081	1,092	1,431
Retail banking | Reclassified Amounts
Disclosure of operating segments [line items]
Assets	(2,000)
Customer deposits	(3,100)
Profit before tax	18	(36)
Corporate and commercial banking
Disclosure of operating segments [line items]
Assets	17,626	18,391	19,858
Customer deposits	24,985	20,546	19,695
Profit before tax	(165)	140	243
Corporate and commercial banking | Reclassified Amounts
Disclosure of operating segments [line items]
Assets	2,000
Customer deposits	3,100
Corporate loans	400
Profit before tax	(9)	24
Corporate and commercial banking | Bottom of range
Disclosure of operating segments [line items]
Annual turnover of business	2
Corporate and commercial banking | Top of range
Disclosure of operating segments [line items]
Annual turnover of business	500
Corporate and investment banking
Disclosure of operating segments [line items]
Annual turnover of business	500
Assets	2,784	4,046	3,244
Customer deposits	6,506	6,102	4,853
Profit before tax	(8)	(36)	(24)
Corporate and investment banking | Reclassified Amounts
Disclosure of operating segments [line items]
Profit before tax	(10)	(1)
Corporate and investment banking | Bottom of range
Disclosure of operating segments [line items]
Annual turnover of business	500
Corporate centre
Disclosure of operating segments [line items]
Assets	79,852	73,345	82,311
Customer deposits	2,049	2,332	2,791
Profit before tax	(303)	(184)	£ (105)
Corporate centre | Reclassified Amounts
Disclosure of operating segments [line items]
Corporate loans	(400)
Profit before tax	£ 1	£ 13

Segments - Summary of Results b

Segments - Summary of Results by Segment (Detail) £ in Millions	12 Months Ended
	Dec. 31, 2020GBP (£)Employee	Dec. 31, 2019GBP (£)Employee	Dec. 31, 2018GBP (£)Employee
Disclosure of operating segments [line items]
Net interest income/(expense)	£ 3,443	£ 3,292	£ 3,603
Non-interest income	532	881	931
Total operating income/(expense)	3,975	4,173	4,534
Operating expenses before credit impairment losses, provisions and charges	(2,452)	(2,499)	(2,579)
Credit impairment losses	(645)	(221)	(153)
Provisions for other liabilities and charges	(273)	(441)	(257)
Total operating credit impairment losses, provisions and charges	(918)	(662)	(410)
Profit before tax	605	1,012	1,545
Revenue from external customers	3,975	4,173	4,534
Inter-segment revenue	3,975	4,173	4,534
Total operating income/(expense)	3,975	4,173	4,534
Revenue from external customers includes the following fee and commission income disaggregated by income type:
Current account and debit card fees	492	758	753
Insurance, protection and investments	65	77	105
Credit cards	67	86	85
Non-banking and other fees	132	191	227
Total fee and commission income	756	1,112	1,170
Fee and commission expense	(371)	(426)	(421)
Net fee and commission income	385	686	749
Customer loans	207,010	205,008	199,586
Total assets	292,332	281,702	283,372
Customer deposits	185,707	171,715	167,315
Total liabilities	£ 276,396	£ 265,685	£ 267,463
Average number of full-time equivalent staff | Employee	21,998	23,570	24,205
Elimination of Intersegment Amounts
Disclosure of operating segments [line items]
Inter-segment revenue	£ 0	£ 0	£ 0
Retail banking
Disclosure of operating segments [line items]
Net interest income/(expense)	3,100	2,827	3,076
Non-interest income	375	691	632
Total operating income/(expense)	3,475	3,518	3,708
Operating expenses before credit impairment losses, provisions and charges	(1,913)	(1,980)	(1,914)
Credit impairment losses	(308)	(156)	(135)
Provisions for other liabilities and charges	(173)	(290)	(228)
Total operating credit impairment losses, provisions and charges	(481)	(446)	(363)
Profit before tax	1,081	1,092	1,431
Revenue from external customers	4,160	4,255	4,361
Total operating income/(expense)	3,475	3,518	3,708
Revenue from external customers includes the following fee and commission income disaggregated by income type:
Current account and debit card fees	441	696	690
Insurance, protection and investments	65	76	105
Credit cards	67	86	85
Non-banking and other fees	15	61	76
Total fee and commission income	588	919	956
Fee and commission expense	(335)	(373)	(382)
Net fee and commission income	253	546	574
Customer loans	183,404	178,762	171,134
Total assets	192,070	185,920	177,959
Customer deposits	152,167	142,735	139,976
Total liabilities	£ 152,715	£ 143,602	£ 140,750
Average number of full-time equivalent staff | Employee	19,151	20,594	20,964
Retail banking | Elimination of Intersegment Amounts
Disclosure of operating segments [line items]
Inter-segment revenue	£ (685)	£ (737)	£ (653)
Corporate and commercial banking
Disclosure of operating segments [line items]
Net interest income/(expense)	364	422	470
Non-interest income	93	109	115
Total operating income/(expense)	457	531	585
Operating expenses before credit impairment losses, provisions and charges	(316)	(324)	(318)
Credit impairment losses	(294)	(45)	(8)
Provisions for other liabilities and charges	(12)	(22)	(16)
Total operating credit impairment losses, provisions and charges	(306)	(67)	(24)
Profit before tax	(165)	140	243
Revenue from external customers	549	633	750
Total operating income/(expense)	457	531	585
Revenue from external customers includes the following fee and commission income disaggregated by income type:
Current account and debit card fees	41	49	49
Insurance, protection and investments	0	0	0
Credit cards	0	0	0
Non-banking and other fees	47	58	65
Total fee and commission income	88	107	114
Fee and commission expense	(19)	(23)	(25)
Net fee and commission income	69	84	89
Customer loans	17,626	18,391	19,858
Total assets	17,626	18,391	19,858
Customer deposits	24,985	20,546	19,695
Total liabilities	£ 25,011	£ 20,572	£ 19,723
Average number of full-time equivalent staff | Employee	2,092	2,151	2,102
Corporate and commercial banking | Elimination of Intersegment Amounts
Disclosure of operating segments [line items]
Inter-segment revenue	£ (92)	£ (102)	£ (165)
Corporate and investment banking
Disclosure of operating segments [line items]
Net interest income/(expense)	55	62	69
Non-interest income	68	70	132
Total operating income/(expense)	123	132	201
Operating expenses before credit impairment losses, provisions and charges	(114)	(130)	(203)
Credit impairment losses	(7)	(22)	(14)
Provisions for other liabilities and charges	(10)	(16)	(8)
Total operating credit impairment losses, provisions and charges	(17)	(38)	(22)
Profit before tax	(8)	(36)	(24)
Revenue from external customers	123	138	245
Total operating income/(expense)	123	132	201
Revenue from external customers includes the following fee and commission income disaggregated by income type:
Current account and debit card fees	10	13	14
Insurance, protection and investments	0	0	0
Credit cards	0	0	0
Non-banking and other fees	66	67	81
Total fee and commission income	76	80	95
Fee and commission expense	(10)	(17)	(14)
Net fee and commission income	66	63	81
Customer loans	2,784	4,041	4,543
Total assets	2,784	4,046	3,244
Customer deposits	6,506	6,102	4,853
Total liabilities	£ 6,517	£ 6,233	£ 4,400
Average number of full-time equivalent staff | Employee	716	804	985
Corporate and investment banking | Elimination of Intersegment Amounts
Disclosure of operating segments [line items]
Inter-segment revenue	£ 0	£ (6)	£ (44)
Corporate centre
Disclosure of operating segments [line items]
Net interest income/(expense)	(76)	(19)	(12)
Non-interest income	(4)	11	52
Total operating income/(expense)	(80)	(8)	40
Operating expenses before credit impairment losses, provisions and charges	(109)	(65)	(144)
Credit impairment losses	(36)	2	4
Provisions for other liabilities and charges	(78)	(113)	(5)
Total operating credit impairment losses, provisions and charges	(114)	(111)	(1)
Profit before tax	(303)	(184)	(105)
Revenue from external customers	(857)	(853)	(822)
Total operating income/(expense)	(80)	(8)	40
Revenue from external customers includes the following fee and commission income disaggregated by income type:
Current account and debit card fees	0	0	0
Insurance, protection and investments	0	1	0
Credit cards	0	0	0
Non-banking and other fees	4	5	5
Total fee and commission income	4	6	5
Fee and commission expense	(7)	(13)	0
Net fee and commission income	(3)	(7)	5
Customer loans	3,196	3,814	4,051
Total assets	79,852	73,345	82,311
Customer deposits	2,049	2,332	2,791
Total liabilities	£ 92,153	£ 95,278	£ 102,590
Average number of full-time equivalent staff | Employee	39	21	154
Corporate centre | Elimination of Intersegment Amounts
Disclosure of operating segments [line items]
Inter-segment revenue	£ 777	£ 845	£ 862

Net Interest Income - Summary o

Net Interest Income - Summary of Net Interest Income (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Interest and similar income:
Loans and advances to customers	£ 4,819	£ 5,230	£ 5,458
Loans and advances to banks	49	137	202
Reverse repurchase agreements – non trading	118	244	124
Other	119	306	282
Total interest and similar income	5,105	5,917	6,066
Interest expense and similar charges:
Deposits by customers	(1,011)	(1,540)	(1,433)
Deposits by banks	(47)	(134)	(117)
Repurchase agreements – non trading	(43)	(126)	(42)
Debt securities in issue	(440)	(678)	(721)
Subordinated liabilities	(111)	(137)	(142)
Other	(10)	(10)	(8)
Total interest expense and similar charges	(1,662)	(2,625)	(2,463)
Net interest income	£ 3,443	£ 3,292	£ 3,603

Net Interest Income - Summary_2

Net Interest Income - Summary of Net Interest Income (Parenthetical) (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Analysis of income and expense [abstract]
Interest income on financial assets at fair value through other comprehensive income	£ 38	£ 155
Interest expense on derivatives hedging debt issuances	451	310
Interest expense on lease liabilities	£ 3	£ 4

Net Fee and Commission Income -

Net Fee and Commission Income - Summary of Net Fee and Commission Income (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Fee And Commission Income [Line Items]
Fee and commission income	£ 756	£ 1,112	£ 1,170
Fee and commission expense	(371)	(426)	(421)
Net fee and commission income	385	686	749
Current account and debit card fees
Net Fee And Commission Income [Line Items]
Fee and commission income	492	758	753
Insurance, protection and investments
Net Fee And Commission Income [Line Items]
Fee and commission income	65	77	105
Credit cards
Net Fee And Commission Income [Line Items]
Fee and commission income	67	86	85
Non-banking and other fees
Net Fee And Commission Income [Line Items]
Fee and commission income	£ 132	£ 191	£ 227

Other Operating Income - Summar

Other Operating Income - Summary of Other Operating Income (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Trading And Other Income [abstract]
Net losses on financial instruments designated at fair value through profit or loss	£ (75)	£ (134)	£ (86)
Net gains on financial instruments mandatorily at fair value through profit or loss	46	70	36
Hedge ineffectiveness	20	8	34
Net profit on sale of financial assets at fair value through other comprehensive income	17	15	19
Income from operating lease assets	126	124	86
Other	13	112	93
Other operating income (expense)	£ 147	£ 195	£ 182

Other Operating Income - Additi

Other Operating Income - Additional Information - For Year-end (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of subordinated liabilities [line items]
Fair value gains/(losses) on net trading and funding of other items by trading book	£ 89	£ 42	£ 22
(Losses)/gains on equity derivatives trading desk	(88)	43	(21)
Net gain on equity index-linked deposits and the related economic hedges	1	1	1
Exchange rate differences recognised	(751)	1,102	(689)
Principally offset from cash flow hedge reserve	809	(1,013)	£ 752
Loss on repurchase of debt securities and subordinated liabilities	24	£ (24)
Vocalink Holdings Limited
Disclosure of subordinated liabilities [line items]
Gain on sale of shares	£ 15

Operating Expenses Before Cre_3

Operating Expenses Before Credit Impairment Losses, Provisions and Charges - Summary of Operating Expenses Before Credit Impairment Losses, Provisions and Charges (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Expenses by nature [abstract]
Wages and salaries	£ 831	£ 852	£ 898
Performance-related payments	97	159	159
Social security costs	101	111	111
Pensions costs: – defined contribution plans	66	66	67
Pension costs: - defined benefit plans	38	35	79
Other share-based payments	0	0	3
Other personnel costs	33	40	52
Employee benefits expense	1,166	1,263	1,369
Other administration expenses	724	693	835
Depreciation, amortisation and impairment	562	543	375
Total operating expenses before impairment losses, provisions and charges	£ 2,452	£ 2,499	£ 2,579

Operating Expenses Before Cre_4

Operating Expenses Before Credit Impairment Losses, Provisions and Charges - Summary of Deferred Performance Awards (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of deferred performance awards [line Items]
Cash	£ 20	£ 27
Shares	19	24
Expenses charged in the year
Disclosure of deferred performance awards [line Items]
Cash	9	10
Shares	8	9
Deferred performance awards	17
Expenses charged in the year | Arising From Awards in Current Year
Disclosure of deferred performance awards [line Items]
Cash	2
Shares	2
Deferred performance awards	4
Expenses charged in the year | Arising From Awards in Prior Year
Disclosure of deferred performance awards [line Items]
Cash	7
Shares	6
Deferred performance awards	13
Expenses deferred to future periods
Disclosure of deferred performance awards [line Items]
Cash	11	17
Shares	11	£ 15
Deferred performance awards	22
Expenses deferred to future periods | Arising From Awards in Current Year
Disclosure of deferred performance awards [line Items]
Cash	2
Shares	2
Deferred performance awards	4
Expenses deferred to future periods | Arising From Awards in Prior Year
Disclosure of deferred performance awards [line Items]
Cash	9
Shares	9
Deferred performance awards	£ 18

Operating Expenses Before Cre_5

Operating Expenses Before Credit Impairment Losses, Provisions and Charges - Summary of Amount of Bonus Awarded to Employees (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of amount of bonus awarded to employees [line items]
Cash award - not deferred	£ 77	£ 130
- deferred	20	27
Share awards - not deferred	3	10
- deferred	19	24
Total discretionary bonus	119	191
Expenses charged in the year
Disclosure of amount of bonus awarded to employees [line items]
Cash award - not deferred	77	130
- deferred	9	10
Share awards - not deferred	3	10
- deferred	8	9
Total discretionary bonus	97	159
Expenses deferred to future periods
Disclosure of amount of bonus awarded to employees [line items]
Cash award - not deferred	0	0
- deferred	11	17
Share awards - not deferred	0	0
- deferred	11	15
Total discretionary bonus	£ 22	£ 32

Operating Expenses Before Cre_6

Operating Expenses Before Credit Impairment Losses, Provisions and Charges - Additional Information (Detail) £ in Millions	Oct. 26, 2018GBP (£)	Dec. 31, 2020GBP (£)Employee	Dec. 31, 2019GBP (£)Employee	Dec. 31, 2018Employee
Operating Expenses Before Credit Impairment Losses Provision And Charges [Line Items]
Increase in liabilities	£ 40	£ 5	£ 5
Allowance included in the scheme liabilities value		£ 50	£ 45
Average number of full-time equivalent staff | Employee		21,998	23,570	24,205
Property, plant and equipment		£ 1,734	£ 1,967
Operating lease assets		100	152
Expense relating to short-term leases for which recognition exemption has been used		10	13
Operating lease assets
Operating Expenses Before Credit Impairment Losses Provision And Charges [Line Items]
Depreciation, property, plant and equipment		92	103
Property, plant and equipment		542	574
IFRS 16
Operating Expenses Before Credit Impairment Losses Provision And Charges [Line Items]
Depreciation for lease assets		£ 58	£ 60

Audit and Other Services - Summ

Audit and Other Services - Summary of Audit and Other Services (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Audit fees:
Fees payable to the Company's auditor and its associates for the audit of the Santander UK group's annual accounts	£ 10	£ 8	£ 7.2
Audit of the Santander UK group's subsidiaries	1.4	1.3	1.1
Total audit fees	11.4	9.3	8.3
Non-audit fees:
Audit-related assurance services	0.8	0.8	0.7
Other assurance services	0.3	0.2	0.1
Other non-audit services	0	0.2	1
Total non-audit fees	£ 1.1	£ 1.2	£ 1.8

Audit and Other Services - Su_2

Audit and Other Services - Summary of Audit and Other Services (Parenthetical) (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of audit fees and non audit fees [Line Items]
Audit-related assurance services	£ 0.8	£ 0.8	£ 0.7
Prior year audit services
Disclosure of audit fees and non audit fees [Line Items]
Auditors fee	0.8	0.1
Audit-related assurance services	£ 0	£ 0.1

Audit and Other Services - Addi

Audit and Other Services - Additional Information (Detail) - GBP (£)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of audit fees and non audit fees [Line Items]
Non -audit fees - Audit fees per US Securities and Exchange Commission guidance	£ 400,000	£ 600,000	£ 100,000
Non- audit fees -Audit Related Fees" per US Securities and Exchange Commission guidance	700,000	400,000	600,000
Audit fees payable for statutory audit	11,400,000	9,300,000	8,300,000
Corporate and Other Borrowers
Disclosure of audit fees and non audit fees [Line Items]
Non- audit fees- All Other Fees" per US Securities and Exchange Commission guidance'	0	200,000	1,100,000
Audit fees payable for statutory audit	£ 24,000	£ 0	£ 150,000

Credit Impairment Losses and _3

Credit Impairment Losses and Provisions - Summary of Impairment Losses and Provisions (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of credit impairment loss and provisions [abstract]
Loans and advances to customers	£ 672	£ 239	£ 189
Recoveries of loans and advances, net of collection costs	(24)	(40)	(42)
Off-balance sheet exposures (See Note 29)	(3)	22	6
Impairment loss on financial assets	645	221	153
Provisions for other liabilities and charges (excluding off-balance sheet credit exposures) (See Note 29)	267	435	257
Provisions for residual value and voluntary termination	6	6	0
Provisions for other liabilities and charges	273	441	257
Total operating impairment losses, provisions and charges	£ 918	£ 662	£ 410

Credit Impairment Losses and _4

Credit Impairment Losses and Provisions - Additional Information (Detail) - GBP (£)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of credit impairment loss and provisions [abstract]
Impairment losses on loans and advances to banks	£ 0	£ 0
Impairment losses on Non-trading reverse repurchase agreements	0	0
Impairment losses on other financial assets at amortised cost	0	0
Impairment losses on financial assets at FVOCI	£ 0	£ 0

Taxation - Disclosure of Tax on

Taxation - Disclosure of Tax on Profit (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Current tax:
UK corporation tax on profit for the year	£ 120	£ 265	£ 408
Adjustments in respect of prior years	(24)	(25)	(20)
Total current tax	96	240	388
Deferred tax:
Charge for the year	34	46	16
Adjustments in respect of prior years	4	(7)	(5)
Total deferred tax	38	39	11
Tax on profit	£ 134	£ 279	£ 399

Taxation - Additional Informati

Taxation - Additional Information (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2016
Income Tax Expense Benefits [line items]
Applicable tax rate (percentage)	19.00%	19.00%	19.00%
Bank corporation tax surcharge				8.00%
Effective tax rate (percentage)	22.10%	27.60%	25.80%
Unrecognised capital losses carried forward	£ 0	£ 0
Banking Entities
Income Tax Expense Benefits [line items]
Applicable tax rate (percentage)	27.00%	27.00%	27.00%
Non-banking Entities
Income Tax Expense Benefits [line items]
Applicable tax rate (percentage)	19.00%	19.00%	19.00%
Deferred tax assets that rely on future profitability excluding timing differences
Income Tax Expense Benefits [line items]
Recognised capital losses carried forward	£ 12	£ 11
Finance (No.2) Act 2016 | Bottom of range | Corporation Tax Rate in 2020
Income Tax Expense Benefits [line items]
Applicable tax rate (percentage)	17.00%

Taxation - Schedule of Tax on P

Taxation - Schedule of Tax on Profit Before Tax Differs from Theoretical Amount that Arise Using Basic Corporation Tax Rate (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Major components of tax expense (income) [abstract]
Profit before tax	£ 605	£ 1,012	£ 1,545
Tax calculated at a tax rate of 19% (2019: 19%, 2018: 19%)	115	192	294
Bank surcharge on profits	31	65	109
Non-deductible preference dividends paid	8	8	8
Non-deductible UK Bank Levy	19	24	20
Non-deductible conduct remediation, fines and penalties	(4)	44	6
Other non-deductible costs and non-taxable income	25	31	30
Effect of change in tax rate on deferred tax provision	6	(14)	(1)
Tax relief on dividends in respect of other equity instruments	(40)	(39)	(42)
Adjustment to prior year provisions	(26)	(32)	(25)
Tax charge	£ 134	£ 279	£ 399

Taxation - Schedule of Tax on_2

Taxation - Schedule of Tax on Profit Before Tax Differs from Theoretical Amount that Arise Using Basic Corporation Tax Rate (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Major components of tax expense (income) [abstract]
Standard tax rate (percentage)	19.00%	19.00%	19.00%

Taxation - Disclosure of Moveme

Taxation - Disclosure of Movements in Current Tax Assets and Liabilities (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Major components of tax expense (income) [abstract]
Assets	£ 200	£ 153
Liabilities	0	0
Beginning Balance	200	153
Income statement charge	(96)	(240)	£ (388)
Other comprehensive income (charge)/credit	1	(4)
Corporate income tax paid	159	292	391
Other movements	0	(1)
Ending Balance	264	200	153
Assets	264	200	153
Liabilities	£ 0	£ 0	£ 0

