Santander UK (STNDF) 6-K 17 Aug 21 2021 Q2 Current report (foreign) Financial data

Cover Page

Cover Page	6 Months Ended
	Jun. 30, 2021
Cover [Abstract]
Document Type	6-K
Amendment Flag	false
Document Period End Date	Jun. 30, 2021
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
Entity Registrant Name	Santander UK plc
Entity Central Index Key	0001087711
Current Fiscal Year End Date	--12-31

Condensed Consolidated Income S

Condensed Consolidated Income Statement - GBP (£) £ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	[1]
Profit (loss) [abstract]
Interest and similar income	£ 2,355	£ 2,592
Interest expense and similar charges	(450)	(1,064)
Net interest income	1,905	1,528
Fee and commission income	305	380
Fee and commission expense	(154)	(180)
Net fee and commission income	151	200
Other operating income	135	100
Total operating income	2,191	1,828
Operating expenses before credit impairment losses, provisions and charges	(1,328)	(1,212)
Credit impairment losses	70	(364)
Provisions for other liabilities and charges	(190)	(64)
Total operating credit impairment losses, provisions and charges	(120)	(428)
Profit from continuing operations before tax	743	188
Tax on profit from continuing operations	(205)	(48)
Profit from continuing operations after tax	538	140
Profit/(loss) from discontinued operations after tax	24	(1)
Profit after tax	562	139
Attributable to:
Equity holders of the parent	545	128
Non-controlling interests	17	11
Profit after tax	£ 562	£ 139
[1]	Adjusted to reflect the presentation of discontinued operations as set out in Note 33.

Condensed Consolidated Statemen

Condensed Consolidated Statement of Comprehensive Income - GBP (£) £ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Statement of comprehensive income [abstract]
Profit after tax	£ 562	£ 139	[1]
Movement in fair value reserve (debt instruments):
Change in fair value	(62)	146
Income statement transfers	59	(166)
Taxation	(1)	5
Other comprehensive income, fair value reserve (debt instruments)	(4)	(15)
Cash flow hedges:
Effective portion of changes in fair value	(618)	2,607
Income statement transfers	377	(2,041)
Taxation	41	(161)
Other comprehensive income, cash flow hedges, total	(200)	405
Currency translation on foreign operations	0	(1)
Net other comprehensive income/(expense) that may be reclassified to profit or loss subsequently	(204)	389
Pension remeasurement:
Change in fair value	745	(376)
Taxation	(248)	97
Pension remeasurement, total	497	(279)
Own credit adjustment:
Change in fair value	0	20
Taxation	0	(5)
Own credit adjustment, total	0	15
Net other comprehensive income/(expense) that will not be reclassified to profit or loss subsequently	497	(264)
Total other comprehensive income net of tax	293	125
Total comprehensive income	855	264
Attributable to:
Equity holders of the parent	837	254
Non-controlling interests	18	10
Total comprehensive income	£ 855	£ 264
[1]	Adjusted to reflect the presentation of discontinued operations as set out in Note 33.

Condensed Consolidated Balance

Condensed Consolidated Balance Sheet - GBP (£) £ in Millions	Jun. 30, 2021	Dec. 31, 2020
Assets
Cash and balances at central banks	£ 39,021	£ 41,250
Financial assets at fair value through profit or loss:
Derivative financial instruments	2,002	3,406
Other financial assets at fair value through profit or loss	179	208
Financial assets at amortised cost:
Loans and advances to customers	207,998	208,750
Loans and advances to banks	1,312	1,682
Reverse repurchase agreements – non trading	18,197	19,599
Other financial assets at amortised cost	530	1,163
Financial assets at fair value through other comprehensive income	6,368	8,950
Interests in other entities	183	172
Intangible assets	1,589	1,646
Property, plant and equipment	1,596	1,734
Current tax assets	282	264
Retirement benefit assets	1,083	495
Other assets	2,629	3,013
Assets held for sale	2,645	0
Total assets	285,614	292,332
Financial liabilities at fair value through profit or loss:
Derivative financial instruments	1,063	1,584
Other financial liabilities at fair value through profit or loss	1,023	1,434
Financial liabilities at amortised cost:
Deposits by customers	193,317	195,135
Deposits by banks	20,502	20,958
Repurchase agreements – non trading	14,089	15,848
Debt securities in issue	29,140	35,566
Subordinated liabilities	2,513	2,556
Other liabilities	2,365	2,337
Provisions	444	464
Deferred tax liabilities	386	111
Retirement benefit obligations	152	403
Liabilities held for sale	3,912	0
Total liabilities	268,906	276,396
Equity
Share capital	3,105	3,105
Share premium	5,620	5,620
Other equity instruments	2,191	2,191
Retained earnings	5,306	4,348
Other reserves	306	510
Total shareholders’ equity	16,528	15,774
Non-controlling interests	180	162
Total equity	16,708	15,936
Total liabilities and equity	£ 285,614	£ 292,332