Taxation - Disclosure of Deferr

Taxation - Disclosure of Deferred Tax Assets and Liabilities Including Movement in Deferred Tax Account (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]
Beginning Balance	£ (149)	£ (223)
Income statement (charge)/credit	(38)	(39)
Transfers/reclassifications	(2)	0
Credited/(charged) to other comprehensive income	78	113
Ending Balance	(111)	(149)
Fair value of financial instruments
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]
Beginning Balance	(52)	(51)
Income statement (charge)/credit	(13)	(1)
Transfers/reclassifications	0	0
Credited/(charged) to other comprehensive income	0	0
Ending Balance	(65)	(52)
Pension remeasurement
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]
Beginning Balance	(96)	(183)
Income statement (charge)/credit	(63)	(44)
Transfers/reclassifications	0	0
Credited/(charged) to other comprehensive income	133	131
Ending Balance	(26)	(96)
Cash flow hedges
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]
Beginning Balance	(58)	(43)
Income statement (charge)/credit	0	0
Transfers/reclassifications	12	22
Credited/(charged) to other comprehensive income	(53)	(37)
Ending Balance	(99)	(58)
Fair value reserve
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]
Beginning Balance	(8)	(13)
Income statement (charge)/credit	0	0
Transfers/reclassifications	(1)	5
Credited/(charged) to other comprehensive income	(2)	0
Ending Balance	(11)	(8)
Tax losses carried forward
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]
Beginning Balance	13	20
Income statement (charge)/credit	2	(7)
Transfers/reclassifications	0	0
Credited/(charged) to other comprehensive income	0	0
Ending Balance	15	13
Accelerated tax depreciation
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]
Beginning Balance	17	(6)
Income statement (charge)/credit	21	23
Transfers/reclassifications	0	0
Credited/(charged) to other comprehensive income	0	0
Ending Balance	38	17
Other temporary differences
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]
Beginning Balance	35	53
Income statement (charge)/credit	15	(10)
Transfers/reclassifications	(13)	(27)
Credited/(charged) to other comprehensive income	0	19
Ending Balance	£ 37	£ 35

Dividends on Ordinary Shares -

Dividends on Ordinary Shares - Disclosure of Dividends on Ordinary Shares Declared and Paid (Detail) - GBP (£) £ / shares in Units, £ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of Dividends on Ordinary Shares [line items]
Dividend per share		£ 0.42	£ 1.02	£ 3.67
Dividends	£ 129	£ 129	£ 315	£ 1,139
Dividends declared and paid			£ 668
First Interim
Disclosure of Dividends on Ordinary Shares [line items]
Dividend per share		£ 0.42	£ 0.53	£ 0.81
Dividends		£ 129	£ 164	£ 250
Second Interim
Disclosure of Dividends on Ordinary Shares [line items]
Dividend per share		£ 0	£ 0.49	£ 2.15
Dividends		£ 0	£ 151	£ 668
Third Interim
Disclosure of Dividends on Ordinary Shares [line items]
Dividend per share		£ 0	£ 0	£ 0.71
Dividends		£ 0	£ 0	£ 221
First dividend paid
Disclosure of Dividends on Ordinary Shares [line items]
Dividends declared and paid			250
Second dividend paid
Disclosure of Dividends on Ordinary Shares [line items]
Dividends declared and paid			£ 221

Derivative Financial Instrume_3

Derivative Financial Instruments - Notional Amounts and Fair Values of Derivatives (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of detailed information about hedges [line items]
Notional amount	£ 197,215	£ 205,172
Fair value assets	3,406	3,316
Fair value liabilities	1,584	1,448
Financial collateral, assets	21,310	25,014
Financial collateral, liabilities	16,772	18,823
Derivatives Held for Trading
Disclosure of detailed information about hedges [line items]
Notional amount	56,251	63,187
Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Notional amount	140,964	141,985
Fair value hedges | Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Notional amount	94,537	96,032
Cash flow hedges | Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Notional amount	46,427	45,953
Exchange Rate Contracts
Disclosure of detailed information about hedges [line items]
Notional amount	42,760	44,133
Exchange Rate Contracts | Derivatives Held for Trading
Disclosure of detailed information about hedges [line items]
Notional amount	14,951	14,149
Exchange Rate Contracts | Fair value hedges | Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Notional amount	789	1,482
Exchange Rate Contracts | Cash flow hedges | Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Notional amount	27,020	28,502
Interest Rate Contracts
Disclosure of detailed information about hedges [line items]
Notional amount	153,315	158,565
Interest Rate Contracts | Derivatives Held for Trading
Disclosure of detailed information about hedges [line items]
Notional amount	40,160	46,564
Interest Rate Contracts | Fair value hedges | Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Notional amount	93,748	94,550
Interest Rate Contracts | Cash flow hedges | Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Notional amount	19,407	17,451
Equity derivative contracts | Fair value hedges | Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Notional amount	0	0
Equity and Credit Contracts
Disclosure of detailed information about hedges [line items]
Notional amount	1,140	2,474
Equity and Credit Contracts | Derivatives Held for Trading
Disclosure of detailed information about hedges [line items]
Notional amount	1,140	2,474
Equity and Credit Contracts | Cash flow hedges | Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Notional amount	0	0
Derivative Assets
Disclosure of detailed information about hedges [line items]
Financial collateral, assets	330	222
Derivative Liabilities
Disclosure of detailed information about hedges [line items]
Financial collateral, liabilities	651	629
Fair Value
Disclosure of detailed information about hedges [line items]
Fair value assets	3,406	3,316
Fair value liabilities	1,584	1,448
Fair Value | Derivatives Held for Trading
Disclosure of detailed information about hedges [line items]
Fair value assets	1,406	1,135
Fair value liabilities	1,015	675
Fair Value | Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Fair value assets	3,754	3,395
Fair value liabilities	2,323	1,987
Fair Value | Fair value hedges | Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Fair value assets	1,309	1,188
Fair value liabilities	1,891	1,490
Fair Value | Cash flow hedges | Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Fair value assets	2,445	2,207
Fair value liabilities	432	497
Fair Value | Exchange Rate Contracts | Derivatives Held for Trading
Disclosure of detailed information about hedges [line items]
Fair value assets	395	134
Fair value liabilities	418	200
Fair Value | Exchange Rate Contracts | Fair value hedges | Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Fair value assets	84	166
Fair value liabilities	6	2
Fair Value | Exchange Rate Contracts | Cash flow hedges | Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Fair value assets	1,978	2,023
Fair value liabilities	409	462
Fair Value | Interest Rate Contracts | Derivatives Held for Trading
Disclosure of detailed information about hedges [line items]
Fair value assets	888	718
Fair value liabilities	542	315
Fair Value | Interest Rate Contracts | Fair value hedges | Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Fair value assets	1,225	1,022
Fair value liabilities	1,885	1,488
Fair Value | Interest Rate Contracts | Cash flow hedges | Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Fair value assets	467	184
Fair value liabilities	23	35
Fair Value | Equity derivative contracts | Fair value hedges | Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Fair value assets	0	0
Fair value liabilities	0	0
Fair Value | Equity and Credit Contracts | Derivatives Held for Trading
Disclosure of detailed information about hedges [line items]
Fair value assets	123	283
Fair value liabilities	55	160
Fair Value | Equity and Credit Contracts | Cash flow hedges | Derivatives Held for Hedging
Disclosure of detailed information about hedges [line items]
Fair value assets	0	0
Fair value liabilities	0	0
Fair Value | Netting adjustments
Disclosure of detailed information about hedges [line items]
Fair value assets	(1,754)	(1,214)
Fair value liabilities	£ (1,754)	£ (1,214)

Derivative Financial Instrume_4

Derivative Financial Instruments - Analysis of the Notional and Fair Values of Derivatives by Trading and Settlement Method (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional amount	£ 197,215	£ 205,172
Exchange Rate Contracts
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional amount	42,760	44,133
Interest Rate Contracts
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional amount	153,315	158,565
Equity and Credit Contracts
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional amount	1,140	2,474
Traded on Recognised Exchanges
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional amount	0	0
Assets traded over the counter	0	0
Liabilities traded over the counter	0	0
Traded on Recognised Exchanges | Exchange Rate Contracts
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional amount	0	0
Assets traded over the counter	0	0
Liabilities traded over the counter	0	0
Traded on Recognised Exchanges | Interest Rate Contracts
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional amount	0	0
Assets traded over the counter	0	0
Liabilities traded over the counter	0	0
Traded on Recognised Exchanges | Equity and Credit Contracts
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional amount	0	0
Assets traded over the counter	0	0
Liabilities traded over the counter	0	0
Settled by Central Counterparties
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional amount	144,343	131,538
Settled by Central Counterparties | Exchange Rate Contracts
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional amount	0	0
Settled by Central Counterparties | Interest Rate Contracts
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional amount	144,343	131,538
Settled by Central Counterparties | Equity and Credit Contracts
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional amount	0	0
Not Settled by Central Counterparties
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional amount	52,872	73,634
Not Settled by Central Counterparties | Exchange Rate Contracts
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional amount	42,760	44,133
Not Settled by Central Counterparties | Interest Rate Contracts
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional amount	8,972	27,027
Not Settled by Central Counterparties | Equity and Credit Contracts
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional amount	1,140	2,474
Traded Over the Counter
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Assets traded over the counter	3,406	3,316
Liabilities traded over the counter	1,584	1,448
Traded Over the Counter | Exchange Rate Contracts
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Assets traded over the counter	2,457	2,324
Liabilities traded over the counter	833	663
Traded Over the Counter | Interest Rate Contracts
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Assets traded over the counter	826	709
Liabilities traded over the counter	696	625
Traded Over the Counter | Equity and Credit Contracts
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Assets traded over the counter	123	283
Liabilities traded over the counter	£ 55	£ 160

Derivative Financial Instrume_5

Derivative Financial Instruments - Summary of Nominal Value in Hedging Instruments (Detail) number in Millions	Dec. 31, 2020
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	54,395
GBP LIBOR
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	47,421
USD LIBOR
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	6,196
Other
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	778
Maturing after transition date
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	26,474
Maturing after transition date | GBP LIBOR
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	23,030
Maturing after transition date | USD LIBOR
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	2,724
Maturing after transition date | Other
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	720
Fair value hedges
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	34,078
Fair value hedges | GBP LIBOR
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	32,223
Fair value hedges | USD LIBOR
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	1,077
Fair value hedges | Other
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	778
Fair value hedges | Maturing after transition date
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	13,359
Fair value hedges | Maturing after transition date | GBP LIBOR
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	12,477
Fair value hedges | Maturing after transition date | USD LIBOR
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	162
Fair value hedges | Maturing after transition date | Other
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	720
Cash flow hedges
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	20,317
Cash flow hedges | GBP LIBOR
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	15,198
Cash flow hedges | USD LIBOR
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	5,119
Cash flow hedges | Other
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	0
Cash flow hedges | Maturing after transition date
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	13,115
Cash flow hedges | Maturing after transition date | GBP LIBOR
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	10,553
Cash flow hedges | Maturing after transition date | USD LIBOR
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	2,562
Cash flow hedges | Maturing after transition date | Other
Disclosure of detailed information about hedged items [line items]
Notional value of hedging instruments	0

Derivative Financial Instrume_6

Derivative Financial Instruments - Summary of Maturity Profile and Average Price/Rate of Hedging Instruments Used in Hedging Strategies (Detail) £ in Millions	Dec. 31, 2020GBP (£)	Dec. 31, 2019GBP (£)
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 197,215	£ 205,172
Interest Rate Contracts
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	153,315	158,565
Interest Rate Contracts | Interest Rate Risks | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	93,475	94,314
Interest Rate Contracts | Interest Rate Risks | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	12,450	6,576
Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	273	236
Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	6,591	10,120
Interest Rate Contracts | Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	366	755
Exchange Rate Contracts
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	42,760	44,133
Exchange Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	789	1,482
Exchange Rate Contracts | Interest Rate And Foreign Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	11,457	16,221
Exchange Rate Contracts | Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	15,563	12,281
Less Than One Month | Interest Rate Contracts | Interest Rate Risks | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 2,429	£ 4,354
Less Than One Month | Interest Rate Contracts | Interest Rate Risks | Fair value hedges | United Kingdom, Pounds
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0069	0.0077
Less Than One Month | Interest Rate Contracts | Interest Rate Risks | Fair value hedges | Euro Member Countries, Euro
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0118	(0.0041)
Less Than One Month | Interest Rate Contracts | Interest Rate Risks | Fair value hedges | United States of America, Dollars
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0187	0
Less Than One Month | Interest Rate Contracts | Interest Rate Risks | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 0	£ 0
Less Than One Month | Interest Rate Contracts | Interest Rate Risks | Cash flow hedges | United Kingdom, Pounds
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0	0
Less Than One Month | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 0	£ 0
Average GBP - EUR exchange rate	0
Average GBP - USD exchange rate	0
Less Than One Month | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges | Euro Member Countries, Euro
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0	0
Less Than One Month | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges | United States of America, Dollars
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0	0
Less Than One Month | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 732	£ 0
Average GBP - EUR exchange rate	0	1.2742
Average GBP - USD exchange rate	1.4649	0
Less Than One Month | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Cash flow hedges | United Kingdom, Pounds
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0201	0.0249
Less Than One Month | Interest Rate Contracts | Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 0	£ 0
Average GBP - EUR exchange rate	0
Average GBP - USD exchange rate	1.2931
Average GBP - JPY exchange rate	0
Less Than One Month | Exchange Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 0	£ 0
Average GBP - EUR exchange rate		0
Average GBP - USD exchange rate		0
Less Than One Month | Exchange Rate Contracts | Interest Rate And Foreign Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	732	£ 812
Less Than One Month | Exchange Rate Contracts | Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	1,439	£ 1,187
Average GBP - EUR exchange rate		0
Average GBP - USD exchange rate		1.2856
Average GBP - JPY exchange rate		0
>1 and ≤ 3 months | Interest Rate Contracts | Interest Rate Risks | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 7,617	£ 5,804
>1 and ≤ 3 months | Interest Rate Contracts | Interest Rate Risks | Fair value hedges | United Kingdom, Pounds
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0065	0.0090
>1 and ≤ 3 months | Interest Rate Contracts | Interest Rate Risks | Fair value hedges | Euro Member Countries, Euro
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0023	0.0029
>1 and ≤ 3 months | Interest Rate Contracts | Interest Rate Risks | Fair value hedges | United States of America, Dollars
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0172	0.0154
>1 and ≤ 3 months | Interest Rate Contracts | Interest Rate Risks | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 897	£ 339
>1 and ≤ 3 months | Interest Rate Contracts | Interest Rate Risks | Cash flow hedges | United Kingdom, Pounds
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0046	0.0076
>1 and ≤ 3 months | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 0	£ 0
Average GBP - EUR exchange rate	0
Average GBP - USD exchange rate	0
>1 and ≤ 3 months | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges | Euro Member Countries, Euro
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0	0
>1 and ≤ 3 months | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges | United States of America, Dollars
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0	0.0238
>1 and ≤ 3 months | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 0	£ 0
Average GBP - EUR exchange rate	0	0
Average GBP - USD exchange rate	0	0
>1 and ≤ 3 months | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Cash flow hedges | United Kingdom, Pounds
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0	0
>1 and ≤ 3 months | Interest Rate Contracts | Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 0	£ 0
Average GBP - EUR exchange rate	0
Average GBP - USD exchange rate	1.3164
Average GBP - JPY exchange rate	137.9769
>1 and ≤ 3 months | Exchange Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 0	£ 755
Average GBP - EUR exchange rate		0
Average GBP - USD exchange rate		1.5110
>1 and ≤ 3 months | Exchange Rate Contracts | Interest Rate And Foreign Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	0	£ 0
>1 and ≤ 3 months | Exchange Rate Contracts | Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	2,015	£ 2,119
Average GBP - EUR exchange rate		1.1444
Average GBP - USD exchange rate		1.2624
Average GBP - JPY exchange rate		145.9275
Later Than Three Months and Not Later Than One Year | Interest Rate Contracts | Interest Rate Risks | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 27,791	£ 27,405
Later Than Three Months and Not Later Than One Year | Interest Rate Contracts | Interest Rate Risks | Fair value hedges | United Kingdom, Pounds
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0082	0.0088
Later Than Three Months and Not Later Than One Year | Interest Rate Contracts | Interest Rate Risks | Fair value hedges | Euro Member Countries, Euro
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0302	0.0221
Later Than Three Months and Not Later Than One Year | Interest Rate Contracts | Interest Rate Risks | Fair value hedges | United States of America, Dollars
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0289	0.0199
Later Than Three Months and Not Later Than One Year | Interest Rate Contracts | Interest Rate Risks | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 2,528	£ 1,066
Later Than Three Months and Not Later Than One Year | Interest Rate Contracts | Interest Rate Risks | Cash flow hedges | United Kingdom, Pounds
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0057	0.0082
Later Than Three Months and Not Later Than One Year | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 0	£ 0
Average GBP - EUR exchange rate	1.1405
Average GBP - USD exchange rate	0
Later Than Three Months and Not Later Than One Year | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges | Euro Member Countries, Euro
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0464	0
Later Than Three Months and Not Later Than One Year | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges | United States of America, Dollars
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0	0
Later Than Three Months and Not Later Than One Year | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 882	£ 3,121
Average GBP - EUR exchange rate	1.3543	1.1689
Average GBP - USD exchange rate	0	1.5357
Later Than Three Months and Not Later Than One Year | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Cash flow hedges | United Kingdom, Pounds
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0318	0.0216
Later Than Three Months and Not Later Than One Year | Interest Rate Contracts | Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 0	£ 0
Average GBP - EUR exchange rate	0
Average GBP - USD exchange rate	1.3231
Average GBP - JPY exchange rate	135.6073
Later Than Three Months and Not Later Than One Year | Exchange Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 132	£ 0
Average GBP - EUR exchange rate		0
Average GBP - USD exchange rate		0
Later Than Three Months and Not Later Than One Year | Exchange Rate Contracts | Interest Rate And Foreign Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	2,583	£ 3,367
Later Than Three Months and Not Later Than One Year | Exchange Rate Contracts | Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	3,877	£ 3,758
Average GBP - EUR exchange rate		1.1167
Average GBP - USD exchange rate		1.2925
Average GBP - JPY exchange rate		143.0857
Later Than One Year and Not Later Than Five Years | Interest Rate Contracts | Interest Rate Risks | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 47,749	£ 43,652
Later Than One Year and Not Later Than Five Years | Interest Rate Contracts | Interest Rate Risks | Fair value hedges | United Kingdom, Pounds
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0073	0.0133
Later Than One Year and Not Later Than Five Years | Interest Rate Contracts | Interest Rate Risks | Fair value hedges | Euro Member Countries, Euro
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0098	0.0136
Later Than One Year and Not Later Than Five Years | Interest Rate Contracts | Interest Rate Risks | Fair value hedges | United States of America, Dollars
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0249	0.0269
Later Than One Year and Not Later Than Five Years | Interest Rate Contracts | Interest Rate Risks | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 7,964	£ 4,671
Later Than One Year and Not Later Than Five Years | Interest Rate Contracts | Interest Rate Risks | Cash flow hedges | United Kingdom, Pounds
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0145	0.0146
Later Than One Year and Not Later Than Five Years | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 236	£ 18
Average GBP - EUR exchange rate	1.1700
Average GBP - USD exchange rate	0
Later Than One Year and Not Later Than Five Years | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges | Euro Member Countries, Euro
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0178	0.0352
Later Than One Year and Not Later Than Five Years | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges | United States of America, Dollars
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0	0
Later Than One Year and Not Later Than Five Years | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 4,062	£ 4,829
Average GBP - EUR exchange rate	1.2525	1.3114
Average GBP - USD exchange rate	1.6085	1.5811
Later Than One Year and Not Later Than Five Years | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Cash flow hedges | United Kingdom, Pounds
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0248	0.0287
Later Than One Year and Not Later Than Five Years | Interest Rate Contracts | Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 366	£ 755
Average GBP - EUR exchange rate	1.1629
Average GBP - USD exchange rate	1.3039
Average GBP - JPY exchange rate	132.2714
Later Than One Year and Not Later Than Five Years | Exchange Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 461	£ 317
Average GBP - EUR exchange rate		1.1781
Average GBP - USD exchange rate		0
Later Than One Year and Not Later Than Five Years | Exchange Rate Contracts | Interest Rate And Foreign Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	6,550	£ 8,009
Later Than One Year and Not Later Than Five Years | Exchange Rate Contracts | Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	7,113	£ 5,217
Average GBP - EUR exchange rate		1.1526
Average GBP - USD exchange rate		1.2991
Average GBP - JPY exchange rate		140.8152
>5 years | Interest Rate Contracts | Interest Rate Risks | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 7,889	£ 13,099
>5 years | Interest Rate Contracts | Interest Rate Risks | Fair value hedges | United Kingdom, Pounds
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0361	0.0300
>5 years | Interest Rate Contracts | Interest Rate Risks | Fair value hedges | Euro Member Countries, Euro
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0234	0.0236
>5 years | Interest Rate Contracts | Interest Rate Risks | Fair value hedges | United States of America, Dollars
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0416	0.0456
>5 years | Interest Rate Contracts | Interest Rate Risks | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 1,061	£ 500
>5 years | Interest Rate Contracts | Interest Rate Risks | Cash flow hedges | United Kingdom, Pounds
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0133	0.0040
>5 years | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 37	£ 218
Average GBP - EUR exchange rate	1.1669
Average GBP - USD exchange rate	0
>5 years | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges | Euro Member Countries, Euro
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0356	0.0212
>5 years | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges | United States of America, Dollars
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0	0
>5 years | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 915	£ 2,170
Average GBP - EUR exchange rate	1.1965	1.2090
Average GBP - USD exchange rate	1.3811	1.4499
>5 years | Interest Rate Contracts | Interest Rate And Foreign Currency Risk | Cash flow hedges | United Kingdom, Pounds
Disclosure of nature and extent of risks arising from financial instruments [line items]
Average fixed interest rate	0.0339	0.0296
>5 years | Interest Rate Contracts | Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 0	£ 0
Average GBP - EUR exchange rate	1.1787
Average GBP - USD exchange rate	0
Average GBP - JPY exchange rate	0
>5 years | Exchange Rate Contracts | Interest Rate And Foreign Currency Risk | Fair value hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 196	£ 410
Average GBP - EUR exchange rate		1.1603
Average GBP - USD exchange rate		0
>5 years | Exchange Rate Contracts | Interest Rate And Foreign Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	1,592	£ 4,033
>5 years | Exchange Rate Contracts | Currency Risk | Cash flow hedges
Disclosure of nature and extent of risks arising from financial instruments [line items]
Notional amount	£ 1,119	£ 0
Average GBP - EUR exchange rate		0
Average GBP - USD exchange rate		0
Average GBP - JPY exchange rate		0

Derivative Financial Instrume_7

Derivative Financial Instruments - Summary of Net Gains Or Losses Arising from Fair Value and Cash Flow Hedges Included in Net Trading and Other Income (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of detailed information about hedges [line items]
Hedge ineffectiveness	£ 20	£ 8	£ 34
Fair Value and Cash Flow Hedges
Disclosure of detailed information about hedges [line items]
(Losses)/gains on hedging instruments	(299)	(360)	4
Gains/(losses) on hedged items attributable to hedged risks	365	414	75
Fair value hedging ineffectiveness	66	54	79
Cash flow hedging ineffectiveness	(46)	(46)	(45)
Hedge ineffectiveness	£ 20	£ 8	£ 34

Derivative Financial Instrume_8

Derivative Financial Instruments - Summary of Information About Hedging Ineffectiveness by Risk Category (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair value hedges
Disclosure of detailed information about hedges [line items]
Changes in FV of hedging instruments to calculate hedge ineffectiveness	£ (299)	£ (360)
Change in value used for calculating hedge ineffectiveness	365	414
Hedge ineffectiveness recognised in income statement	66	54
Fair value hedges | Interest Rate Risks
Disclosure of detailed information about hedges [line items]
Changes in FV of hedging instruments to calculate hedge ineffectiveness	(358)	(264)
Change in value used for calculating hedge ineffectiveness	385	284
Hedge ineffectiveness recognised in income statement	27	20
Fair value hedges | Interest Rate And Foreign Currency Risk
Disclosure of detailed information about hedges [line items]
Changes in FV of hedging instruments to calculate hedge ineffectiveness	59	(96)
Change in value used for calculating hedge ineffectiveness	(20)	130
Hedge ineffectiveness recognised in income statement	39	34
Cash flow hedges
Disclosure of detailed information about hedges [line items]
Changes in FV of hedging instruments to calculate hedge ineffectiveness	925	(903)
Change in value used for calculating hedge ineffectiveness	(972)	857
Hedge ineffectiveness recognised in income statement	(46)	(46)
Changes in value of hedging instrument recognised in OCI	(971)	857
Amount reclassified from cash flow hedging reserve to income statement	£ 808	(1,012)
Cash flow hedges | Interest Rate Risks
Disclosure of detailed information about hedges [line items]
Income statement line item affected by the reclassification	Net interest income
Changes in FV of hedging instruments to calculate hedge ineffectiveness	£ 185	34
Hedge ineffectiveness recognised in income statement	6	1
Changes in value of hedging instrument recognised in OCI	(179)	(33)
Amount reclassified from cash flow hedging reserve to income statement	£ 33	13
Cash flow hedges | Interest Rate And Foreign Currency Risk
Disclosure of detailed information about hedges [line items]
Income statement line item affected by the reclassification	Net interest income/other operating income
Changes in FV of hedging instruments to calculate hedge ineffectiveness	£ 782	(604)
Hedge ineffectiveness recognised in income statement	(48)	(43)
Changes in value of hedging instrument recognised in OCI	(830)	561
Amount reclassified from cash flow hedging reserve to income statement	£ 773	(709)
Cash flow hedges | Currency Risk
Disclosure of detailed information about hedges [line items]
Income statement line item affected by the reclassification	Net interest income/other operating income
Changes in FV of hedging instruments to calculate hedge ineffectiveness	£ (42)	(333)
Hedge ineffectiveness recognised in income statement	(4)	(4)
Changes in value of hedging instrument recognised in OCI	38	329
Amount reclassified from cash flow hedging reserve to income statement	£ 2	(316)
Cash flow hedges | Equity Risks
Disclosure of detailed information about hedges [line items]
Income statement line item affected by the reclassification	Operating expenses
Changes in FV of hedging instruments to calculate hedge ineffectiveness	£ 0	0
Hedge ineffectiveness recognised in income statement	0	0
Changes in value of hedging instrument recognised in OCI	0	0
Amount reclassified from cash flow hedging reserve to income statement	£ 0	£ 0

Derivative Financial Instrume_9

Derivative Financial Instruments - Additional Information (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of nature and extent of risks arising from financial instruments [abstract]
Ceased cash flow hedge accounting	£ 4	£ 4

Derivative Financial Instrum_10

Derivative Financial Instruments - Summary of Reconciliation by Risk Category of Components of Equity and Analysis of Other Comprehensive Income Items Resulting from Hedge Accounting (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of detailed information about hedges [line items]
Beginning Balance	£ 481	£ 326
Effective portion of changes in fair value	971	(857)
Income statement transfers	(808)	1,012
Ending Balance	644	481
Interest Rate Risks
Disclosure of detailed information about hedges [line items]
Effective portion of changes in fair value	179	33
Income statement transfers	(33)	(13)
Foreign Currency Risk
Disclosure of detailed information about hedges [line items]
Effective portion of changes in fair value	(38)	(329)
Income statement transfers	(2)	316
Equity Risks
Disclosure of detailed information about hedges [line items]
Effective portion of changes in fair value	0	0
Income statement transfers	0	0
Interest Rate And Foreign Currency Risk
Disclosure of detailed information about hedges [line items]
Effective portion of changes in fair value	830	(561)
Income statement transfers	£ (773)	£ 709

Derivative Financial Instrum_11

Derivative Financial Instruments - Summary of Details of Hedged Exposures by Hedging Strategies (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of detailed information about hedges [line items]
Cash flow hedge reserve	£ 644	£ 481	£ 326
Fair value hedges
Disclosure of detailed information about hedges [line items]
Carrying value of assets	58,019	52,375
Accumulated amount of FV hedge adjustments on hedged item in carrying value of hedged item liabilities	(649)	(608)
Accumulated amount of FV hedge adjustments for portfolio hedge of interest rate risks assets	1,038	800
Accumulated amount of FV hedge adjustments on balance sheet for discontinued hedges	424	186
Change in value used for calculating hedge ineffectiveness	365	414
Fair value hedges | Interest Rate Risks
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	385	284
Fair value hedges | Interest Rate Risks | Deposits by customers
Disclosure of detailed information about hedges [line items]
Carrying value of liabilities	(7,309)	(9,944)
Accumulated amount of FV hedge adjustments on hedged item in carrying value of hedged item liabilities	(158)	(85)
Accumulated amount of FV hedge adjustments for portfolio hedge of interest rate risks liabilities	(10)	4
Accumulated amount of FV hedge adjustments on balance sheet for discontinued hedges	(77)	(11)
Change in value used for calculating hedge ineffectiveness	(73)	(110)
Fair value hedges | Interest Rate Risks | Deposits by banks
Disclosure of detailed information about hedges [line items]
Carrying value of liabilities	0	(517)
Accumulated amount of FV hedge adjustments on hedged item in carrying value of hedged item liabilities	0	(16)
Accumulated amount of FV hedge adjustments for portfolio hedge of interest rate risks liabilities	0	0
Accumulated amount of FV hedge adjustments on balance sheet for discontinued hedges	0	(17)
Change in value used for calculating hedge ineffectiveness	0	(1)
Fair value hedges | Interest Rate Risks | Debt Securities
Disclosure of detailed information about hedges [line items]
Carrying value of liabilities	(5,885)	(8,099)
Accumulated amount of FV hedge adjustments on hedged item in carrying value of hedged item liabilities	(375)	(303)
Accumulated amount of FV hedge adjustments for portfolio hedge of interest rate risks liabilities	(137)	(166)
Accumulated amount of FV hedge adjustments on balance sheet for discontinued hedges	(239)	(300)
Change in value used for calculating hedge ineffectiveness	(61)	(50)
Fair value hedges | Interest Rate Risks | Subordinated liabilities
Disclosure of detailed information about hedges [line items]
Carrying value of liabilities	(636)	(707)
Accumulated amount of FV hedge adjustments on hedged item in carrying value of hedged item liabilities	(185)	(181)
Accumulated amount of FV hedge adjustments for portfolio hedge of interest rate risks liabilities	(41)	(48)
Accumulated amount of FV hedge adjustments on balance sheet for discontinued hedges	(166)	(204)
Change in value used for calculating hedge ineffectiveness	(27)	(19)
Fair value hedges | Interest Rate And Foreign Currency Risk
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	(20)	130
Fair value hedges | Interest Rate And Foreign Currency Risk | Debt Securities
Disclosure of detailed information about hedges [line items]
Carrying value of liabilities	(621)	(1,396)
Accumulated amount of FV hedge adjustments on hedged item in carrying value of hedged item liabilities	(94)	(135)
Accumulated amount of FV hedge adjustments for portfolio hedge of interest rate risks liabilities	0	0
Accumulated amount of FV hedge adjustments on balance sheet for discontinued hedges	(76)	(122)
Change in value used for calculating hedge ineffectiveness	(34)	136
Fair value hedges | Interest Rate And Foreign Currency Risk | Subordinated liabilities
Disclosure of detailed information about hedges [line items]
Carrying value of liabilities	(3)	(7)
Accumulated amount of FV hedge adjustments on hedged item in carrying value of hedged item liabilities	3	7
Accumulated amount of FV hedge adjustments for portfolio hedge of interest rate risks liabilities	0	0
Accumulated amount of FV hedge adjustments on balance sheet for discontinued hedges	3	7
Change in value used for calculating hedge ineffectiveness	(1)	(2)
Cash flow hedges
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	(972)	857
Cash flow hedge reserve	644	481
Balances on cash flow hedge reserve where hedge accounting is no longer applied	(1)	8
Cash flow hedges | Interest Rate Risks | Deposits by banks
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	7	3
Cash flow hedge reserve	(2)	(2)
Balances on cash flow hedge reserve where hedge accounting is no longer applied	0	0
Cash flow hedges | Interest Rate Risks | Debt Securities
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	0	0
Cash flow hedge reserve	0	0
Balances on cash flow hedge reserve where hedge accounting is no longer applied	0	0
Cash flow hedges | Interest Rate Risks | Repurchase agreements - non trading
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	1	0
Cash flow hedge reserve	(1)	0
Balances on cash flow hedge reserve where hedge accounting is no longer applied	0	0
Cash flow hedges | Currency Risk | Deposits by customers
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	(5)	3
Cash flow hedge reserve	14	0
Balances on cash flow hedge reserve where hedge accounting is no longer applied	0	0
Cash flow hedges | Currency Risk | Deposits by banks
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	0	4
Cash flow hedge reserve	0	0
Balances on cash flow hedge reserve where hedge accounting is no longer applied	0	0
Cash flow hedges | Currency Risk | Debt Securities
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	(15)	177
Cash flow hedge reserve	(60)	(3)
Balances on cash flow hedge reserve where hedge accounting is no longer applied	0	0
Cash flow hedges | Currency Risk | Repurchase agreements - non trading
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	(15)	0
Cash flow hedge reserve	0	0
Balances on cash flow hedge reserve where hedge accounting is no longer applied	0	0
Cash flow hedges | Equity Risks | Other Liabilities
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	0	0
Cash flow hedge reserve	0	0
Balances on cash flow hedge reserve where hedge accounting is no longer applied	0	(1)
Cash flow hedges | Interest Rate And Foreign Currency Risk | Deposits by customers
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	(132)	0
Cash flow hedge reserve	87	0
Balances on cash flow hedge reserve where hedge accounting is no longer applied	0	0
Loans and advances to customers | Fair value hedges | Interest Rate Risks
Disclosure of detailed information about hedges [line items]
Carrying value of assets	54,118	43,098
Accumulated amount of FV hedge adjustments on hedged item in carrying value of hedged item assets	0	0
Accumulated amount of FV hedge adjustments for portfolio hedge of interest rate risks assets	1,189	870
Accumulated amount of FV hedge adjustments on balance sheet for discontinued hedges	892	630
Change in value used for calculating hedge ineffectiveness	334	258
Loans and advances to customers | Cash flow hedges | Interest Rate Risks
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	(183)	(34)
Cash flow hedge reserve	165	21
Balances on cash flow hedge reserve where hedge accounting is no longer applied	1	(11)
Other financial assets at amortised cost | Fair value hedges | Interest Rate Risks
Disclosure of detailed information about hedges [line items]
Carrying value of assets	772	6,627
Accumulated amount of FV hedge adjustments on hedged item in carrying value of hedged item assets	0	0
Accumulated amount of FV hedge adjustments for portfolio hedge of interest rate risks assets	36	142
Accumulated amount of FV hedge adjustments on balance sheet for discontinued hedges	13	121
Change in value used for calculating hedge ineffectiveness	121	83
Reverse repo agreement non trading | Fair value hedges | Interest Rate Risks
Disclosure of detailed information about hedges [line items]
Carrying value of assets	12,149	17,121
Accumulated amount of FV hedge adjustments on hedged item in carrying value of hedged item assets	0	0
Accumulated amount of FV hedge adjustments for portfolio hedge of interest rate risks assets	1	(2)
Accumulated amount of FV hedge adjustments on balance sheet for discontinued hedges	0	0
Change in value used for calculating hedge ineffectiveness	3	(2)
Other financial assets at fair value through other comprehensive income | Fair value hedges | Interest Rate Risks
Disclosure of detailed information about hedges [line items]
Carrying value of assets	5,129	5,944
Accumulated amount of FV hedge adjustments on hedged item in carrying value of hedged item assets	155	102
Accumulated amount of FV hedge adjustments for portfolio hedge of interest rate risks assets	0	0
Accumulated amount of FV hedge adjustments on balance sheet for discontinued hedges	74	82
Change in value used for calculating hedge ineffectiveness	88	125
Other financial assets at fair value through other comprehensive income | Fair value hedges | Interest Rate And Foreign Currency Risk
Disclosure of detailed information about hedges [line items]
Carrying value of assets	299	241
Accumulated amount of FV hedge adjustments on hedged item in carrying value of hedged item assets	5	3
Accumulated amount of FV hedge adjustments for portfolio hedge of interest rate risks assets	0	0
Accumulated amount of FV hedge adjustments on balance sheet for discontinued hedges	0	0
Change in value used for calculating hedge ineffectiveness	15	(4)
Loans and advances to banks | Cash flow hedges | Interest Rate Risks
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	(2)	(2)
Cash flow hedge reserve	1	0
Balances on cash flow hedge reserve where hedge accounting is no longer applied	0	0
Reverse repurchase agreements - non-trading | Cash flow hedges | Interest Rate Risks
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	(2)	0
Cash flow hedge reserve	1	0
Balances on cash flow hedge reserve where hedge accounting is no longer applied	0	0
Other financial assets measured at fair value through other comprehensive income | Cash flow hedges | Currency Risk
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	40	(122)
Cash flow hedge reserve	6	3
Balances on cash flow hedge reserve where hedge accounting is no longer applied	0	0
Highly probable forecast transactions | Cash flow hedges | Currency Risk
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	33	267
Cash flow hedge reserve	3	2
Balances on cash flow hedge reserve where hedge accounting is no longer applied	0	0
Subordinated Liabilities and loans and advances to customers | Cash flow hedges | Interest Rate And Foreign Currency Risk | Subordinated Liabilities and loans and advances to customers
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	(130)	(69)
Cash flow hedge reserve	194	180
Balances on cash flow hedge reserve where hedge accounting is no longer applied	0	0
Debt securities in issue and loans and advances to customers | Cash flow hedges | Interest Rate And Foreign Currency Risk | Debt securities in issue and loans and advances to customers
Disclosure of detailed information about hedges [line items]
Change in value used for calculating hedge ineffectiveness	(569)	630
Cash flow hedge reserve	236	280
Balances on cash flow hedge reserve where hedge accounting is no longer applied	£ (2)	£ 20

Other Financial Assets at Fai_3

Other Financial Assets at Fair Value Through Profit or Loss - Summary of Other Financial Assets at Fair Value Through Profit or Loss (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of other financial assets at fair value through profit or loss [Line Items]
Financial assets at fair value through profit or loss	£ 208	£ 386
Loans and advances to customers
Disclosure of other financial assets at fair value through profit or loss [Line Items]
Financial assets at fair value through profit or loss	99	92
Debt securities
Disclosure of other financial assets at fair value through profit or loss [Line Items]
Financial assets at fair value through profit or loss	109	294
Equity Securities
Disclosure of other financial assets at fair value through profit or loss [Line Items]
Financial assets at fair value through profit or loss	0	0
Reverse repurchase agreements - non-trading
Disclosure of other financial assets at fair value through profit or loss [Line Items]
Financial assets at fair value through profit or loss	0	0
Loans to Housing Associations | Loans and advances to customers
Disclosure of other financial assets at fair value through profit or loss [Line Items]
Financial assets at fair value through profit or loss	13	12
Other Loans | Loans and advances to customers
Disclosure of other financial assets at fair value through profit or loss [Line Items]
Financial assets at fair value through profit or loss	£ 86	£ 80

Other Financial Assets at Fai_4

Other Financial Assets at Fair Value Through Profit or Loss - Additional Information (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of other financial assets at fair value through profit or loss [Line Items]
Financial assets designated at FVTPL	£ 13	£ 12
Financial assets mandatorily at FVTPL	195	374
Net (loss)/gain attributable to changes in credit risk for loans and advances at fair value through profit or loss	0	0	£ (1)
Cumulative net loss attributable to changes in credit risk for loans and advances at fair value through profit or loss	(2)	(2)
Debt securities
Disclosure of other financial assets at fair value through profit or loss [Line Items]
Senior credit linked notes presented on a net basis	2,100
Senior tranches of credit linked notes netted against senior cash deposits	£ 825	£ 1,500

Loans and Advances to Custome_3

Loans and Advances to Customers - Summary of Net Loans and Advances to Customers (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Net Loans and Advances to Customers [abstract]
Loans secured on residential properties	£ 166,714	£ 165,356
Corporate loans	24,474	27,043
Finance leases	6,554	6,264
Secured advances	0	0
Other unsecured loans	9,933	7,096
Amounts due from fellow Banco Santander subsidiaries and joint ventures	2,425	2,366
Amounts due from Santander UK Group Holdings plc	7	8
Amounts due from subsidiaries	0	0
Loans and advances to customers	210,107	208,133
Credit impairment loss allowances on loans and advances to customers	(1,303)	(785)
RV and voluntary termination provisions on finance leases	(54)	(61)
Net loans and advances to customers	£ 208,750	£ 207,287

Loans and Advances to Custome_4

Loans and Advances to Customers - Summary of Finance Lease and Hire Purchase Contract Receivables (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of finance lease and hire purchase contract receivables [line items]
Gross investment	£ 7,210	£ 6,992
Unearned finance income	(656)	(728)
Net investment	6,554	6,264
Not later than one year
Disclosure of finance lease and hire purchase contract receivables [line items]
Gross investment	3,468	3,060
Unearned finance income	(297)	(303)
Net investment	3,171	2,757
Later than one year and not later than two years
Disclosure of finance lease and hire purchase contract receivables [line items]
Gross investment	1,829	2,046
Unearned finance income	(173)	(207)
Net investment	1,656	1,839
Later than two years and not later than three years
Disclosure of finance lease and hire purchase contract receivables [line items]
Gross investment	1,099	1,157
Unearned finance income	(106)	(119)
Net investment	993	1,038
Later than three years and not later than four years
Disclosure of finance lease and hire purchase contract receivables [line items]
Gross investment	575	541
Unearned finance income	(55)	(54)
Net investment	520	487
Later than four years and not later than five years
Disclosure of finance lease and hire purchase contract receivables [line items]
Gross investment	231	41
Unearned finance income	(25)	(4)
Net investment	206	37
>5 years
Disclosure of finance lease and hire purchase contract receivables [line items]
Gross investment	8	147
Unearned finance income	0	(41)
Net investment	£ 8	£ 106

Loans and Advances To Custome_5

Loans and Advances To Customers - Additional Information (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Loans and Advances to Customers [abstract]
Unguaranteed residual value of leases	£ 3,552	£ 3,512
Contingent rent income	0	0	£ 0
Finance income on the net investment in finance leases	£ 308	£ 299	£ 346