Condensed Consolidated Cash Flo

Condensed Consolidated Cash Flow Statement - GBP (£) £ in Millions		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020
Cash flows from operating activities
Profit after tax		£ 562	£ 139	[1]
Adjustments for:
Non-cash items included in profit		645	778
Change in operating assets		251	(4,172)
Change in operating liabilities	[2]	2,725	10,697
Corporation taxes paid		(153)	(110)
Effects of exchange rate differences		(534)	891
Net cash flows from operating activities		3,496	8,223
Cash flows from investing activities
Purchase of property, plant and equipment and intangible assets		(350)	(151)
Proceeds from sale of property, plant and equipment and intangible assets		272	92
Purchase of financial assets at amortised cost and financial assets at fair value through other comprehensive income		(1,016)	(2,626)
Proceeds from sale and redemption of financial assets at amortised cost and financial assets at fair value through other comprehensive income		3,789	2,581
Net cash flows from investing activities		2,695	(104)
Cash flows from financing activities
Issue of other equity instruments		210	0
Issue of debt securities and subordinated notes		36	4,097
Issuance costs of debt securities and subordinated notes		(3)	(10)
Repayment of debt securities and subordinated notes		(8,031)	(6,133)
Repurchase of other equity instruments		(210)	0
Dividends paid on preference shares and other equity instruments		(83)	(82)
Principal elements of lease payments	[2]	(13)	(28)
Net cash flows from financing activities		(8,094)	(2,156)
Change in cash and cash equivalents		(1,903)	5,963
Cash and cash equivalents at beginning of the period		45,582	27,817
Effects of exchange rate changes on cash and cash equivalents		(10)	94
Cash and cash equivalents at the end of the period		43,669	33,874
Cash and cash equivalents consist of:
Cash and balances at central banks		39,021	30,276
Less: regulatory minimum cash balances		(897)	(772)
Cash and bank balances at central banks less regulatory minimum cash balances		38,124	29,504
Other cash equivalents		5,545	4,370
Cash and cash equivalents at the end of the period		£ 43,669	£ 33,874
[1]	Adjusted to reflect the presentation of discontinued operations as set out in Note 33.
[2]	For H121, Principal elements of lease payments are included as a separate line item in the Condensed Consolidated Cash Flow Statement. As a result, cash flows of £28m for H120 have been reclassified from 'Change in operating liabilities' within operating activities to 'Principal elements of lease payments' within financing activities.

Condensed Consolidated Cash F_2

Condensed Consolidated Cash Flow Statement (Parenthetical) £ in Millions	6 Months Ended
	Jun. 30, 2020GBP (£)
Statement [Line Items]
Principal elements of lease payments	£ 28	[1]
Change in operating liabilities	10,697	[1]
Change in operating liabilities
Statement [Line Items]
Change in operating liabilities	£ 28
[1]	For H121, Principal elements of lease payments are included as a separate line item in the Condensed Consolidated Cash Flow Statement. As a result, cash flows of £28m for H120 have been reclassified from 'Change in operating liabilities' within operating activities to 'Principal elements of lease payments' within financing activities.