Securitisations and Covered B_3

Securitisations and Covered Bonds - Analysis of Securitisations and Covered Bonds (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of securitisations and covered bonds [line items]
Gross assets	£ 33,432	£ 36,391
Notes and debentures issued	21,615	22,783
Assets charged as security for liabilities	4,549	4,786
Securitisations and covered bond	21,615	22,783
Securitisation Programmes
Disclosure of securitisations and covered bonds [line items]
Gross assets	9,762	13,068
Notes and debentures issued	2,330	3,779
Assets charged as security for liabilities	4,549	4,786
Securitisation Programmes | Master Trust Structures
Disclosure of securitisations and covered bonds [line items]
Gross assets	8,113	11,046
Notes and debentures issued	1,119	2,226
Assets charged as security for liabilities	4,091	4,221
Securitisation Programmes | Other Securitisation Structures
Disclosure of securitisations and covered bonds [line items]
Gross assets	1,649	2,022
Notes and debentures issued	1,211	1,553
Assets charged as security for liabilities	458	565
Holmes | Securitisation Programmes | Master Trust Structures
Disclosure of securitisations and covered bonds [line items]
Gross assets	3,073	4,262
Notes and debentures issued	829	1,931
Assets charged as security for liabilities	334	463
Fosse | Securitisation Programmes | Master Trust Structures
Disclosure of securitisations and covered bonds [line items]
Gross assets	2,258	3,708
Notes and debentures issued	290	295
Assets charged as security for liabilities	1,402	1,404
Langton | Securitisation Programmes | Master Trust Structures
Disclosure of securitisations and covered bonds [line items]
Gross assets	2,782	3,076
Notes and debentures issued	0	0
Assets charged as security for liabilities	2,355	2,354
Motor | Securitisation Programmes | Other Securitisation Structures
Disclosure of securitisations and covered bonds [line items]
Gross assets	189	490
Notes and debentures issued	104	324
Assets charged as security for liabilities	97	197
Auto ABS UK Loans | Securitisation Programmes | Other Securitisation Structures
Disclosure of securitisations and covered bonds [line items]
Gross assets	1,460	1,532
Notes and debentures issued	1,107	1,229
Assets charged as security for liabilities	361	368
Euro 35bn Global Covered Bond Programme | Covered Bond Programme
Disclosure of securitisations and covered bonds [line items]
Gross assets	23,670	23,323
Notes and debentures issued	19,285	19,004
Assets charged as security for liabilities	0	0
Notes in issue held by group	£ 0	£ 0

Securitisations and Covered B_4

Securitisations and Covered Bonds - Summary of Issuances and Redemptions of Securitisation and Covered Bond Programme (Detail) - GBP (£) £ in Billions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Internal
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	£ 0	£ 1.5
Internal and external redemptions	0.4	0.8
Internal | Master Trust Structures | Holmes
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0
Internal and external redemptions	0.3	0
Internal | Master Trust Structures | Fosse
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	1.4
Internal and external redemptions	0	0
Internal | Master Trust Structures | Langton
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0
Internal and external redemptions	0	0
Internal | Other Securitisation Structures | Motor
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0
Internal and external redemptions	0.1	0.2
Internal | Other Securitisation Structures | Auto ABS UK Loans
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0.1
Internal and external redemptions	0	0.1
Internal | Covered Bond Programme
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0
Internal and external redemptions	0	0.5
External
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	3.3	3.2
Internal and external redemptions	3.9	3.2
External | Master Trust Structures | Holmes
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0
Internal and external redemptions	0.9	1.1
External | Master Trust Structures | Fosse
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0.1
Internal and external redemptions	0	0
External | Master Trust Structures | Langton
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0
Internal and external redemptions	0	0
External | Other Securitisation Structures | Motor
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0
Internal and external redemptions	0.2	0.4
External | Other Securitisation Structures | Auto ABS UK Loans
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0.3	0.2
Internal and external redemptions	0.1	0.2
External | Covered Bond Programme
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	3	2.9
Internal and external redemptions	£ 2.7	£ 1.5

Securitisations and Covered B_5

Securitisations and Covered Bonds - Additional Information (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Holmes
Disclosure of gross asset securitised [line items]
Beneficial interest in residential mortgage loans	£ 1,000	£ 2,100
Cash deposits	186	283
Fosse
Disclosure of gross asset securitised [line items]
Beneficial interest in residential mortgage loans	1,700	1,700
Cash deposits	0	0
Langton
Disclosure of gross asset securitised [line items]
Beneficial interest in residential mortgage loans	£ 2,400	£ 2,400

Transfers of Financial Assets_3

Transfers of Financial Assets Not Qualifying for Derecognition - Carrying Amount of Financial Assets that Did Not Qualify for Derecognition and their Associated Financial Liabilities (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of continuing involvement in derecognised financial assets [line items]
Carrying amount of financial assets not derecognised in their entirety	£ 9,495	£ 17,779
Recognised liabilities representing continuing involvement in derecognised financial assets	(4,422)	(10,661)
Securities lending agreements
Disclosure of continuing involvement in derecognised financial assets [line items]
Carrying amount of financial assets not derecognised in their entirety	918	195
Recognised liabilities representing continuing involvement in derecognised financial assets	(752)	(143)
Securitisations
Disclosure of continuing involvement in derecognised financial assets [line items]
Carrying amount of financial assets not derecognised in their entirety	6,980	9,992
Recognised liabilities representing continuing involvement in derecognised financial assets	(2,330)	(3,779)
Sale and Repurchase Agreements
Disclosure of continuing involvement in derecognised financial assets [line items]
Carrying amount of financial assets not derecognised in their entirety	1,597	7,592
Recognised liabilities representing continuing involvement in derecognised financial assets	£ (1,340)	£ (6,739)

Reverse Repurchase Agreements_3

Reverse Repurchase Agreements - Non Trading - Summary of Reverse Repurchase Agreements - Non Trading (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of reserve repurchase agreements non-trading [line items]
Reverse repurchase agreements – non trading	£ 19,599	£ 23,636
Agreements with banks
Disclosure of reserve repurchase agreements non-trading [line items]
Reverse repurchase agreements – non trading	1,258	2,161
Agreements with customers
Disclosure of reserve repurchase agreements non-trading [line items]
Reverse repurchase agreements – non trading	£ 18,341	£ 21,475

Other Financial Assets At Amo_3

Other Financial Assets At Amortised Cost - Summary of Other Financial Assets at Amortised Cost (Detail) - Other Financial Assets - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Financial assets at amortised cost [line items]
Financial assets at amortised cost	£ 1,163	£ 7,056
Asset backed securities
Financial assets at amortised cost [line items]
Financial assets at amortised cost	491	532
Debt securities
Financial assets at amortised cost [line items]
Financial assets at amortised cost	£ 672	£ 6,524

Financial Assets At Fair Valu_3

Financial Assets At Fair Value Through Other Comprehensive Income - Summary of Financial Assets at Fair Value Through Other Comprehensive Income (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of financial assets at fair value through other comprehensive income [line items]
Financial assets at fair value through other comprehensive income	£ 8,950	£ 9,747
Debt securities
Disclosure of financial assets at fair value through other comprehensive income [line items]
Financial assets at fair value through other comprehensive income	8,929	9,691
Loans and advances to customers
Disclosure of financial assets at fair value through other comprehensive income [line items]
Financial assets at fair value through other comprehensive income	£ 21	£ 56

Interests in Other Entities - S

Interests in Other Entities - Schedule of Interests in Other Entities (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Investments accounted for using equity method [abstract]
Subsidiaries	£ 0	£ 0
Joint ventures	172	117
Total	£ 172	£ 117

Interests In Other Entities - A

Interests In Other Entities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of subsidiaries [line items]
Proportion of ownership interest in subsidiary	100.00%
Subsidiaries with material non-controlling interests
Disclosure of subsidiaries [line items]
Proportion of ownership interest in subsidiary	50.00%	50.00%

Interests in Other Entities -_2

Interests in Other Entities - Schedule of Subsidiaries With Significant Non-Controlling Interests (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of subsidiaries [line items]
Profit attributable to non-controlling interests	£ 19	£ 19	£ 22
Accumulated non-controlling interests of the subsidiary	162	160
Total assets	292,332	281,702	283,372
Total liabilities	276,396	265,685	267,463
Profit for the year	471	733	1,146
Total comprehensive income for the year	211	395	£ 1,549
Subsidiaries with material non-controlling interests
Disclosure of subsidiaries [line items]
Profit attributable to non-controlling interests	19	19
Accumulated non-controlling interests of the subsidiary	162	160
Dividends paid to non-controlling interests	15	12
Total assets	3,451	3,228
Total liabilities	3,127	2,905
Profit for the year	38	40
Total comprehensive income for the year	£ 33	£ 40

Interests In Other Entities - I

Interests In Other Entities - Interests in Unconsolidated Structured Entities - Additional Information (Detail) £ in Millions	12 Months Ended
	Dec. 31, 2020GBP (£)	Dec. 31, 2019GBP (£)
Disclosure of information about unconsolidated subsidiaries [line items]
Number of credit protection vehicles	3	4
Senior credit linked notes issued	£ 2,160	£ 3,766
Junior credit linked notes issued	£ 678	825
Santander (UK) Common Investment Fund
Disclosure of information about unconsolidated subsidiaries [line items]
Name of subsidiary	The Santander (UK) Common Investment Fund
Defined benefit assets and obligations	£ 13,553	£ 12,446

Interests In Other Entities -_3

Interests In Other Entities - Interests in Joint Ventures - Additional Information (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of joint ventures [line items]
Carrying value of interest in joint venture	£ 172	£ 117
Joint ventures
Disclosure of joint ventures [line items]
Profit after tax	20	30
Carrying value of interest in joint venture	172	117
Commitments and contingent liabilities	£ 0	£ 0

Intangible Assets - Schedule of

Intangible Assets - Schedule of Goodwill (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of goodwill [line items]
Carrying amount	£ 1,203	£ 1,203
Gross carrying amount
Disclosure of goodwill [line items]
Carrying amount	1,269	1,269
Accumulated impairment
Disclosure of goodwill [line items]
Carrying amount	£ (66)	£ (66)

Intangible Assets - Additional

Intangible Assets - Additional Information (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of detailed information about intangible assets [abstract]
Goodwill impairment recognised	£ 0	£ 0
Intangible assets under development	99	197
Internally generated computer software	£ 68	£ 123

Intangible Assets - Schedule _2

Intangible Assets - Schedule of Goodwill for Cash Generating Units (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of information for cash-generating units [line items]
Goodwill	£ 1,203	£ 1,203
Personal Financial Services
Disclosure of information for cash-generating units [line items]
Goodwill	£ 1,169	£ 1,169
Discount rate percentage	13.60%	9.80%
Growth rate percentage	1.60%	1.60%
Private Banking
Disclosure of information for cash-generating units [line items]
Goodwill	£ 30	£ 30
Discount rate percentage	8.90%	9.80%
Growth rate percentage	1.60%	1.60%
Other
Disclosure of information for cash-generating units [line items]
Goodwill	£ 4	£ 4
Discount rate percentage	13.60%	9.80%
Growth rate percentage	1.60%	1.60%

Intangible Assets - Reasonably

Intangible Assets - Reasonably Possible Changes in Key Assumptions (Details)	12 Months Ended
	Dec. 31, 2020
Disclosure of detailed information about intangible assets [abstract]
Value in use estimate, increase (decrease) in cash flows	(500.00%)
Increase (decrease) in discount rate applied to cash flow projections	(1.00%)

Intangible Assets - Sensitivity

Intangible Assets - Sensitivity of VIU Changes to Current Assumptions (Details) £ in Millions	12 Months Ended
	Dec. 31, 2020GBP (£)
Disclosure of Goodwill [table]
Carrying value	£ 6,758
Value in use	8,602
Headroom	£ 1,844
Increase in post tax discount rate	239.00%
Decrease in cash flows	22.00%

Intangible Assets - Schedule _3

Intangible Assets - Schedule of Other Intangibles (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of detailed information about intangible assets [line items]
Beginning balance	£ 563	£ 605
Additions	102	173
Disposals	0	14
Charge	(197)	(191)
Impairment	(25)	(10)
Ending balance	443	563
Gross carrying amount
Disclosure of detailed information about intangible assets [line items]
Beginning balance	1,249	1,090
Additions	102	173
Disposals	47	14
Charge	0	0
Impairment	0	0
Ending balance	1,304	1,249
Accumulated amortisation/ impairment
Disclosure of detailed information about intangible assets [line items]
Beginning balance	(686)	(485)
Additions	0	0
Disposals	(47)	0
Charge	(197)	(191)
Impairment	(25)	(10)
Ending balance	£ (861)	£ (686)

Property, Plant and Equipment_2

Property, Plant and Equipment (Details) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	£ 1,967
Property, plant and equipment at end of period	1,734	£ 1,967
Accelerated depreciation expense	9	2
Construction in progress	55	16
Property
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	816
Property, plant and equipment at end of period	783	816
Office fixtures and equipment
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	420
Property, plant and equipment at end of period	307	420
Computer software
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	5
Property, plant and equipment at end of period	2	5
Operating lease assets
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	574
Property, plant and equipment at end of period	542	574
Right-of-use assets
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	152
Property, plant and equipment at end of period	100	152
Gross carrying amount
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	4,095	3,940
Additions	299	331
Disposals	(373)	(176)
Property, plant and equipment at end of period	4,021	4,095
Gross carrying amount | Property
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	1,270	1,291
Additions	61	5
Disposals	(59)	(26)
Property, plant and equipment at end of period	1,272	1,270
Gross carrying amount | Office fixtures and equipment
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	1,436	1,398
Additions	43	70
Disposals	(104)	(32)
Property, plant and equipment at end of period	1,375	1,436
Gross carrying amount | Computer software
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	439	438
Additions	2	1
Disposals	(5)	0
Property, plant and equipment at end of period	436	439
Gross carrying amount | Operating lease assets
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	738	604
Additions	185	251
Disposals	(203)	(117)
Property, plant and equipment at end of period	720	738
Gross carrying amount | Right-of-use assets
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	212	209
Additions	8	4
Disposals	(2)	(1)
Property, plant and equipment at end of period	218	212
Accumulated depreciation
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	(2,128)	(1,900)
Additions	(340)	(341)
Disposals	181	113
Property, plant and equipment at end of period	(2,287)	(2,128)
Accumulated depreciation | Property
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	(454)	(429)
Additions	(79)	(39)
Disposals	44	14
Property, plant and equipment at end of period	(489)	(454)
Accumulated depreciation | Office fixtures and equipment
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	(1,016)	(903)
Additions	(111)	(161)
Disposals	59	48
Property, plant and equipment at end of period	(1,068)	(1,016)
Accumulated depreciation | Computer software
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	(434)	(434)
Additions	0	0
Disposals	0	0
Property, plant and equipment at end of period	(434)	(434)
Accumulated depreciation | Operating lease assets
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	(164)	(134)
Additions	(92)	(81)
Disposals	78	51
Property, plant and equipment at end of period	(178)	(164)
Accumulated depreciation | Right-of-use assets
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	(60)	0
Additions	(58)	(60)
Disposals	0	0
Property, plant and equipment at end of period	£ (118)	£ (60)

Other Financial Liabilities a_3

Other Financial Liabilities at Fair Value Through Profit or Loss - Summary of Other Financial Liabilities at Fair Value Through Profit or Loss (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of other financial liabilities at fair value through profit or loss [line items]
Financial liabilities at fair value	£ 1,434	£ 1,713
US$30bn Euro Medium Term Note Programme
Disclosure of other financial liabilities at fair value through profit or loss [line items]
Financial liabilities at fair value	102	159
Structured Notes Programmes
Disclosure of other financial liabilities at fair value through profit or loss [line items]
Financial liabilities at fair value	805	809
Eurobonds
Disclosure of other financial liabilities at fair value through profit or loss [line items]
Financial liabilities at fair value	150	137
Structured Deposits
Disclosure of other financial liabilities at fair value through profit or loss [line items]
Financial liabilities at fair value	375	435
Collateral and associated financial guarantees
Disclosure of other financial liabilities at fair value through profit or loss [line items]
Financial liabilities at fair value	2	173
Repurchase agreements - non trading
Disclosure of other financial liabilities at fair value through profit or loss [line items]
Financial liabilities at fair value	£ 0	£ 0

Other Financial Liabilities a_4

Other Financial Liabilities at Fair Value Through Profit or Loss - Additional Information (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of other financial liabilities at fair value through profit or loss [line items]
Net gain (loss) on financial liabilities designated at fair value, attributable to changes in credit risk	£ (3)	£ (77)	£ 84
Accumulated gain (loss) on financial liabilities designated at fair value, attributable to changes in credit risk	(3)	0
Difference between carrying amount of financial liability and amount contractually required to pay at maturity to holder of obligation	11	4
Collateral and associated financial guarantees
Disclosure of other financial liabilities at fair value through profit or loss [line items]
Cash deposits presented on a net basis		2,100
Cash deposits netted against senior credit linked notes	£ 825	£ 1,500

Deposits by Customers - Schedul

Deposits by Customers - Schedule of Deposits by Customers (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of deposits from customers [line items]
Deposits by customers	£ 195,135	£ 181,883
Demand and time deposits
Disclosure of deposits from customers [line items]
Deposits by customers	185,879	171,736
Amounts due to other Santander UK Group Holdings plc subsidiaries
Disclosure of deposits from customers [line items]
Deposits by customers	59	44
Amounts due to Santander UK Group Holdings plc
Disclosure of deposits from customers [line items]
Deposits by customers	7,883	8,869
Amounts Due to Fellow Banco Santander Subsidiaries and Joint Ventures
Disclosure of deposits from customers [line items]
Deposits by customers	£ 1,314	£ 1,234

Deposits by Customers - Sched_2

Deposits by Customers - Schedule of Deposits by Customers (Parenthetical) (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of deposits from customers [line items]
Deposits by customers	£ 195,135	£ 181,883
Equity Index Linked Deposits
Disclosure of deposits from customers [line items]
Deposits by customers	577	1,139
Capital amount guaranteed or protected	577	1,139
Capital amount of return guaranteed	£ 2	£ 18

Deposits by Banks - Schedule of

Deposits by Banks - Schedule of Deposits by Banks (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of deposits from banks [line items]
Deposits by banks	£ 20,958	£ 14,353
Items in Course of Transmission
Disclosure of deposits from banks [line items]
Deposits by banks	375	337
Deposits Held as Collateral
Disclosure of deposits from banks [line items]
Deposits by banks	2,063	2,169
Other Deposits
Disclosure of deposits from banks [line items]
Deposits by banks	18,519	11,847
Amounts due to Santander UK subsidiaries
Disclosure of deposits from banks [line items]
Deposits by banks	£ 1	£ 0

Deposits by Banks - Schedule _2

Deposits by Banks - Schedule of Deposits by Banks (Parenthetical) (Detail) - GBP (£) £ in Billions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of deposits from banks [abstract]
Drawdown from TFS	£ 6.3	£ 10.8
Drawdown from TFSME	£ 11.7	£ 0

Repurchase Agreements - Non T_3

Repurchase Agreements - Non Trading - Summary of Repurchase Agreements Non Trading (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure Of Repurchase Agreements [line items]
Repurchase agreements - non trading	£ 15,848	£ 18,286
Agreements with banks
Disclosure Of Repurchase Agreements [line items]
Repurchase agreements - non trading	6,358	10,227
Agreements with customers
Disclosure Of Repurchase Agreements [line items]
Repurchase agreements - non trading	£ 9,490	£ 8,059

Debt Securities In Issue - Summ

Debt Securities In Issue - Summary of Debt Securities in Issue (Detail) £ in Millions	Dec. 31, 2020EUR (€)	Dec. 31, 2020USD ($)	Dec. 31, 2020GBP (£)	Dec. 31, 2019EUR (€)	Dec. 31, 2019USD ($)	Dec. 31, 2019GBP (£)
Debt securities in issue [line items]
Notional amount			£ 197,215			£ 205,172
Debt securities			35,566			41,129
Medium Term Notes
Debt securities in issue [line items]
Debt securities			8,212			12,466
Euro 35bn Global Covered Bond Programme
Debt securities in issue [line items]
Notional amount | €	€ 35,000,000,000			€ 35,000,000,000
Debt securities			19,285			19,004
US$20bn Commercial Paper Programmes
Debt securities in issue [line items]
Notional amount | $		$ 20,000,000,000			$ 20,000,000,000
Debt securities			2,824			3,014
Certificates of deposit
Debt securities in issue [line items]
Debt securities			2,858			2,806
Credit linked notes
Debt securities in issue [line items]
Credit linked notes			57			60
Securitisation Programmes
Debt securities in issue [line items]
Debt securities			2,330			3,779
US$30bn Euro Medium Term Note Programme | Medium Term Notes
Debt securities in issue [line items]
Notional amount | $		30,000,000,000			30,000,000,000
Debt securities			1,694			4,679
Euro 30bn Euro Medium Term Note Programme | Medium Term Notes
Debt securities in issue [line items]
Notional amount	30,000,000,000	30,000,000,000		€ 30,000,000,000	$ 30,000,000,000
Debt securities			388			1,896
US SEC-registered Debt Programme – Santander UK plc | Medium Term Notes
Debt securities in issue [line items]
Debt securities			4,723			5,891
Euro 750m Senior Unsecured Notes | Medium Term Notes
Debt securities in issue [line items]
Notional amount | €	€ 750,000,000
Debt securities			673			0
US$1bn Senior Unsecured Notes | Medium Term Notes
Debt securities in issue [line items]
Notional amount | $		$ 1,000,000,000
Debt securities			£ 734			£ 0