Condensed Consolidated Statem_2

Condensed Consolidated Statement of Changes in Equity - GBP (£) £ in Millions	Total		Total	Share capital	Share premium	Other equity instruments	Fair value	Cash flow hedging	Currency translation	Retained earnings	Non-controlling interests
Beginning balance at Dec. 31, 2019	£ 16,017		£ 15,857	£ 3,105	£ 5,620	£ 2,191	£ 23	£ 371	£ 1	£ 4,546	£ 160
Profit after tax	139	[1]	128							128	11
Other comprehensive income, net of tax:
Fair value reserve (debt instruments)	(15)		(15)				(15)
Cash flow hedges	405		405					405
Pension remeasurement	(279)		(278)							(278)	(1)
Own credit adjustment	15		15							15
Currency translation on foreign operations	(1)		(1)						(1)
Total comprehensive income	264		254				(15)	405	(1)	(135)	10
Dividends on preference shares and other equity instruments	(82)		(82)							(82)
Ending balance at Jun. 30, 2020	16,199		16,029	3,105	5,620	2,191	8	776	0	4,329	170
Beginning balance at Dec. 31, 2020	15,936		15,774	3,105	5,620	2,191	28	481	1	4,348	162
Profit after tax	562		545							545	17
Other comprehensive income, net of tax:
Fair value reserve (debt instruments)	(4)		(4)				(4)
Cash flow hedges	(200)		(200)					(200)
Pension remeasurement	497		496							496	1
Own credit adjustment	0
Total comprehensive income	855		837				(4)	(200)	0	1,041	18
Issue of other equity instruments	210		210			210
Repurchase of other equity instruments	(210)		(210)			(210)
Dividends on preference shares and other equity instruments	(83)		(83)							(83)
Ending balance at Jun. 30, 2021	£ 16,708		£ 16,528	£ 3,105	£ 5,620	£ 2,191	£ 24	£ 281	£ 1	£ 5,306	£ 180
[1]	Adjusted to reflect the presentation of discontinued operations as set out in Note 33.

Accounting Policies

Accounting Policies	6 Months Ended
	Jun. 30, 2021
Disclosure Of Significant Accounting Policies [Abstract]
Accounting Policies	1. ACCOUNTING POLICIES The financial information in these Condensed Consolidated Interim Financial Statements is unaudited and does not constitute statutory accounts as defined in section 434 of the UK Companies Act 2006. Statutory accounts for the year ended 31 December 2020 have been delivered to the Registrar of Companies. The Condensed Consolidated Interim Financial Statements reflect all adjustments that, in the opinion of management, are necessary for a fair statement of the results of operations for the interim period. All such adjustments to the financial information are of a normal, recurring nature. Because the results from common banking activities are so closely related and responsive to changes in market conditions, the results for any interim period are not necessarily indicative of the results that can be expected for the year. On 31 December 2020, International Financial Reporting Standards (IFRSs) as adopted by the European Union at that date was brought into UK law and became UK-adopted International Accounting Standards (IAS), with future changes being subject to endorsement by the UK Endorsement Board. Santander UK plc (the Company) and its subsidiaries (collectively Santander UK or the Santander UK group) transitioned to UK-adopted International Accounting Standards in its consolidated financial statements on 1 January 2021. This change constitutes a change in accounting framework. However, there is no impact on recognition, measurement or disclosure in the period reported as a result of the change in framework. The Condensed Consolidated Interim Financial Statements have been prepared in accordance with UK adopted IAS 34 ‘Interim Financial Reporting', IAS 34 ‘Interim Financial Reporting’ as issued by the International Accounting Standards Board (IASB), IAS 34 ‘Interim Financial Reporting’ as adopted by the EU, and the Disclosure Guidance and Transparency Rules sourcebook of the UK's Financial Conduct Authority (FCA). They do not include all the information and disclosures normally required for full annual financial statements and should be read in conjunction with the Consolidated Financial Statements of Santander UK for the year ended 31 December 2020 which were prepared in accordance with international accounting standards in conformity with the requirements of the Companies Act 2006 and IFRSs adopted pursuant to Regulation (EC) No 1606/2002 as it applies in the European Union. Those consolidated financial statements were also prepared in accordance with IFRSs as issued by the IASB, including interpretations issued by the IFRS Interpretations Committee, as there were no applicable differences from IFRSs as issued by the IASB for the periods presented. At 30 June 2021, there were no differences between IFRSs adopted by the UK, IFRSs issued by the IASB, and IFRSs adopted by the EU in terms of their application to Santander UK. The financial statements for Santander UK for the year ended 31 December 2021 will be prepared in accordance with IFRS as adopted by the UK, IFRSs as issued by the IASB, including interpretations issued by the IFRS Interpretations Committee, and IFRSs as adopted by the EU. Except as noted below, the same accounting policies, presentation and methods of computation are followed in these Condensed Consolidated Interim Financial Statements as were applied in the presentation of Santander UK’s 2020 Annual Report. Non-current assets held for sale Non-current assets (or disposal groups) are classified as held for sale when their carrying amount is to be recovered principally through a sale transaction rather than continuing use. In order to be classified as held for sale, the asset must be available for immediate sale in its present condition subject only to terms that are usual and customary and the sale must be highly probable. Non-current assets (or disposal groups) held for sale are measured at the lower of carrying amount and fair value less cost to sell, with the exception of financial instruments which remain governed by the requirements of IFRS 9, and are held at their IFRS 9 carrying value. For details of the disposal of assets and liabilities in connection with the transfer of the Corporate & Investment Banking (CIB) business to the London Branch of Banco Santander SA (SLB), and for details on the sale of Retail Banking residential mortgage assets, see Note 33. Discontinued operations A discontinued operation is a component of the Santander UK group that has been disposed of or is classified as held for sale and that represents a separate major line of business or geographical area of operations, is part of a single co-ordinated plan to dispose of such a line of business or area of operations, or is a subsidiary acquired exclusively with a view to resale. The results of discontinued operations are presented separately in the Condensed Consolidated Income Statement. Prior periods have been restated accordingly. Future accounting developments At 30 June 2021, for the Santander UK group, there were no significant new or revised standards and interpretations, and amendments thereto, which have been issued, including those which are not yet effective or which have otherwise not been early adopted where permitted. Going concern In light of the continuing economic uncertainty, the Directors updated their going concern assessment in preparing these Condensed Consolidated Interim Financial Statements. After making enquiries, the Directors have a reasonable expectation that Santander UK has adequate resources to continue in operational existence for at least twelve months from the date of this report and, therefore, having reassessed the principal risks and uncertainties, the Directors consider it appropriate for the Condensed Consolidated Interim Financial Statements to be prepared on a going concern basis. In making their going concern assessment in connection with preparing the Condensed Consolidated Interim Financial Statements, the Directors considered a wide range of information that included Santander UK’s long-term business and strategic plans, forecasts and projections, estimated capital, funding and liquidity requirements, contingent liabilities and the reasonably possible changes in trading performance arising from potential economic, market and product developments. The Directors' assessment specifically considered the impacts of Covid-19, including for the following areas: – Economic scenarios and probability weights: These underpin our ECL impairment allowances and are discussed in detail in the Credit risk section of the Risk review. The Directors reviewed the economic scenarios to ensure that they captured the wide range of potential outcomes for the UK economy. Whereas the outlook for 2021 is more positive following the success of the vaccination programme and cautious optimism that the worst of the health crisis has passed, the downside scenarios continue to encapsulate different potential outcomes including higher-for-longer unemployment, and emergence of Covid-19 variants that are more resistant to existing vaccines leading to further lockdowns and a slower recovery that is more akin to the ‘U’ shape of past recessions. – Liquidity: Santander UK remains in a strong liquidity position and Covid-19 did not trigger a liquidity stress. – Capital: Santander UK remains strongly capitalised and Covid-19 did not trigger a capital stress. – Customers: Santander UK has supported many thousands of individuals and businesses affected by Covid-19 with a range of measures, including payment holidays on mortgages, personal loans and credit cards as well as taking an active part in UK Government loan schemes to help businesses. The majority of customers who took payment holidays have resumed payments. We continue to work with those customers who remain affected to understand their individual situations and, where necessary, help them resume payments. – Operations: All our operations continue to operate effectively with a significant majority of staff working from home and the majority of branches remaining open and returning to more normal hours. The current operating model could be sustained indefinitely with additional resilience being continuously implemented and is not affecting our ability to operate all our services, or raising concerns about our Business Continuity Planning, and – Key suppliers: Suppliers continue to be closely monitored in line with our Third Party Risk Management Framework. No significant service issues have been reported across our cohort of critical suppliers, and no material issues have been reported across our broader (non-critical) supply chain. Isolated supplier impacts have been seen, but these are being managed. CRITICAL JUDGEMENTS AND ACCOUNTING ESTIMATES The preparation of the Condensed Consolidated Interim Financial Statements requires management to make judgements and accounting estimates that affect the reported amount of assets and liabilities at the date of the Condensed Consolidated Interim 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 the course of preparing the Condensed Consolidated Interim 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 losses Key judgements – Determining an appropriate definition of default – Establishing the criteria for a significant increase in credit risk (SICR) and, for corporate borrowers, internal credit risk rating – Applying post model adjustments – Assessing individual corporate Stage 3 exposures Key estimates – Forward-looking multiple economic scenario assumptions – Probability weights assigned to multiple economic scenarios For more on each of these key judgements and estimates, including the impact of Covid-19 on them, see 'Management judgements and estimates 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 Key judgements – Determining whether a present obligation exists – Assessing the likely outcome of future legal decisions Key estimates – Probability, timing, nature and amount of any outflows that may arise from past events Note 26 'Contingent liabilities and commitments' 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. These judgements 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, or to calculate any resulting sensitivities. For more on each of these key judgements and estimates, see Notes 24 and 26. c) Pensions Key judgements – Setting the criteria for constructing the corporate bond yield curve used to determine the discount rate – Determining the methodology for setting the inflation assumption Key estimates – Discount rate applied to future cash flows – Rate of price inflation – Expected lifetime of the schemes' members For more on each of these key judgements and estimates, including the impact of Covid-19 on them, see Note 25. Sensitivity of defined benefit pension scheme estimates For detailed disclosures see ‘Actuarial assumption sensitivities’ in Note 25. d) Goodwill Key judgements: – Determining the basis of goodwill impairment calculation assumptions, including management's planning assumptions considering internal capital allocations needed to support Santander UK's strategy, current market conditions and the macro-economic outlook. Key estimates: – Forecast cash flows for cash generating units, including estimated allocations of regulatory capital – Growth rate beyond initial cash flow projections – Discount rates which factor in risk-free rates and applicable risk premiums These are variables subject to fluctuations in external market rates and economic conditions beyond management’s control For more on each of these key judgements and estimates, see Note 16. Sensitivity of goodwill