Subordinated Liabilities - Sche

Subordinated Liabilities - Schedule of Subordinated Liabilities (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of Subordinated Liabilities [Abstract]
£325m Sterling preference shares	£ 344	£ 344
Undated subordinated liabilities	557	581
Dated subordinated liabilities	1,655	2,603
Subordinated liabilities	2,556	3,528
Loss on repurchase of debt securities and subordinated liabilities	£ 24	£ (24)

Subordinated Liabilities - Addi

Subordinated Liabilities - Additional Information (Detail) - GBP (£)	Dec. 31, 2020	Dec. 31, 2019
Disclosure of subordinated liabilities [line items]
Defaults of principal, interest or other breaches with respect to subordinated liabilities	£ 0	£ 0
Repayment or purchase of subordinated liabilities before stated maturity allowed	£ 0	£ 0
Interest rate		5.875%
7.125% 30 Year Step-up Perpetual Callable Subordinated Notes
Disclosure of subordinated liabilities [line items]
Interest rate	7.125%	7.125%
7.125% 30 Year Step-up Perpetual Callable Subordinated Notes | Santander UK Plc
Disclosure of subordinated liabilities [line items]
Interest rate	7.125%
10.0625% Exchangeable Capital Securities
Disclosure of subordinated liabilities [line items]
Interest rate	10.0625%	10.0625%
10.0625% Exchangeable Capital Securities | Santander UK Plc
Disclosure of subordinated liabilities [line items]
Interest rate	10.0625%
7.375% 20 Year Step-up perpetual callable subordinated notes
Disclosure of subordinated liabilities [line items]
Interest rate	7.375%	7.375%
7.375% 20 Year Step-up perpetual callable subordinated notes | Santander UK Plc
Disclosure of subordinated liabilities [line items]
Interest rate	7.375%
10.375% exchangeable subordinated capital securities
Disclosure of subordinated liabilities [line items]
Interest rate	10.375%
10.375% non-cumulative non-redeemable sterling preference shares
Disclosure of subordinated liabilities [line items]
Par value per share	£ 1

Subordinated Liabilities - Summ

Subordinated Liabilities - Summary of Undated Subordinated Liabilities (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of subordinated liabilities [line items]
Interest rate		5.875%
Undated subordinated liabilities	£ 557	£ 581
10.0625% Exchangeable Capital Securities
Disclosure of subordinated liabilities [line items]
Interest rate	10.0625%	10.0625%
First call date	n/a
Undated subordinated liabilities	£ 205	£ 205
7.375% 20 Year Step-up perpetual callable subordinated notes
Disclosure of subordinated liabilities [line items]
Interest rate	7.375%	7.375%
Notes maturity, year	20 years	20 years
First call date	2020
Undated subordinated liabilities	£ 0	£ 15
7.125% 30 Year Step-up Perpetual Callable Subordinated Notes
Disclosure of subordinated liabilities [line items]
Interest rate	7.125%	7.125%
Notes maturity, year	30 years	30 years
First call date	2030
Undated subordinated liabilities	£ 352	£ 361

Subordinated Liabilities - Su_2

Subordinated Liabilities - Summary of Dated Subordinated Liabilities (Detail) £ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2020GBP (£)	Dec. 31, 2019USD ($)	Dec. 31, 2019GBP (£)
Disclosure of subordinated liabilities [line items]
Interest rate			5.875%	5.875%
Dated subordinated liabilities		£ 1,655		£ 2,603
5% Subordinated Notes 2023
Disclosure of subordinated liabilities [line items]
Interest rate	5.00%	5.00%	5.00%	5.00%
Notional amount | $	$ 1,500,000,000		$ 1,500,000,000
Maturity period	2023
Dated subordinated liabilities		£ 542		£ 1,132
4.75% Subordinated Notes 2025
Disclosure of subordinated liabilities [line items]
Interest rate	4.75%	4.75%	4.75%	4.75%
Notional amount | $	$ 1,000,000,000		$ 1,000,000,000
Maturity period	2025
Dated subordinated liabilities		£ 536		£ 763
7.95% Subordinated Notes 2029
Disclosure of subordinated liabilities [line items]
Interest rate	7.95%	7.95%	7.95%	7.95%
Notional amount | $	$ 1,000,000,000		$ 1,000,000,000
Maturity period	2029
Dated subordinated liabilities		£ 242		£ 280
6.50% Subordinated Notes 2030
Disclosure of subordinated liabilities [line items]
Interest rate	6.50%	6.50%	6.50%	6.50%
Maturity period	2030
Dated subordinated liabilities		£ 31		£ 40
5.875% Subordinated Notes 2031
Disclosure of subordinated liabilities [line items]
Interest rate	5.875%	5.875%
Maturity period	2031
Dated subordinated liabilities		£ 10		£ 9
5.625% Subordinated Notes 2045
Disclosure of subordinated liabilities [line items]
Interest rate	5.625%	5.625%	5.625%	5.625%
Notional amount | $	$ 500,000,000		$ 500,000,000
Maturity period	2045
Dated subordinated liabilities		£ 294		£ 379

Other Liabilities (Details)

Other Liabilities (Details) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Subclassifications of assets, liabilities and equities [abstract]
Lease liabilities	£ 97	£ 137
Other	2,240	2,207
Other liabilities	£ 2,337	£ 2,344

Provisions - Summary of Provisi

Provisions - Summary of Provisions Reconciliation (Detail)	12 Months Ended
	Dec. 31, 2020GBP (£)
Disclosure of other provisions [line items]
Beginning balance	£ 572,000,000
Additional provisions	289,000,000
Provisions released	(25,000,000)
Utilisation and other	(382,000,000)
Recharge	10,000,000
Ending balance	464,000,000
Sub-total ≤ 1 year
Disclosure of other provisions [line items]
Ending balance	323,000,000
More than One Year
Disclosure of other provisions [line items]
Ending balance	141,000,000
PPI
Disclosure of other provisions [line items]
Beginning balance	189,000,000
Additional provisions	0
Provisions released	0
Utilisation and other	(113,000,000)
Recharge	0
Ending balance	76,000,000
PPI | Sub-total ≤ 1 year
Disclosure of other provisions [line items]
Ending balance	76,000,000
PPI | More than One Year
Disclosure of other provisions [line items]
Ending balance	0
Other products
Disclosure of other provisions [line items]
Beginning balance	25,000,000
Additional provisions	0
Provisions released	(15,000,000)
Utilisation and other	(2,000,000)
Recharge	0
Ending balance	8,000,000
Other products | Sub-total ≤ 1 year
Disclosure of other provisions [line items]
Ending balance	2,000,000
Other products | More than One Year
Disclosure of other provisions [line items]
Ending balance	6,000,000
FSCS And Bank Levy
Disclosure of other provisions [line items]
Beginning balance	46,000,000
Additional provisions	72,000,000
Provisions released	0
Utilisation and other	(94,000,000)
Recharge	10,000,000
Ending balance	34,000,000
FSCS And Bank Levy | Sub-total ≤ 1 year
Disclosure of other provisions [line items]
Ending balance	34,000,000
FSCS And Bank Levy | More than One Year
Disclosure of other provisions [line items]
Ending balance	0
Vacant Property
Disclosure of other provisions [line items]
Beginning balance	59,000,000
Additional provisions	9,000,000
Provisions released	(6,000,000)
Utilisation and other	(16,000,000)
Recharge	0
Ending balance	46,000,000
Vacant Property | Sub-total ≤ 1 year
Disclosure of other provisions [line items]
Ending balance	25,000,000
Vacant Property | More than One Year
Disclosure of other provisions [line items]
Ending balance	21,000,000
Off- balance sheet ECL
Disclosure of other provisions [line items]
Beginning balance	78,000,000
Additional provisions	0
Provisions released	(3,000,000)
Utilisation and other	0
Recharge	0
Ending balance	75,000,000
Off- balance sheet ECL | Sub-total ≤ 1 year
Disclosure of other provisions [line items]
Ending balance	75,000,000
Off- balance sheet ECL | More than One Year
Disclosure of other provisions [line items]
Ending balance	0
Regulatory and other
Disclosure of other provisions [line items]
Beginning balance	175,000,000
Additional provisions	208,000,000
Provisions released	(1,000,000)
Utilisation and other	(157,000,000)
Recharge	0
Ending balance	225,000,000
Regulatory and other | Sub-total ≤ 1 year
Disclosure of other provisions [line items]
Ending balance	111,000,000
Regulatory and other | More than One Year
Disclosure of other provisions [line items]
Ending balance	£ 114,000,000

Provisions - Additional Informa

Provisions - Additional Information - For Year-end (Detail)	12 Months Ended
	Dec. 31, 2020GBP (£)Complaints	Dec. 31, 2019GBP (£)	Dec. 31, 2018GBP (£)
Future expected provision number
Provisions	£ 464,000,000	£ 572,000,000
Complaints to date | Complaints	4,600,000
Complaints reviewing | Complaints	3,500
Additional provisions	£ 289,000,000
PPI
Future expected provision number
Provisions	76,000,000	189,000,000
Additional provisions	0
Regulatory Related Uk Bank Levy
Future expected provision number
Provisions	£ 34,000,000	£ 46,000,000
Bank levy rate	0.14%	0.15%
Cost of UK Bank Levy Incurred	£ 72,000,000	£ 86,000,000	£ 69,000,000
Amount paid as UK Bank Levy	88,000,000	90,000,000
Other products
Future expected provision number
Provisions	8,000,000	25,000,000
Additional provisions	0
Property
Future expected provision number
Transformation charge	1,000,000
Compliance with Consumer Credit Act
Future expected provision number
Provisions	47,000,000	£ 68,000,000
Regulatory Other Provisions
Future expected provision number
Charge related to operational risk and provisions	17,000,000
Additional provisions	65,000,000
Operational Risk Provisions
Future expected provision number
Additional provisions	£ 91,000,000

Retirement Benefit Plans - Summ

Retirement Benefit Plans - Summary of Retirement Benefit Plans (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets/(liabilities)
Total net assets	£ 92	£ 389
Unfunded Pension And Post Retirement Medical Benefits
Assets/(liabilities)
Surplus (deficit) in plan	(42)	(41)
Surplus | Funded Defined Benefit Pension Scheme
Assets/(liabilities)
Surplus (deficit) in plan	495	669
Deficit | Funded Defined Benefit Pension Scheme
Assets/(liabilities)
Surplus (deficit) in plan	£ (361)	£ (239)

Retirement Benefit Plans - Su_2

Retirement Benefit Plans - Summary of Remeasurement Losses/(Gains) Recognised In Other Comprehensive Income (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of net defined benefit liability (asset) [abstract]
Pension remeasurement	£ 505	£ 522	£ (470)

Retirement Benefit Plans - Addi

Retirement Benefit Plans - Additional Information - Year-End (Detail) £ in Millions	Oct. 26, 2018GBP (£)	Dec. 31, 2020GBP (£)Directors	Dec. 31, 2019GBP (£)Directors	Dec. 31, 2018GBP (£)	Aug. 31, 2019GBP (£)	Mar. 01, 2015
Disclosure of defined benefit plans [line items]
Pensions costs: – defined contribution plans		£ 66	£ 66	£ 67
Percentage of employee covered defined benefit pension schemes		11.00%	11.00%
Number of directors selected | Directors			5
Number of directors nominated | Directors			5
Actuarial valuation deficit					£ 1,136
Increase (decrease) in GMP allowance		£ 5	£ 5
GMP allowance included in scheme liabilities		50	45
Contribution to scheme based on agreement		£ 236	203
Forward curve term		30 years
Increase (decrease) in net defined benefit liability (asset)		£ 35	£ 20	£ 83
Percentage of pension pay increase		2.90%	2.90%	2.90%		1.00%
Percentage improvements in membership profile retirement benefits plan		0.15%
Improvements to life expectancy of female employees		1.25%
Improvements to life expectancy of male employees		1.25%
Average duration of defined benefit obligation		19.4 years	18.8 years
Bottom of range
Disclosure of defined benefit plans [line items]
Period over which the forward rate is used for computing retirement benefits		15 years
Top of range
Disclosure of defined benefit plans [line items]
Period over which the forward rate is used for computing retirement benefits		30 years
Actuarial Assumption of Expected Rates of Inflation
Disclosure of defined benefit plans [line items]
Increase (decrease) in net defined benefit liability (asset)		£ 64
Percentage of reasonably possible increase in actuarial assumption		0.25%
Santander CF Trustee Limited
Disclosure of defined benefit plans [line items]
Number of trustee directors | Directors		5
Santander (UK) Group Pension Scheme Trustees Limited
Disclosure of defined benefit plans [line items]
Number of trustee directors | Directors		2
Equity collar
Disclosure of defined benefit plans [line items]
Gross notional value		£ 1,076	£ 1,560
Currency futures
Disclosure of defined benefit plans [line items]
Gross notional value		2,378	2,079
Present value of defined benefit obligations
Disclosure of defined benefit plans [line items]
Estimated increase in liabilities in relation to judgment on GMP equalisation	£ 40
Deficit Repair Contributions
Disclosure of defined benefit plans [line items]
Contribution to scheme based on agreement		£ 187	£ 153
Santander UK Plc
Disclosure of defined benefit plans [line items]
Number of trustee directors | Directors		3
Santander UK Group | Deficit Group Section Contributions Plan
Disclosure of defined benefit plans [line items]
Contributions to defined benefit plans		£ 187
Maturity date for contribution		31 March 2026

Retirement Benefit Plans - Tota

Retirement Benefit Plans - Total Defined Benefit Plan Amount Charged to Income Statement (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of net defined benefit liability (asset) [abstract]
Net interest income	£ (10)	£ (23)	£ (7)
Current service cost	36	34	41
Past service and GMP costs	1	1	41
Administration costs	8	8	8
Total	£ 35	£ 20	£ 83

Retirement Benefit Plans - Su_3

Retirement Benefit Plans - Summary of Amounts Recognised in Other Comprehensive Income (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of net defined benefit liability (asset) [abstract]
Return on plan assets (excluding amounts included in net interest expense)	£ (1,328)	£ (855)	£ 246
Actuarial losses/(gains) arising from changes in demographic assumptions	34	42	(56)
Actuarial (gains)/losses arising from experience adjustments	(141)	(42)	15
Actuarial losses/(gains) arising from changes in financial assumptions	1,940	1,377	(675)
Pension remeasurement	£ 505	£ 522	£ (470)

Retirement Benefit Plans - Disc

Retirement Benefit Plans - Disclosure of Movements in Present Value of Defined Benefit Obligations (Detail) - GBP (£) £ in Millions	Oct. 26, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of net defined benefit liability (asset) [line items]
Current service cost paid by Santander UK plc		£ 36	£ 34	£ 41
Interest cost		(10)	(23)	(7)
Past service cost		1	1	41
Remeasurement due to actuarial movements arising from:
Changes in demographic assumptions		34	42	(56)
– Experience adjustments		(141)	(42)	15
Changes in financial assumptions		1,940	1,377	(675)
Present value of defined benefit obligations
Disclosure of net defined benefit liability (asset) [line items]
Beginning balance		(12,158)	(10,804)
Current service cost paid by Santander UK plc		(24)	(22)
Current service cost paid by subsidiaries		(12)	(12)
Interest cost		(253)	(308)
Employer salary sacrifice contributions		(2)	(9)
Past service cost		(1)	(1)
GMP equalisation cost	£ (40)
Remeasurement due to actuarial movements arising from:
Changes in demographic assumptions		(34)	(42)
– Experience adjustments		141	42
Changes in financial assumptions		(1,940)	(1,377)
Benefits paid		396	375
Ending balance		£ (13,887)	£ (12,158)	£ (10,804)

Retirement Benefit Plans - Di_2

Retirement Benefit Plans - Disclosure of Movements in Fair Value of Scheme Assets (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of net defined benefit liability (asset) [line items]
Interest cost	£ (10)	£ (23)	£ (7)
Return on plan assets (excluding amounts included in net interest expense)	(1,328)	(855)	246
Fair value of scheme assets
Disclosure of net defined benefit liability (asset) [line items]
Beginning balance	12,547	11,532
Interest cost	263	331
Contributions paid by employer and scheme members	245	212
Contributions paid by fellow Banco Santander subsidiaries	0	0
Administration costs paid	(8)	(8)
Return on plan assets (excluding amounts included in net interest expense)	1,328	855
Benefits paid	(396)	(375)
Ending balance	£ 13,979	£ 12,547	£ 11,532

Retirement Benefit Plans - Su_4

Retirement Benefit Plans - Summary of Composition and Fair Value of Plan Assets (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of net defined benefit liability (asset) [line items]
UK equities	£ 40	£ 128
Overseas equities	2,275	2,675
Corporate bonds	1,578	1,777
Government fixed interest bonds	1,618	2,710
Government index-linked bonds	6,695	4,543
Property	1,454	1,332
Derivatives	312	94
Cash	1,161	984
Repurchase agreements	(2,198)	(3,263)
Other	1,044	1,567
Total	£ 13,979	£ 12,547
UK equities	0.00%	1.00%
Overseas equities	16.00%	21.00%
Corporate bonds	11.00%	15.00%
Government fixed interest bonds	12.00%	22.00%
Government index-linked bonds	48.00%	35.00%
Property	10.00%	11.00%
Derivatives	2.00%	1.00%
Cash	8.00%	8.00%
Repurchase agreements	(15.00%)	(26.00%)
Other	8.00%	12.00%
Total	100.00%	100.00%
Quoted Prices in Active Markets
Disclosure of net defined benefit liability (asset) [line items]
UK equities	£ 40	£ 128
Overseas equities	1,271	1,742
Corporate bonds	1,121	1,333
Government fixed interest bonds	1,618	2,710
Government index-linked bonds	6,695	4,543
Property	0	0
Derivatives	0	0
Cash	0	0
Repurchase agreements	0	0
Other	0	0
Total	£ 10,745	£ 10,456
UK equities	0.00%	1.00%
Overseas equities	9.00%	14.00%
Corporate bonds	8.00%	11.00%
Government fixed interest bonds	12.00%	22.00%
Government index-linked bonds	48.00%	35.00%
Property	0.00%	0.00%
Derivatives	0.00%	0.00%
Cash	0.00%	0.00%
Repurchase agreements	0.00%	0.00%
Other	0.00%	0.00%
Total	77.00%	83.00%
Prices Not Quoted in Active Markets
Disclosure of net defined benefit liability (asset) [line items]
UK equities	£ 0	£ 0
Overseas equities	1,004	933
Corporate bonds	457	444
Government fixed interest bonds	0	0
Government index-linked bonds	0	0
Property	1,454	1,332
Derivatives	312	94
Cash	1,161	984
Repurchase agreements	(2,198)	(3,263)
Other	1,044	1,567
Total	£ 3,234	£ 2,091
UK equities	0.00%	0.00%
Overseas equities	7.00%	7.00%
Corporate bonds	3.00%	4.00%
Government fixed interest bonds	0.00%	0.00%
Government index-linked bonds	0.00%	0.00%
Property	10.00%	11.00%
Derivatives	2.00%	1.00%
Cash	8.00%	8.00%
Repurchase agreements	(15.00%)	(26.00%)
Other	8.00%	12.00%
Total	23.00%	17.00%

Retirement Benefit Plans - Su_5

Retirement Benefit Plans - Summary of Principal Actuarial Assumptions Used for Defined Benefit Schemes (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Mar. 01, 2015
Disclosure of net defined benefit liability (asset) [abstract]
Discount rate for scheme liabilities	1.30%	2.10%	2.90%
General price inflation	3.00%	3.00%	3.20%
General salary increase	1.00%	1.00%	1.00%
Expected rate of pension increase	2.90%	2.90%	2.90%	1.00%
Longevity at 60 for current pensioners, on the valuation date:
- Males	27 years 6 months	27 years 3 months 18 days	27 years 3 months 18 days
- Females	30 years	29 years 9 months 18 days	30 years 1 month 6 days
Longevity at 60 for future pensioners currently aged 40, on the valuation date:
- Males	29 years	28 years 10 months 24 days	28 years 8 months 12 days
- Females	31 years 6 months	31 years 3 months 18 days	31 years 7 months 6 days

Retirement Benefit Plans - Su_6

Retirement Benefit Plans - Summary of Actuarial Assumption Sensitivities (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Actuarial Assumption of Discount Rates
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Description of changes in methods and assumptions	25 bps increase
Increase decrease in defined benefit obligation	£ (662)	£ (564)
Actuarial Assumption of Expected Rates of Inflation
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Description of changes in methods and assumptions	25 bps increase
Increase decrease in defined benefit obligation	£ 365	407
Actuarial Assumption of Expected Rates of Mortality
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Description of changes in methods and assumptions	Each additional year of longevity assumed
Increase decrease in defined benefit obligation	£ 515	£ 419

Retirement Benefit Plans - Di_3

Retirement Benefit Plans - Disclosure of Benefits Expected To Be Paid (Detail) £ in Millions	Dec. 31, 2020GBP (£)
Sub-total ≤ 1 year
Disclosure of net defined benefit liability (asset) [line items]
Benefits expected to be paid	£ 359
>1 and ≤ 2 years
Disclosure of net defined benefit liability (asset) [line items]
Benefits expected to be paid	327
2 - 3 years
Disclosure of net defined benefit liability (asset) [line items]
Benefits expected to be paid	336
3 - 4 years
Disclosure of net defined benefit liability (asset) [line items]
Benefits expected to be paid	359
4 - 5 years
Disclosure of net defined benefit liability (asset) [line items]
Benefits expected to be paid	382
Five Years Ending 2029
Disclosure of net defined benefit liability (asset) [line items]
Benefits expected to be paid	£ 2,139

Contingent Liabilities and Co_3

Contingent Liabilities and Commitments - Summary of Contingent Liabilities and Commitments (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of contingent liabilities and commitments [line items]
Guarantees and formal standby facilities, credit lines and other commitments	£ 43,160	£ 41,595
Contingent Liability for Guarantees
Disclosure of contingent liabilities and commitments [line items]
Guarantees given to third parties	939	1,198
Loan commitments
Disclosure of contingent liabilities and commitments [line items]
Formal standby facilities, credit lines and other commitments	42,221	40,397
Formal standby facilities, credit lines and other commitments	£ 42,221	£ 40,397

Contingent Liabilities and Co_4

Contingent Liabilities and Commitments - Additional Information (Detail) £ in Millions	Nov. 02, 2015	Dec. 31, 2020GBP (£)	Dec. 31, 2020EUR (€)	Dec. 31, 2020EUR (€)	Dec. 30, 2020GBP (£)	Jul. 31, 2020GBP (£)	Dec. 31, 2019GBP (£)
Disclosure of contingent liabilities and commitments [line items]
Unused provision reversed, other provisions		£ 25
Additional provisions		289
Provisions		464					£ 572
UK&I Banks
Disclosure of contingent liabilities and commitments [line items]
Preferred stock issued, threshold amount | €			€ 0
AXA France | Mis-Selling Losses Claimed
Disclosure of contingent liabilities and commitments [line items]
Amount claimed by litigation counterparty					£ 631
FSCS
Disclosure of contingent liabilities and commitments [line items]
Unused provision reversed, other provisions		0
Provisions							0
Compliance with Consumer Credit Act
Disclosure of contingent liabilities and commitments [line items]
Provisions		47					£ 68
Genworth Financial International Holdings Inc
Disclosure of contingent liabilities and commitments [line items]
Amount awarded to other party						£ 624
Santander UK Group
Disclosure of contingent liabilities and commitments [line items]
Liability arising through disallowance of tax allowance		£ 152
Visa Europe Ltd
Disclosure of contingent liabilities and commitments [line items]
Visa Europe Ltd sale to Visa Inc	100.00%
Bottom of range
Disclosure of contingent liabilities and commitments [line items]
Losses on litigation settlements | €			€ 1,000,000,000
Top of range
Disclosure of contingent liabilities and commitments [line items]
Santander UK liability under indemnity cap | €				€ 39,850,000

Share Capital - Schedule of Sha

Share Capital - Schedule of Share Capital Issued and Fully Paid (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Jan. 01, 2020	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018
Disclosure of classes of share capital [line items]
Issued and fully paid share capital	£ 3,105	£ 3,105	£ 3,105	£ 3,119	£ 3,119	£ 3,119
Redemption of preference shares, amount	£ 0	£ (14)
Ordinary Share Capital
Disclosure of classes of share capital [line items]
Number of shares issued and fully paid (in shares)	31,051,768,866	31,051,768,866	31,051,768,866	31,051,768,866	31,051,768,866	31,051,768,866
Issued and fully paid share capital	£ 3,105	£ 3,105	£ 3,105	£ 3,105	£ 3,105	£ 3,105
Number of redemption of preference shares (in shares)	0	0
Redemption of preference shares, amount	£ 0	£ 0
Three Hundred Million Preference Shares
Disclosure of classes of share capital [line items]
Number of shares issued and fully paid (in shares)	0	0	0	13,780	13,780	13,780
Issued and fully paid share capital	£ 0	£ 0	£ 0	£ 14	£ 14	£ 14
Number of redemption of preference shares (in shares)	0	(13,780)
Redemption of preference shares, amount	£ 0	£ (14)

Share Capital - Schedule of S_2

Share Capital - Schedule of Share Capital Issued and Fully Paid (Parenthetical) (Detail) - GBP (£)	Dec. 31, 2020	Dec. 31, 2019
Disclosure of classes of share capital [line items]
Notional amount	£ 197,215,000,000	£ 205,172,000,000
Ordinary Share Capital
Disclosure of classes of share capital [line items]
Par value per share	£ 0.10
Three Hundred Million Preference Shares
Disclosure of classes of share capital [line items]
Par value per share	£ 1,000
Notional amount	£ 300,000,000

Share Capital - Summary of Shar

Share Capital - Summary of Share Premium (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Jan. 01, 2019
Disclosure of classes of share capital [abstract]
Share premium	£ 5,620	£ 5,620	£ 5,620	£ 5,620

Share Capital - Additional Info

Share Capital - Additional Information (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of classes of share capital [abstract]
Sterling preference shares, Notional amount	£ 325	£ 325

Other Equity Instruments - Summ

Other Equity Instruments - Summary of Other Equity Instruments (Detail) £ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2020GBP (£)	Dec. 31, 2020GBP (£)	Dec. 31, 2019GBP (£)
Disclosure of other equity instruments [line items]
Other equity instruments		£ 2,191	£ 2,191
300m Step-up Callable Perpetual Reserve Capital Instruments
Disclosure of other equity instruments [line items]
Capital securities	£ 300	£ 300
Interest rate		0.07037	0.07037
Other equity interest maturity period		2026-02	2026-02
Other equity instruments		£ 235	£ 235
500m Fixed Rate Reset Perpetual AT1 Capital Securities
Disclosure of other equity instruments [line items]
Capital securities	500	£ 500
Interest rate		0.0675	0.0675
Other equity interest maturity period		2024-06	2024-06
Other equity instruments		£ 496	£ 496
750m Fixed Rate Reset Perpetual AT1 Capital Securities
Disclosure of other equity instruments [line items]
Capital securities	750	£ 750
Interest rate		0.07375	0.07375
Other equity interest maturity period		2022-06	2022-06
Other equity instruments		£ 750	£ 750
500m Fixed Rate Reset Perpetual AT1 Capital Securities
Disclosure of other equity instruments [line items]
Capital securities	500	£ 500
Interest rate		0.0518	0.0518
Other equity interest maturity period		2021-03	2021-03
Other equity instruments		£ 210	£ 210
500m Fixed Rate Reset Perpetual AT1 Capital Securities
Disclosure of other equity instruments [line items]
Capital securities	£ 500	£ 500
Interest rate		0.0630	0.0630
Other equity interest maturity period		2025-03	2025-03
Other equity instruments		£ 500	£ 500

Other Equity Instruments - Addi

Other Equity Instruments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
300m Fixed Rate Reset Perpetual AT1 Capital Securities
Disclosure of other equity instruments [line items]
Reserve Capital Instruments bear interest rate reset period	5 years
Reserve Capital Instruments benchmark gilt rate period	5 years
500m Fixed Rate Reset Perpetual AT1 Capital Securities
Disclosure of other equity instruments [line items]
Distribution rate resets, period	5 years
Common equity tier 1 capital ratio minimum	7.00%
Interest rate	0.0675	0.0675
750m Fixed Rate Reset Perpetual AT1 Capital Securities
Disclosure of other equity instruments [line items]
Interest rate	0.07375	0.07375
500m Fixed Rate Reset Perpetual AT1 Capital Securities
Disclosure of other equity instruments [line items]
Interest rate	0.0518	0.0518
500m Fixed Rate Reset Perpetual AT1 Capital Securities
Disclosure of other equity instruments [line items]
Interest rate	0.0630	0.0630

Non-Controlling Interests - Sch

Non-Controlling Interests - Schedule of Amounts Attributable to Non-Controlling Interests (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of noncontrolling interests [line items]
Non-controlling interests	£ 162	£ 160
PSA Finance UK Limited
Disclosure of noncontrolling interests [line items]
Non-controlling interests	£ 162	£ 160

Changes in Liabilities Arisin_3

Changes in Liabilities Arising From Financing Activities - Summary of Changes in Liabilities Arising from Financing Activities (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of reconciliation of liabilities arising from financing activities [line items]
Beginning Balance	£ 46,848	£ 52,284
Cash flows from financing activities	(8,152)	(4,112)
Cash flows from operating activities	1,201	(529)
Unrealised foreign exchange	398	(1,370)
Other changes	18	575
Ending Balance	40,313	46,848
Debt Securities
Disclosure of reconciliation of liabilities arising from financing activities [line items]
Beginning Balance	41,129	46,692
Cash flows from financing activities	(7,201)	(3,839)
Cash flows from operating activities	1,201	(529)
Unrealised foreign exchange	376	(1,320)
Other changes	61	125
Ending Balance	35,566	41,129
Subordinated liabilities
Disclosure of reconciliation of liabilities arising from financing activities [line items]
Beginning Balance	3,528	3,601
Cash flows from financing activities	(659)	0
Cash flows from operating activities	0	0
Unrealised foreign exchange	22	(50)
Other changes	(335)	(23)
Ending Balance	2,556	3,528
Other equity instruments
Disclosure of reconciliation of liabilities arising from financing activities [line items]
Beginning Balance	2,191	1,991
Cash flows from financing activities	0	196
Cash flows from operating activities	0	0
Unrealised foreign exchange	0	0
Other changes	0	4
Ending Balance	2,191	2,191
Dividends Paid
Disclosure of reconciliation of liabilities arising from financing activities [line items]
Beginning Balance	0	0
Cash flows from financing activities	(292)	(469)
Cash flows from operating activities	0	0
Unrealised foreign exchange	0	0
Other changes	292	469
Ending Balance	£ 0	£ 0

Assets Charged as Security fo_3

Assets Charged as Security for Liabilities and Collateral Accepted as Security for Assets - Financial Assets Charged as Security Under On-Balance Sheet and Off-Balance (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of assets pledged as security [line items]
Assets charged as security for liabilities	£ 4,549	£ 4,786
On-Balance Sheet
Disclosure of assets pledged as security [line items]
Assets charged as security for liabilities	62,557	63,025
On-Balance Sheet | Cash and Balances at Central Banks
Disclosure of assets pledged as security [line items]
Assets charged as security for liabilities	985	1,080
On-Balance Sheet | Loans and advances to banks
Disclosure of assets pledged as security [line items]
Assets charged as security for liabilities	550	403
On-Balance Sheet | Loans and advances to customers | Securitisations and Covered Bonds
Disclosure of assets pledged as security [line items]
Assets charged as security for liabilities	31,138	36,225
On-Balance Sheet | Loans and advances to customers | Other
Disclosure of assets pledged as security [line items]
Assets charged as security for liabilities	23,655	16,282
On-Balance Sheet | Other financial assets at amortised cost
Disclosure of assets pledged as security [line items]
Assets charged as security for liabilities	648	3,026
On-Balance Sheet | Financial assets at fair value through other comprehensive income
Disclosure of assets pledged as security [line items]
Assets charged as security for liabilities	5,581	6,009
Off-Balance Sheet
Disclosure of assets pledged as security [line items]
Assets charged as security for liabilities	£ 24,701	£ 15,098

Assets Charged as Security fo_4

Assets Charged as Security for Liabilities and Collateral Accepted as Security for Assets - Additional Information (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of assets pledged as security [line items]
Assets charged as security for liabilities	£ 4,549	£ 4,786
Gross assets charged as security for liabilities	33,432	36,391
Notes issued under securitisation and covered bond programmes retained internally	21,615	22,783
Gross assets	33,432	36,391
Securities Lending
Disclosure of assets pledged as security [line items]
Assets charged as security for liabilities	30,040	21,563
Collateral Accepted as Security for Assets	4,538	4,676
Internally Retained | Securitisations and Covered Bonds
Disclosure of assets pledged as security [line items]
Notes issued under securitisation and covered bond programmes retained internally	4,530	4,728
Third Party Bilateral Secured Funding Transactions | Securitisations and Covered Bonds
Disclosure of assets pledged as security [line items]
Assets charged as security for liabilities	1,114	1,581
Cash | Derivative Financial Instruments
Disclosure of assets pledged as security [line items]
Assets charged as security for liabilities	1,598	1,716
Collateral Accepted as Security for Assets	2,063	2,169
Purchase And Resale Agreements
Disclosure of assets pledged as security [line items]
Collateral Accepted as Security for Assets	25,972	20,444
Subsidiaries | Sale and Repurchase Agreements
Disclosure of assets pledged as security [line items]
Assets charged as security for liabilities	25,874	20,686
Subsidiaries | Loans and advances to customers securitisations and covered bonds
Disclosure of assets pledged as security [line items]
Assets charged as security for liabilities	1,392	2,067
Santander UK Plc | Internally Retained | Securitisations and Covered Bonds
Disclosure of assets pledged as security [line items]
Internally retained securitisation and covered bond issuances available for creating collateral	£ 2,294	£ 166

Assets Charged as Security fo_5

Assets Charged as Security for Liabilities and Collateral Accepted as Security for Assets - Schedule of Collateral Held as Security for Assets (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
On-Balance Sheet
Disclosure of assets pledged as security [line items]
Total on-balance sheet/ off-balance sheet	£ 2,063	£ 2,169
On-Balance Sheet | Deposits by banks
Disclosure of assets pledged as security [line items]
Total on-balance sheet/ off-balance sheet	2,063	2,169
Off-Balance Sheet
Disclosure of assets pledged as security [line items]
Total on-balance sheet/ off-balance sheet	£ 30,510	£ 25,120

Share-Based Compensation - Addi

Share-Based Compensation - Additional Information (Detail)	1 Months Ended	12 Months Ended			156 Months Ended
	Mar. 31, 2019	Dec. 31, 2020GBP (£)£ / sharesshares	Dec. 31, 2019GBP (£)£ / sharesshares	Dec. 31, 2015	Dec. 31, 2020GBP (£)invitation
Disclosure of terms and conditions of share-based payment arrangement [line items]
Price determination period for options granted for share based payments arrangement		3 days
Expiration term of options granted for share based payments arrangement		6 months
Number of sharesave schemes launched | invitation					13
Weighted average share price | £ / shares		£ 2.92	£ 3.18
Weighted average grant-date fair value		£ 0.21	£ 0.49		£ 0.21
Liabilities arising from share-based payment transactions		500,000	2,400,000		500,000
Liabilities arising from share-based payment transactions, vested liabilities		£ 0	£ 1,400,000		0
Number of shares outstanding | shares		3,254,900	2,396,909
Employees
Disclosure of terms and conditions of share-based payment arrangement [line items]
Deferred shares description		Any deferred awards are dependent on continued employment or subject to Santander's discretion for leavers. For 2018 and 2019 bonus awards, deferral of the award is over a three, five or seven-year period from the anniversary of the initial award. Deferred bonus awards in shares are subject to an additional one-year retention period from the point of delivery.
Cash | Employees
Disclosure of terms and conditions of share-based payment arrangement [line items]
Annual bonus percentage		50.00%
Shares | Employees
Disclosure of terms and conditions of share-based payment arrangement [line items]
Annual bonus percentage		50.00%
Long-Term Incentive Plan (LTIP)
Disclosure of terms and conditions of share-based payment arrangement [line items]
Percent of other equity instruments vested in share-based payment arrangement	65.78%
Deferral term of conditional awards		3 years		3 years
Performance cycle of conditional awards				1 year
Share Incentive Plans
Disclosure of terms and conditions of share-based payment arrangement [line items]
Vesting requirements for sharebased payment arrangement		no vesting conditions attached to these shares, and no restrictions as to when the shares can be removed from the trust. However, if a participant chooses to sell the shares before the end of five years, they will be liable for the taxable benefit received when the shares are taken out of the trust. The shares can be released from trust after five years free of income tax and national insurance contributions. 2,396,909 shares were outstanding at 31 December 2019 (2018: 2,346,108 shares).
Maximum investment value per tax year		£ 1,800			1,800
Percentage of maximum salary that can be invested		10.00%
Bottom of range
Disclosure of terms and conditions of share-based payment arrangement [line items]
Estimated employee saving on share based compensation plan		£ 5			5
Terms of options		3 years
Deferred shares description		40
Top of range
Disclosure of terms and conditions of share-based payment arrangement [line items]
Estimated employee saving on share based compensation plan		£ 500			500
Terms of options		5 years
Deferred shares description		60
Variable Pay of Less Than 500,000
Disclosure of terms and conditions of share-based payment arrangement [line items]
Estimated employee saving on share based compensation plan		£ 500,000			£ 500,000
Variable Pay of Less Than 500,000 | Employees
Disclosure of terms and conditions of share-based payment arrangement [line items]
Percentage of defer annual bonus		40.00%
Variable Pay at or Above 500,000 | Employees
Disclosure of terms and conditions of share-based payment arrangement [line items]
Percentage of defer annual bonus		60.00%

Share-Based Compensation - Summ

Share-Based Compensation - Summary of Movement in Share Options (Detail) - Sharesave Schemes number in Thousands	12 Months Ended
	Dec. 31, 2020£ / shares	Dec. 31, 2019£ / shares
Disclosure of terms and conditions of share-based payment arrangement [line items]
Number of options outstanding beginning balance	23,373	26,838
Number of options granted	11,642	9,594
Number of options exercised	(860)	(7,978)
Number of options forfeited/expired	(12,993)	(5,081)
Number of options outstanding ending balance	21,162	23,373
Number of options exercisable	1,805	2,519
Weighted average exercise price outstanding beginning balance	£ 3.03	£ 3.12
Weighted average exercise price granted	1.65	2.83
Weighted average exercise price exercised	2.75	2.83
Weighted average exercise price forfeited/expired	2.96	3.42
Weighted average exercise price outstanding ending balance	2.32	3.03
Weighted average exercise price exercisable ending balance	£ 3.59	£ 3.62

Share-Based Compensation - Su_2

Share-Based Compensation - Summary of Range of Exercise Prices and Weighted Average Remaining Contractual Life of the Options Outstanding (Detail) - Sharesave Schemes - £ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]
Option outstanding, Weighted average exercise price	£ 2.32	£ 3.03	£ 3.12
1 to 2
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]
Options outstanding , Weighted average remaining contractual life years	4 years
Option outstanding, Weighted average exercise price	£ 1.65
2 to 3
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]
Options outstanding , Weighted average remaining contractual life years	2 years	3 years
Option outstanding, Weighted average exercise price	£ 2.81	£ 2.80
3 to 4
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]
Options outstanding , Weighted average remaining contractual life years	1 year	2 years
Option outstanding, Weighted average exercise price	£ 3.38	£ 3.38
4 to 5
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]
Options outstanding , Weighted average remaining contractual life years	1 year	2 years
Option outstanding, Weighted average exercise price	£ 4.02	£ 4.13

Share-Based Compensation - Su_3

Share-Based Compensation - Summary of Range of Exercise Prices and Weighted Average Remaining Contractual Life of the Options Outstanding (Parenthetical) (Detail) - Sharesave Schemes	Dec. 31, 2020£ / shares
1 to 2 | Bottom of range
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]
Range of exercise prices	£ 1
1 to 2 | Top of range
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]
Range of exercise prices	2
2 to 3 | Bottom of range
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]
Range of exercise prices	2
2 to 3 | Top of range
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]
Range of exercise prices	3
3 to 4 | Bottom of range
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]
Range of exercise prices	3
3 to 4 | Top of range
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]
Range of exercise prices	4
4 to 5 | Bottom of range
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]
Range of exercise prices	4
4 to 5 | Top of range
Disclosure of number and weighted average remaining contractual life of outstanding share options [line items]
Range of exercise prices	£ 5

Share-Based Compensation - Su_4

Share-Based Compensation - Summary of Movement in Value of Conditional Awards in LTIPs (Detail) - 2015 LTIP - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of terms and conditions of share-based payment arrangement [line items]
Conditional awards at the beginning of the year	£ 0	£ 6,374
Conditional award payments made	0	4,578
Conditional awards forfeited or cancelled	0	(1,796)
Conditional awards outstanding at the end of the year	£ 0	£ 0

Transactions With Directors a_3

Transactions With Directors and Other Key Management Personnel - Summary of Remuneration of the Directors and Other Key Management Personnel (Detail) - GBP (£)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure Of Remuneration Of Directors And Other Key Management Personnel [Abstract]
Salaries and fees	£ 5,353,980	£ 5,025,665	£ 5,028,434
Performance-related payments	1,458,911	3,864,965	5,194,317
Other fixed remuneration (pension and other allowances & non-cash benefits)	1,107,348	1,367,069	1,467,011
Expenses	6,772	42,526	25,198
Total remuneration	7,927,011	10,300,225	11,714,960
Short-term employee benefits	18,432,698	21,925,975	24,445,189
Post-employment benefits	2,084,645	3,590,466	2,399,261
Total Compensation	£ 20,517,343	£ 25,516,441	£ 26,844,450

Transactions With Directors a_4

Transactions With Directors and Other Key Management Personnel - Summary of Remuneration of the Directors and Other Key Management Personnel (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2020GBP (£)Individuals	Dec. 31, 2019GBP (£)Individuals	Dec. 31, 2018GBP (£)Individuals
Disclosure of directors and key management remuneration [line items]
Number of Key management personnel who receive termination benefit | Individuals	0	1	2
Number of previously employed individuals for buyouts of deferred performance related payments | Individuals	2
Deferred performance related payments of shares		£ 0
Termination payment	£ 0	£ 1,076,435	£ 847,388
Director
Disclosure of directors and key management remuneration [line items]
Number of previously employed individuals for buyouts of deferred performance related payments | Individuals	1
Deferred performance related payments of shares	£ 1,293,678
Deferred performance related payments of shares vested	£ 242,605
Key management personnel of entity or parent
Disclosure of directors and key management remuneration [line items]
Number of previously employed individuals for buyouts of deferred performance related payments | Individuals	1
Deferred performance related payments of shares	£ 924,133
Deferred performance related payments of shares vested	£ 60,500

Transactions With Directors a_5

Transactions With Directors and Other Key Management Personnel - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020GBP (£)DirectorSecurityLoan	Dec. 31, 2019GBP (£)DirectorSecurityLoan	Dec. 31, 2018GBP (£)
Disclosure of transactions between related parties [line items]
Remuneration excluding pension contribution	£ 2,134,134	£ 3,725,993
Remuneration excluding pension contribution performance related	726,040	1,989,900
Ex gratia pensions paid to former Directors	366,248	335,202	£ 87,300
Sharedealing transactions through the Santander UK group's	£ 0	£ 0
Number of Directors undertook sharedealing transactions | Director	0	0
Directors who held any interest in the shares | Director	0	0
Number of loan made to directors | SecurityLoan	7	9
Directors loans principal amount outstanding	£ 1,829,267	£ 1,767,066
Directors who exercised or were granted any rights to subscribe for shares | Director	0	0
Other key management personnel
Disclosure of transactions between related parties [line items]
Number of loan made to key management personnel | SecurityLoan	5	9
Loans made to key management personnel	£ 1,811,171	£ 3,153,343

Transactions With Directors a_6

Transactions With Directors and Other Key Management Personnel - Summary of Transactions with Directors, Other Key Management Personnel (Detail) - Key management personnel of entity or parent £ in Thousands	12 Months Ended
	Dec. 31, 2020GBP (£)DepositsLoans	Dec. 31, 2019GBP (£)DepositsLoans
Disclosure of directors and key management remuneration [line items]
Number of secured loans, unsecured loans and overdrafts, beginning balance | Loans	18	16
Number of secured loans, unsecured loans and overdrafts, net movements | Loans	(6)	2
Number of secured loans, unsecured loans and overdrafts, ending balance | Loans	12	18
Beginning balance	£ 4,920	£ 3,035
Net movements	(1,280)	1,885
Ending balance	£ 3,640	£ 4,920
Number of deposit, bank and instant access accounts and investments, beginning balance | Deposits	32	30
Number of deposit, bank and instant access accounts and investments, net movements | Deposits	(9)	2
Number of deposit, bank and instant access accounts and investments, ending balance | Deposits	23	32
Beginning balance	£ 11,975	£ 10,963
Net movements	(3,780)	1,012
Ending balance	£ 8,195	£ 11,975

Related Party Disclosures - Sum

Related Party Disclosures - Summary of Transactions With Related Parties (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of transactions between related parties [line items]
Interest, fees and other income received	£ (213)	£ (232)	£ (189)
Interest, fees and other expenses paid	578	756	670
Amounts owed by related parties	4,022	3,966
Amounts owed to related parties	(13,865)	(13,629)
Ultimate parent
Disclosure of transactions between related parties [line items]
Interest, fees and other income received	(119)	(130)	(72)
Interest, fees and other expenses paid	105	266	217
Amounts owed by related parties	1,557	1,560
Amounts owed to related parties	(2,151)	(2,143)
Immediate parent
Disclosure of transactions between related parties [line items]
Interest, fees and other income received	(7)	(7)	(3)
Interest, fees and other expenses paid	316	317	275
Amounts owed by related parties	8	8
Amounts owed to related parties	(10,121)	(10,012)
Fellow Subsidiaries
Disclosure of transactions between related parties [line items]
Interest, fees and other income received	(58)	(66)	(86)
Interest, fees and other expenses paid	157	173	178
Amounts owed by related parties	223	204
Amounts owed to related parties	(559)	(544)
Associates and joint ventures
Disclosure of transactions between related parties [line items]
Interest, fees and other income received	(29)	(29)	(28)
Interest, fees and other expenses paid	0	0	£ 0
Amounts owed by related parties	2,234	2,194
Amounts owed to related parties	£ (1,034)	£ (930)

Related Party Disclosures - Add

Related Party Disclosures - Additional Information (Detail) - GBP (£) £ in Millions	1 Months Ended	12 Months Ended
	Jul. 31, 2018	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019
Disclosure of transactions between related parties [line items]
Other assets			£ 3,013	£ 2,520
Liabilities		£ 267,463	£ 276,396	£ 265,685
Decrease in RWAs due to transfers	£ 5,500
Dividend paid	668
Banco Santander London Branch
Disclosure of transactions between related parties [line items]
Customer loans	1,400
Other assets	21,500
Liabilities	20.7
Banco Santander London Branch | Derivative Financial Instruments
Disclosure of transactions between related parties [line items]
Other assets	19,700
Liabilities	£ 18.8
Santander UK Group Holdings Plc
Disclosure of transactions between related parties [line items]
Sale Of Share Capital Issued Percentage		100.00%
Consideration paid (received)		£ 337

Financial Instruments - Analysi

Financial Instruments - Analysis of Fair Value of Financial Instruments Carried at Amortised (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Loans and advances to customers	£ 208,750	£ 207,287
Loans and advances to banks	1,682	1,855
Reverse repurchase agreements - non trading	19,599	23,636
Deposits by customers	195,135	181,883
Deposits by banks	20,958	14,353
Repurchase agreements - non trading	15,848	18,286
Debt securities in issue	35,566	41,129
Subordinated liabilities	2,556	3,528
Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Financial assets	233,761	244,395
Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Total financial liabilities	271,522	261,582
Deposits by customers | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Deposits by customers	195,242	182,013
Deposits by banks | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Deposits by banks	20,967	14,363
Repurchase agreements - non trading | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Repurchase agreements - non trading	15,847	18,292
Debt Securities | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Debt securities in issue	36,397	42,694
Subordinated liabilities | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Subordinated liabilities	3,069	4,220
Loans and advances to customers | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Loans and advances to customers	211,279	211,796
Loans and advances to banks | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Loans and advances to banks	1,682	1,855
Reverse repurchase agreements - non-trading | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Reverse repurchase agreements - non trading	19,608	23,634
Other financial assets at amortised cost | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Other financial assets at amortised cost	1,192	7,110
Level 1 | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Financial assets	799	6,575
Level 1 | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Total financial liabilities	0	0
Level 1 | Deposits by customers | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Deposits by customers	0	0
Level 1 | Deposits by banks | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Deposits by banks	0	0
Level 1 | Repurchase agreements - non trading | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Repurchase agreements - non trading	0	0
Level 1 | Debt Securities | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Debt securities in issue	0	0
Level 1 | Subordinated liabilities | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Subordinated liabilities	0	0
Level 1 | Loans and advances to customers | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Loans and advances to customers	0	0
Level 1 | Loans and advances to banks | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Loans and advances to banks	0	0
Level 1 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Reverse repurchase agreements - non trading	0	0
Level 1 | Other financial assets at amortised cost | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Other financial assets at amortised cost	799	6,575
Level 2 | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Financial assets	21,457	25,908
Level 2 | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Total financial liabilities	74,703	79,257
Level 2 | Deposits by customers | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Deposits by customers	108	95
Level 2 | Deposits by banks | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Deposits by banks	20,951	13,956
Level 2 | Repurchase agreements - non trading | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Repurchase agreements - non trading	15,847	18,292
Level 2 | Debt Securities | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Debt securities in issue	34,967	42,694
Level 2 | Subordinated liabilities | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Subordinated liabilities	2,830	4,220
Level 2 | Loans and advances to customers | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Loans and advances to customers	0	0
Level 2 | Loans and advances to banks | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Loans and advances to banks	1,682	1,739
Level 2 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Reverse repurchase agreements - non trading	19,382	23,634
Level 2 | Other financial assets at amortised cost | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Other financial assets at amortised cost	393	535
Level 3 | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Financial assets	211,505	211,912
Level 3 | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Total financial liabilities	196,819	182,325
Level 3 | Deposits by customers | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Deposits by customers	195,134	181,918
Level 3 | Deposits by banks | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Deposits by banks	16	407
Level 3 | Repurchase agreements - non trading | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Repurchase agreements - non trading	0	0
Level 3 | Debt Securities | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Debt securities in issue	1,430	0
Level 3 | Subordinated liabilities | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Subordinated liabilities	239	0
Level 3 | Loans and advances to customers | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Loans and advances to customers	211,279	211,796
Level 3 | Loans and advances to banks | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Loans and advances to banks	0	116
Level 3 | Reverse repurchase agreements - non-trading | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Reverse repurchase agreements - non trading	226	0
Level 3 | Other financial assets at amortised cost | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Other financial assets at amortised cost	0	0
Fair Value | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Financial assets	231,194	239,834
Fair Value | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Total financial liabilities	270,063	259,179
Fair Value | Deposits by customers | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Deposits by customers	195,135	181,883
Fair Value | Deposits by banks | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Deposits by banks	20,958	14,353
Fair Value | Repurchase agreements - non trading | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Repurchase agreements - non trading	15,848	18,286
Fair Value | Debt Securities | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Debt securities in issue	35,566	41,129
Fair Value | Subordinated liabilities | Financial Liabilities at Amortised Cost, Category
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Subordinated liabilities	2,556	3,528
Fair Value | Loans and advances to customers | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Loans and advances to customers	208,750	207,287
Fair Value | Loans and advances to banks | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Loans and advances to banks	1,682	1,855
Fair Value | Reverse repurchase agreements - non-trading | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Reverse repurchase agreements - non trading	19,599	23,636
Fair Value | Other financial assets at amortised cost | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Other financial assets at amortised cost	£ 1,163	£ 7,056

Financial Instruments - Fair Va

Financial Instruments - Fair Values of Financial Instruments Measured at Fair Value on a Recurring Basis (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 12,564	£ 13,449
Liabilities at fair value	3,018	3,161
Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	1,434	1,713
Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	1,584	1,448
Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	3,406	3,316
Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	208	386
Financial assets at fair value through other comprehensive income
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	8,950	9,747
Level 1
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	8,501	9,209
Liabilities at fair value	0	0
Level 1 | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	0	0
Level 1 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	0	0
Level 1 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	0	0
Level 1 | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	0	0
Level 1 | Financial assets at fair value through other comprehensive income
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	8,501	9,209
Level 2
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	3,766	3,723
Liabilities at fair value	2,978	3,068
Level 2 | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	1,426	1,652
Level 2 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	1,552	1,416
Level 2 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	3,338	3,241
Level 2 | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	0	0
Level 2 | Financial assets at fair value through other comprehensive income
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	428	482
Level 3
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	297	517
Liabilities at fair value	40	93
Level 3 | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	8	61
Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	32	32
Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	68	75
Level 3 | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	208	386
Level 3 | Financial assets at fair value through other comprehensive income
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 21	56
Exchange Rate Contracts | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Valuation technique	A
Liabilities at fair value	£ 833	664
Exchange Rate Contracts | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 2,457	2,323
Valuation technique	A
Exchange Rate Contracts | Level 1 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	£ 0	0
Exchange Rate Contracts | Level 1 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	0	0
Exchange Rate Contracts | Level 2 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	833	660
Exchange Rate Contracts | Level 2 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	2,455	2,317
Exchange Rate Contracts | Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	0	4
Exchange Rate Contracts | Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 2	6
Interest Rate Contracts | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Valuation technique	A & C
Liabilities at fair value	£ 2,450	1,838
Interest Rate Contracts | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 2,580	1,924
Valuation technique	A & C
Interest Rate Contracts | Level 1 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	£ 0	0
Interest Rate Contracts | Level 1 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	0	0
Interest Rate Contracts | Level 2 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	2,447	1,836
Interest Rate Contracts | Level 2 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	2,566	1,915
Interest Rate Contracts | Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	3	2
Interest Rate Contracts | Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	14	9
Equity and Credit Contracts
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 51	52
Equity and Credit Contracts | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Valuation technique	B & D
Liabilities at fair value	£ 55	160
Equity and Credit Contracts | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 123	283
Valuation technique	B & D
Equity and Credit Contracts | Level 1 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	£ 0	0
Equity and Credit Contracts | Level 1 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	0	0
Equity and Credit Contracts | Level 2 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	26	134
Equity and Credit Contracts | Level 2 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	71	223
Equity and Credit Contracts | Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	29	26
Equity and Credit Contracts | Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	52	60
Netting | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	(1,754)	(1,214)
Netting | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	(1,754)	(1,214)
Netting | Level 1 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	0	0
Netting | Level 1 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	0	0
Netting | Level 2 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	(1,754)	(1,214)
Netting | Level 2 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	(1,754)	(1,214)
Netting | Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	0	0
Netting | Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	0	0
Debt securities | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 109	294
Valuation technique	A, B & D
Debt securities | Financial assets at fair value through other comprehensive income
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 8,929	9,691
Valuation technique	D
Debt securities | Level 1 | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 0	0
Debt securities | Level 1 | Financial assets at fair value through other comprehensive income
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	8,501	9,209
Debt securities | Level 2 | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	0	0
Debt securities | Level 2 | Financial assets at fair value through other comprehensive income
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	428	482
Debt securities | Level 3 | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	109	294
Debt securities | Level 3 | Financial assets at fair value through other comprehensive income
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	0	0
Equity securities | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value
Valuation technique	B
Equity securities | Level 1 | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value
Equity securities | Level 2 | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value
Equity securities | Level 3 | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value
Loans and advances to customers | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 99	92
Valuation technique	A
Loans and advances to customers | Financial assets at fair value through other comprehensive income
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 21	56
Valuation technique	D
Loans and advances to customers | Level 1 | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 0	0
Loans and advances to customers | Level 1 | Financial assets at fair value through other comprehensive income
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	0	0
Loans and advances to customers | Level 2 | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	0	0
Loans and advances to customers | Level 2 | Financial assets at fair value through other comprehensive income
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	0	0
Loans and advances to customers | Level 3 | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	99	92
Loans and advances to customers | Level 3 | Financial assets at fair value through other comprehensive income
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 21	56
Debt Securities | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Valuation technique	A
Liabilities at fair value	£ 1,057	1,105
Debt Securities | Level 1 | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	0	0
Debt Securities | Level 2 | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	1,051	1,099
Debt Securities | Level 3 | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	£ 6	6
Reverse repurchase agreements - non-trading | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Valuation technique	A
Liabilities at fair value
Reverse repurchase agreements - non-trading | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value
Valuation technique	A
Reverse repurchase agreements - non-trading | Level 1 | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value
Reverse repurchase agreements - non-trading | Level 1 | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value
Reverse repurchase agreements - non-trading | Level 2 | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value
Reverse repurchase agreements - non-trading | Level 2 | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value
Reverse repurchase agreements - non-trading | Level 3 | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value
Reverse repurchase agreements - non-trading | Level 3 | Other Financial assets at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value
Structured Deposits | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Valuation technique	A
Liabilities at fair value	£ 375	435
Structured Deposits | Level 1 | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	0	0
Structured Deposits | Level 2 | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	375	406
Structured Deposits | Level 3 | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	£ 0	29
Collateral and associated financial guarantees | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Valuation technique	D
Liabilities at fair value	£ 2	173
Collateral and associated financial guarantees | Level 1 | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	0	0
Collateral and associated financial guarantees | Level 2 | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	0	147
Collateral and associated financial guarantees | Level 3 | Other Financial Liabilities at fair value through profit or loss, category
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	£ 2	£ 26

Financial Instruments - Summary

Financial Instruments - Summary of Fair Value Adjustment (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of financial assets [line items]
Fair value adjustments	£ 29	£ 17
Bid-Offer and Trade Specific Adjustments
Disclosure of financial assets [line items]
Fair value adjustments	(8)	(12)
Uncertainty
Disclosure of financial assets [line items]
Fair value adjustments	23	17
Credit Risk Adjustment
Disclosure of financial assets [line items]
Fair value adjustments	11	6
Funding Fair Value Adjustment
Disclosure of financial assets [line items]
Fair value adjustments	3	6
Risk Related Adjustments
Disclosure of financial assets [line items]
Fair value adjustments	29	17
Model-Related
Disclosure of financial assets [line items]
Fair value adjustments	0	0
Day One Profit
Disclosure of financial assets [line items]
Fair value adjustments	£ 0	£ 0

Financial Instruments - Additio

Financial Instruments - Additional Information (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disclosure of financial assets [abstract]
Exposure to wrong-way risk	£ 0	£ 0
Fair value of financial instruments transferred from Level 1 to Level 2	0	0
Fair value of financial instruments transferred from Level 2 to Level 1	0	0
Fair value of financial instruments transferred from Level 2 to Level 3	0	0
Fair value of financial instruments transferred from Level 3 to Level 2	0	0
Fair value of financial instruments transferred from Level 1 to Level 2	0	0
Fair value of financial instruments transferred from Level 2 to Level 1	0	0
Fair value of financial instruments transferred from Level 2 to Level 3	0	0
Fair value of financial instruments transferred from Level 3 to Level 2	£ 0	£ 0

Financial Instruments - Analy_2

Financial Instruments - Analysis of Financial Instruments Valued Using Internal Models Based on Information Other Than Market Data (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of financial assets and liabilities [line items]
FVTPL	£ 208	£ 386
Financial assets at fair value through other comprehensive income	8,950	9,747
Total net assets	257	424
Derivative liabilities and other Level 3 liabilities, fair value movements recognised in profit/(loss)	0	0	£ 0
Total (expense)/ income	17	(5)	6
Level 3
Disclosure of financial assets and liabilities [line items]
Financial assets and liabilities	249	442
Derivative liabilities and other Level 3 liabilities, fair value movements recognised in profit/(loss)	11	(16)	7
Other Level 3 | Level 3
Disclosure of financial assets and liabilities [line items]
Other Level 3 assets	17	23
Other Level 3 liabilities	(9)	(41)
Financial asset liabilities	7	16	(2)
Derivative assets and other Level 3 assets, fair value movements recognised in profit/(loss)	(1)	(5)	1
Reversionary property securities | Equity and Credit Contracts
Disclosure of financial assets and liabilities [line items]
Derivative assets	51	52
Derivative assets and other Level 3 assets, fair value movements recognised in profit/(loss)	3	2	30
Reversionary property securities | Debt Securities
Disclosure of financial assets and liabilities [line items]
FVTPL	107	120
Derivative assets and other Level 3 assets, fair value movements recognised in profit/(loss)	6	(17)	(28)
Loans and advances to customers | Roll-up Mortgage Portfolio
Disclosure of financial assets and liabilities [line items]
FVTPL	56	51
Derivative assets and other Level 3 assets, fair value movements recognised in profit/(loss)	6	0	8
Loans and advances to customers | Other Loans
Disclosure of financial assets and liabilities [line items]
FVTPL	43	41
Derivative assets and other Level 3 assets, fair value movements recognised in profit/(loss)	3	1	2
Loans and advances to customers | Other Loans | Financial assets at fair value through other comprehensive income
Disclosure of financial assets and liabilities [line items]
Financial assets at fair value through other comprehensive income	21	56
Derivative assets and other Level 3 assets, fair value movements recognised in profit/(loss)	(4)	(2)	(5)
Credit linked notes | Debt Securities
Disclosure of financial assets and liabilities [line items]
FVTPL	2	174
Derivative assets and other Level 3 assets, fair value movements recognised in profit/(loss)	(16)	7	13
Property Related Options and Forwards | Equity Securities
Disclosure of financial assets and liabilities [line items]
Derivative liabilities	(29)	(26)
Derivative assets and other Level 3 assets, fair value movements recognised in profit/(loss)	(3)	0	0
Credit Protection Guarantees | Financial Guarantees
Disclosure of financial assets and liabilities [line items]
FVTPL	(2)	(26)
Derivative liabilities and other Level 3 liabilities, fair value movements recognised in profit/(loss)	£ 16	£ (7)	£ (13)

Financial Instruments - Assets

Financial Instruments - Assets and Liabilities Reconciliation of Fair Value Measurements in Level 3 of the Fair Value Hierarchy (Detail) - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of fair value measurement of assets [line items]
Beginning balance	£ 281,702	£ 283,372
Beginning balance	(265,685)	(267,463)
Total gains/(losses) recognised in profit or loss, fair value movements	0	0	£ 0
Transfers in	0	0
Transfers in	0	0
Transfers out	0	0
Transfers out	0	0
Ending Balance	292,332	281,702	283,372
Ending Balance	(276,396)	(265,685)	(267,463)
Level 3 | Fair Value Through P&L
Disclosure of fair value measurement of assets [line items]
Beginning balance	386	976
Total gains/(losses) recognised in profit or loss, fair value movements	(1)	(9)
Total gains/(losses) recognised in profit or loss, foreign exchange and other movements	(5)	6
Transfers in	0	11
Transfers out	0
Netting	(42)	(430)
Additions	0	188
Sales	(19)	0
Settlements	(111)	(356)
Ending Balance	208	386	976
Gains/(losses) recognised in profit or loss relating to assets and liabilities held at the end of the year	(6)	(3)
Sales, fair value measurement, assets	19	0
Level 3 | Financial assets at FVOCI
Disclosure of fair value measurement of assets [line items]
Beginning balance	56	73
Total gains/(losses) recognised in profit or loss, fair value movements	(4)	(2)
Total gains/(losses) recognised in profit or loss, foreign exchange and other movements	0	0
Transfers in	0	0
Transfers out	0
Netting	0	0
Additions	0	0
Sales	(19)	0
Settlements	(12)	(15)
Ending Balance	21	56	73
Gains/(losses) recognised in profit or loss relating to assets and liabilities held at the end of the year	(4)	(2)
Sales, fair value measurement, assets	19	0
Level 3 | Financial assets at Fair Value Class
Disclosure of fair value measurement of assets [line items]
Beginning balance	517	1,143
Total gains/(losses) recognised in profit or loss, fair value movements	5	7
Total gains/(losses) recognised in profit or loss, foreign exchange and other movements	(5)	6
Transfers in	1	11
Transfers out	0
Netting	(42)	(430)
Additions	2	190
Sales	(38)	0
Settlements	(143)	(410)
Ending Balance	297	517	1,143
Gains/(losses) recognised in profit or loss relating to assets and liabilities held at the end of the year	0	13
Sales, fair value measurement, assets	38	0
Level 3 | Derivative Financial Instruments
Disclosure of fair value measurement of assets [line items]
Beginning balance	75	94
Total gains/(losses) recognised in profit or loss, fair value movements	10	18
Total gains/(losses) recognised in profit or loss, foreign exchange and other movements	0	0
Transfers in	1	0
Transfers out	0
Netting	0	0
Additions	2	2
Sales	0	0
Settlements	(20)	(39)
Ending Balance	68	75	94
Gains/(losses) recognised in profit or loss relating to assets and liabilities held at the end of the year	10	18
Sales, fair value measurement, assets	0	0
Derivative Liabilities | Level 3
Disclosure of fair value measurement of assets [line items]
Beginning balance	(32)	(66)
Total gains/(losses) recognised in profit or loss, fair value movements	(6)	(6)
Total gains/(losses) recognised in profit or loss, foreign exchange and other movements	0	0
Transfers in	0	0
Transfers out	0
Netting	0	0
Additions	0	0
Sales	0	0
Settlements	6	40
Ending Balance	(32)	(32)	(66)
Gains/(losses) recognised in profit or loss relating to assets and liabilities held at the end of the year	(6)
Gains/(losses) recognised in profit or loss relating to assets and liabilities held at the end of the year		(6)
Sales	0	0
Fair Value Through P&L | Derivative Liabilities | Level 3
Disclosure of fair value measurement of assets [line items]
Beginning balance	(61)	(49)
Total gains/(losses) recognised in profit or loss, fair value movements	18	(6)
Total gains/(losses) recognised in profit or loss, foreign exchange and other movements	8	(6)
Transfers in	0	0
Transfers out	28
Netting	0	0
Additions	(2)	(3)
Sales	0	0
Settlements	1	3
Ending Balance	(8)	(61)	(49)
Gains/(losses) recognised in profit or loss relating to assets and liabilities held at the end of the year	26
Gains/(losses) recognised in profit or loss relating to assets and liabilities held at the end of the year		(12)
Sales	0	0
Financial Liabilities at Fair Value, Class | Derivative Liabilities | Level 3
Disclosure of fair value measurement of assets [line items]
Beginning balance	(93)	(115)
Total gains/(losses) recognised in profit or loss, fair value movements	12	(12)
Total gains/(losses) recognised in profit or loss, foreign exchange and other movements	8	(6)
Transfers in	0	0
Transfers out	28
Netting	0	0
Additions	(2)	(3)
Sales	0	0
Settlements	7	43
Ending Balance	(40)	(93)	£ (115)
Gains/(losses) recognised in profit or loss relating to assets and liabilities held at the end of the year	20
Gains/(losses) recognised in profit or loss relating to assets and liabilities held at the end of the year		(18)
Sales	£ 0	£ 0

Financial Instruments - Effects

Financial Instruments - Effects of Changes in Significant Unobservable Assumptions to Reasonably Possible Alternatives (Level 3) (Detail) £ in Millions	12 Months Ended
	Dec. 31, 2020GBP (£)Index	Dec. 31, 2019GBP (£)Index
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Fair value	£ 12,564	£ 13,449
Fair value	(3,018)	(3,161)
Equity and Credit Contracts
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Fair value	£ 51	£ 52
Assumption description	HPI Forward growth rate	HPI Forward growth rate
Shift	1.00%	1.00%
Favourable changes	£ 8	£ 8
Unfavourable changes	£ (8)	£ (8)
Equity and Credit Contracts | Reversionary property derivatives
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Shift	10.00%	10.00%
Favourable changes	£ 7	£ 7
Unfavourable changes	£ (7)	£ (7)
HPI spot rate index level | Index	445	802
Equity Contracts [Member]
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Fair value	£ (29)	£ (26)
Assumption description	HPI Forward growth rate	HPI Forward growth rate
Shift	1.00%	1.00%
Favourable changes	£ 2	£ 2
Unfavourable changes	£ (2)	£ (2)
Equity Contracts [Member] | Property Related Options and Forwards
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Shift	10.00%	10.00%
Favourable changes	£ 3	£ 3
Unfavourable changes	£ (3)	£ (3)
HPI spot rate index level | Index	433	758
Bottom of range | Equity and Credit Contracts
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	0.00%	0.00%
Bottom of range | Equity Contracts [Member] | Property Related Options and Forwards
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	0.00%	0.00%
Top of range | Equity and Credit Contracts
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	5.00%	5.00%
Top of range | Equity Contracts [Member] | Property Related Options and Forwards
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	5.00%	5.00%
Weighted Average | Equity and Credit Contracts
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	2.57%	2.57%
Weighted Average | Equity Contracts [Member]
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	2.42%	2.44%
Fair Value Through P&L | Debt Securities
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Fair value	£ 107	£ 120
Assumption description	HPI Forward growth rate	HPI Forward growth rate
Shift	1.00%	1.00%
Favourable changes	£ 1	£ 0
Unfavourable changes	£ (1)	£ 0
Fair Value Through P&L | Debt Securities | Reversionary property securities
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Shift	10.00%	10.00%
Favourable changes	£ 5	£ 6
Unfavourable changes	£ (5)	£ (6)
HPI spot rate index level | Index	445	802
Fair Value Through P&L | Loans and advances to customers | Roll-up Mortgage Portfolio
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Fair value	£ 56	£ 51
Assumption description	HPI Forward growth rate	HPI Forward growth rate
Shift	1.00%	1.00%
Favourable changes	£ 2	£ 2
Unfavourable changes	(2)	(2)
Fair Value Through P&L | Loans and advances to customers | Other Loans
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Fair value	£ 43	£ 41
Assumption description	Credit spreads	Credit spreads
Shift	0.20%	20.00%
Favourable changes	£ 0	£ 0
Unfavourable changes	£ 0	£ 0
Fair Value Through P&L | Bottom of range | Debt Securities
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	0.00%	0.00%
Fair Value Through P&L | Bottom of range | Loans and advances to customers | Roll-up Mortgage Portfolio
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	0.00%	0.00%
Fair Value Through P&L | Bottom of range | Loans and advances to customers | Other Loans
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	0.07%	0.00%
Fair Value Through P&L | Top of range | Debt Securities
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	5.00%	5.00%
Fair Value Through P&L | Top of range | Loans and advances to customers | Roll-up Mortgage Portfolio
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	5.00%	5.00%
Fair Value Through P&L | Top of range | Loans and advances to customers | Other Loans
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	1.55%	1.00%
Fair Value Through P&L | Weighted Average | Debt Securities
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	2.57%	2.57%
Fair Value Through P&L | Weighted Average | Loans and advances to customers | Roll-up Mortgage Portfolio
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	2.69%	2.69%
Fair Value Through P&L | Weighted Average | Loans and advances to customers | Other Loans
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	0.44%	0.35%
Financial assets at fair value through other comprehensive income
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Fair value	£ 8,950	£ 9,747
Financial assets at fair value through other comprehensive income | Loans and advances to customers
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Fair value	£ 21	£ 56
Assumption description	Credit spreads	Credit spreads
Shift	20.00%	20.00%
Favourable changes	£ 0
Unfavourable changes	£ 0
Financial assets at fair value through other comprehensive income | Loans and advances to customers | Other Loans
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Favourable changes		£ 0
Unfavourable changes		£ 0
Financial assets at fair value through other comprehensive income | Bottom of range | Loans and advances to customers | Other Loans
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	0.15%	0.00%
Financial assets at fair value through other comprehensive income | Top of range | Loans and advances to customers | Other Loans
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	0.53%	1.00%
Financial assets at fair value through other comprehensive income | Weighted Average | Loans and advances to customers
Effect of changes in significant unobservable assumptions to reasonably possible alternatives - Level 3 [line items]
Assumption value	0.32%	0.51%

Financial Instruments - Maturit

Financial Instruments - Maturities of Undiscounted Cash Flows for Financial Liabilities and Off Balance Sheet Commitments (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure Of Maturity Analysis For Financial Liabilities [line items]
Other financial liabilities at fair value through profit or loss	£ 1,434	£ 1,713
Deposits by customers	195,135	181,883
Deposits by banks	20,958	14,353
Repurchase agreements - non trading	15,848	18,286
Debt securities in issue	35,566	41,129
Subordinated liabilities	2,556	3,528
Lease liabilities	97	137
Undiscounted Cash Flow
Disclosure Of Maturity Analysis For Financial Liabilities [line items]
Derivative financial instruments	1,632	1,495
Other financial liabilities at fair value through profit or loss	1,437	1,796
Deposits by customers	195,563	182,655
Deposits by banks	21,360	14,519
Repurchase agreements - non trading	15,850	18,308
Debt securities in issue	36,513	42,828
Subordinated liabilities	4,061	5,870
Lease liabilities	109	155
Total financial liabilities	276,525	267,626
Off-balance sheet commitments given(1)	43,160	41,595
Undiscounted Cash Flow | On Demand
Disclosure Of Maturity Analysis For Financial Liabilities [line items]
Derivative financial instruments	0	9
Other financial liabilities at fair value through profit or loss	0	1
Deposits by customers	176,911	160,833
Deposits by banks	2,410	2,711
Repurchase agreements - non trading	0	6
Debt securities in issue	0	0
Subordinated liabilities	0	0
Lease liabilities	0	0
Total financial liabilities	179,321	163,560
Off-balance sheet commitments given(1)	21,054	20,158
Undiscounted Cash Flow | Not Later Than Three Months
Disclosure Of Maturity Analysis For Financial Liabilities [line items]
Derivative financial instruments	279	243
Other financial liabilities at fair value through profit or loss	1	6
Deposits by customers	3,443	2,531
Deposits by banks	1,912	486
Repurchase agreements - non trading	15,350	15,878
Debt securities in issue	4,993	7,129
Subordinated liabilities	30	239
Lease liabilities	0	0
Total financial liabilities	26,008	26,512
Off-balance sheet commitments given(1)	4,493	2,927
Undiscounted Cash Flow | Later Than Three Months and Not Later Than One Year
Disclosure Of Maturity Analysis For Financial Liabilities [line items]
Derivative financial instruments	133	293
Other financial liabilities at fair value through profit or loss	107	203
Deposits by customers	5,453	6,476
Deposits by banks	2,596	4,764
Repurchase agreements - non trading	500	1,578
Debt securities in issue	9,897	8,702
Subordinated liabilities	92	131
Lease liabilities	17	45
Total financial liabilities	18,795	22,192
Off-balance sheet commitments given(1)	4,974	5,742
Undiscounted Cash Flow | Later Than One Year and Not Later Than Five Years
Disclosure Of Maturity Analysis For Financial Liabilities [line items]
Derivative financial instruments	582	418
Other financial liabilities at fair value through profit or loss	570	617
Deposits by customers	7,483	9,770
Deposits by banks	14,230	6,338
Repurchase agreements - non trading	0	846
Debt securities in issue	14,083	18,100
Subordinated liabilities	1,515	1,539
Lease liabilities	56	58
Total financial liabilities	38,519	37,686
Off-balance sheet commitments given(1)	11,493	11,490
Undiscounted Cash Flow | >5 years
Disclosure Of Maturity Analysis For Financial Liabilities [line items]
Derivative financial instruments	638	532
Other financial liabilities at fair value through profit or loss	759	969
Deposits by customers	2,273	3,045
Deposits by banks	212	220
Repurchase agreements - non trading	0	0
Debt securities in issue	7,540	8,897
Subordinated liabilities	2,424	3,961
Lease liabilities	36	52
Total financial liabilities	13,882	17,676
Off-balance sheet commitments given(1)	£ 1,146	£ 1,278

Offsetting Financial Assets a_3

Offsetting Financial Assets and Liabilities - Summary of Information about Impact of Offsetting of Financial Assets and Liabilities (Detail) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019
Disclosure of offsetting of financial assets and liabilities [line items]
Gross amounts, assets	£ 38,609	£ 35,005
Amounts offset, assets	(9,312)	(4,138)
Net amounts reported on the balance sheet, assets	29,297	30,867
Financial instruments, assets	(911)	(1,305)
Financial collateral, assets	21,310	25,014
Net amount, assets	7,076	4,548
Assets not subject to enforceable netting arrangements, assets	204,140	205,227
Balance sheet total, assets	233,437	236,094
Gross amounts, liabilities	36,753	33,973
Amounts offset, liabilities	(9,312)	(4,138)
Net amounts reported on the balance sheet, liabilities	27,441	29,835
Financial instruments, liabilities	(911)	(1,305)
Financial collateral, liabilities	16,772	18,823
Net amount, liabilities	9,758	9,707
Assets not subject to enforceable netting arrangements, liabilities	206,084	186,135
Balance sheet total, liabilities	233,525	215,970
Derivative Financial Assets
Disclosure of offsetting of financial assets and liabilities [line items]
Gross amounts, assets	5,071	4,446
Amounts offset, assets	(1,754)	(1,214)
Net amounts reported on the balance sheet, assets	3,317	3,232
Financial instruments, assets	(782)	(768)
Financial collateral, assets	1,840	1,915
Net amount, assets	695	549
Assets not subject to enforceable netting arrangements, assets	89	84
Balance sheet total, assets	3,406	3,316
Amortised cost
Disclosure of offsetting of financial assets and liabilities [line items]
Gross amounts, assets	26,084	25,312
Amounts offset, assets	(6,485)	(1,676)
Net amounts reported on the balance sheet, assets	19,599	23,636
Financial instruments, assets	(129)	(537)
Financial collateral, assets	19,470	23,099
Net amount, assets	0	0
Assets not subject to enforceable netting arrangements, assets	0	0
Balance sheet total, assets	19,599	23,636
Gross amounts, liabilities	22,333	19,962
Amounts offset, liabilities	(6,485)	(1,676)
Net amounts reported on the balance sheet, liabilities	15,848	18,286
Financial instruments, liabilities	(129)	(537)
Financial collateral, liabilities	15,719	17,749
Net amount, liabilities	0	0
Assets not subject to enforceable netting arrangements, liabilities	0	0
Balance sheet total, liabilities	15,848	18,286
Fair Value
Disclosure of offsetting of financial assets and liabilities [line items]
Gross amounts, assets	0	0
Amounts offset, assets	0	0
Net amounts reported on the balance sheet, assets	0	0
Financial instruments, assets	0	0
Financial collateral, assets	0	0
Net amount, assets	0	0
Assets not subject to enforceable netting arrangements, assets	0	0
Balance sheet total, assets	0	0
Gross amounts, liabilities	0	0
Amounts offset, liabilities	0	0
Net amounts reported on the balance sheet, liabilities	0	0
Financial instruments, liabilities	0	0
Financial collateral, liabilities	0	0
Net amount, liabilities	0	0
Assets not subject to enforceable netting arrangements, liabilities	0	0
Balance sheet total, liabilities	0	0
Loans and advances to customers
Disclosure of offsetting of financial assets and liabilities [line items]
Gross amounts, assets	7,454	5,247
Amounts offset, assets	(1,073)	(1,248)
Net amounts reported on the balance sheet, assets	6,381	3,999
Financial instruments, assets	0	0
Financial collateral, assets	0	0
Net amount, assets	6,381	3,999
Assets not subject to enforceable netting arrangements, assets	204,051	205,143
Balance sheet total, assets	210,432	209,142
Derivative Financial Liabilities
Disclosure of offsetting of financial assets and liabilities [line items]
Gross amounts, liabilities	3,261	2,616
Amounts offset, liabilities	(1,754)	(1,214)
Net amounts reported on the balance sheet, liabilities	1,507	1,402
Financial instruments, liabilities	(782)	(768)
Financial collateral, liabilities	551	572
Net amount, liabilities	174	62
Assets not subject to enforceable netting arrangements, liabilities	77	46
Balance sheet total, liabilities	1,584	1,448
Deposits by customers and banks
Disclosure of offsetting of financial assets and liabilities [line items]
Gross amounts, liabilities	11,159	11,395
Amounts offset, liabilities	(1,073)	(1,248)
Net amounts reported on the balance sheet, liabilities	10,086	10,147
Financial instruments, liabilities	0	0
Financial collateral, liabilities	502	502
Net amount, liabilities	9,584	9,645
Assets not subject to enforceable netting arrangements, liabilities	206,007	186,089
Balance sheet total, liabilities	£ 216,093	£ 196,236

Events After the Balance Shee_2

Events After the Balance Sheet Date (Details)	12 Months Ended
	Dec. 31, 2020
Disclosure of non-adjusting events after reporting period [abstract]
Description of nature of non-adjusting event after reporting period	There have been no significant events between 31 December 2020 and the date of approval of these financial statements which would require a change to or additional disclosure in the financial statements.

Interest Rate Benchmark Refor_2

Interest Rate Benchmark Reform - Amounts Impacted by IBOR Reform (Details) - GBP (£) £ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure of expected impact of initial application of new standards or interpretations [abstract]
Derivative financial instruments	£ 3,406	£ 3,316
Other financial assets at fair value through profit or loss	208	386
Other financial assets at amortised cost	1,163	7,056
Financial assets at fair value through other comprehensive income	8,950	9,747
Assets	292,332	281,702	£ 283,372
Derivative financial instruments	1,584	1,448
Other financial liabilities at fair value through profit or loss	1,434	1,713
Liabilities	276,396	£ 265,685	£ 267,463
GBP LIBOR
Disclosure of expected impact of initial application of new standards or interpretations [abstract]
Derivative financial instruments	33,486
Other financial assets at fair value through profit or loss	968
Other financial assets at amortised cost	15,062
Financial assets at fair value through other comprehensive income	428
Assets	49,944
Derivative financial instruments	35,217
Other financial liabilities at fair value through profit or loss	1,129
Financial liabilities at amortised cost	2,354
Liabilities	38,700
Off-balance sheet commitments given(1)	11,400
USD LIBOR
Disclosure of expected impact of initial application of new standards or interpretations [abstract]
Derivative financial instruments	4,514
Other financial assets at fair value through profit or loss	22
Other financial assets at amortised cost	1,191
Financial assets at fair value through other comprehensive income	0
Assets	5,727
Derivative financial instruments	5,205
Other financial liabilities at fair value through profit or loss	69
Financial liabilities at amortised cost	1,319
Liabilities	6,593
Off-balance sheet commitments given(1)	2,126
Other
Disclosure of expected impact of initial application of new standards or interpretations [abstract]
Derivative financial instruments	2,149
Other financial assets at fair value through profit or loss	0
Other financial assets at amortised cost	90
Financial assets at fair value through other comprehensive income	0
Assets	2,239
Derivative financial instruments	88
Other financial liabilities at fair value through profit or loss	0
Financial liabilities at amortised cost	0
Liabilities	88
Off-balance sheet commitments given(1)	573
Total
Disclosure of expected impact of initial application of new standards or interpretations [abstract]
Derivative financial instruments	40,149
Other financial assets at fair value through profit or loss	990
Other financial assets at amortised cost	16,343
Financial assets at fair value through other comprehensive income	428
Assets	57,910
Derivative financial instruments	40,510
Other financial liabilities at fair value through profit or loss	1,198
Financial liabilities at amortised cost	3,673
Liabilities	45,381
Off-balance sheet commitments given(1)	£ 14,099

Interest Rate Benchmark Refor_3

Interest Rate Benchmark Reform - Derivatives Directly Affected (Details) number in Millions	Dec. 31, 2020
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	54,395
Maturing after transition date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	26,474
GBP LIBOR
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	47,421
GBP LIBOR | Maturing after transition date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	23,030
USD LIBOR
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	6,196
USD LIBOR | Maturing after transition date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	2,724
Other
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	778
Other | Maturing after transition date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	720
Cash flow hedges
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	20,317
Cash flow hedges | Maturing after transition date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	13,115
Cash flow hedges | GBP LIBOR
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	15,198
Cash flow hedges | GBP LIBOR | Maturing after transition date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	10,553
Cash flow hedges | USD LIBOR
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	5,119
Cash flow hedges | USD LIBOR | Maturing after transition date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	2,562
Cash flow hedges | Other
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	0
Cash flow hedges | Other | Maturing after transition date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	0
Fair value hedges
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	34,078
Fair value hedges | Maturing after transition date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	13,359
Fair value hedges | GBP LIBOR
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	32,223
Fair value hedges | GBP LIBOR | Maturing after transition date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	12,477
Fair value hedges | USD LIBOR
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	1,077
Fair value hedges | USD LIBOR | Maturing after transition date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	162
Fair value hedges | Other
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	778
Fair value hedges | Other | Maturing after transition date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	720

Transfer of Mortgages to Sant_2

Transfer of Mortgages to Santander Financial Services PLC (Details) - Santander Financial Services PLC - Mortgages - GBP (£) £ in Millions	1 Months Ended	12 Months Ended
	Oct. 31, 2020	Dec. 31, 2020
Disclosure of financial assets [line items]
Book value of financial assets transferred	£ 3,163
Proceeds from transfer of financial assets	3,174
Price premium on transfer of financial assets	11	£ 11
Significant costs related to transfer of financial assets	£ 0
Allowance account for credit losses of financial assets, reversal for transfer of financial assets		0.3
Loss on reversal of hedge accounting for transferred assets		14
Gain (Loss) On Transfer Of Financial Assets, Net		£ 3