Segments

Segments	6 Months Ended
	Jun. 30, 2021
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. As reflected in the 2020 Annual Report, the segmental basis of presentation in these Condensed Consolidated Interim Financial Statements changed following a management review of our structure. At 31 December 2020 this resulted in customer assets of £2.0bn and customer deposits of £3.1bn being transferred from Business Banking (in Retail Banking) to CCB, non-core corporate mortgages of £0.4bn transferring from Corporate Centre to CCB, and a number of smaller business lines transferring from CIB to Corporate Centre. For H120, this resulted in an increase in profit before tax in Retail Banking of £28m, a decrease in CCB of £31m, a decrease in CIB of £4m, and an increase of £7m in Corporate Centre. The net impact for Santander UK was nil. Proposed transfer of the CIB business to SLB Following a hearing held on 23 June 2021, the High Court of England and Wales has sanctioned the transfer of substantially all the CIB business of Santander UK to SLB by way of a Part VII banking business transfer scheme. The transfers of CIB business to SLB under the scheme are scheduled to take place before the end of 2021. Management considered the related business transfers to be highly probable at 30 June 2021. As such, the Santander UK group reclassified as held for sale the assets and liabilities relating to the CIB business. For more details, see Note 33. At 30 June 2021, the CIB business met the requirements for presentation as discontinued operations. For more details, see Note 33. Results by segment Half year to Retail Banking Corporate & Commercial Banking Corporate & Investment Banking Corporate Centre Total 30 June 2021 £m £m £m £m £m Net interest income/(expense) 1,741 203 — (39) 1,905 Non-interest income 189 51 — 46 286 Total operating income 1,930 254 — 7 2,191 Operating expenses before credit impairment losses, provisions and charges (922) (177) — (229) (1,328) Credit impairment losses 48 22 — — 70 Provisions for other liabilities and charges (67) (5) — (118) (190) Total operating credit impairment losses, provisions and charges (19) 17 — (118) (120) Profit/(loss) from continuing operations before tax 989 94 — (340) 743 Revenue from external customers 2,197 273 — (279) 2,191 Inter-segment revenue (267) (19) — 286 — Total operating income 1,930 254 — 7 2,191 Revenue from external customers includes the following fee and commission income disaggregated by income type: (1) – Current account and debit card fees 185 23 — — 208 – Insurance, protection and investments 33 — — — 33 – Credit cards 29 — — — 29 – Non-banking and other fees (2) 6 29 — — 35 Total fee and commission income 253 52 — — 305 Fee and commission expense (140) (10) — (4) (154) Net fee and commission income/(expense) 113 42 — (4) 151 30 June 2021 Customer loans 186,706 17,055 1,913 2,673 208,347 Total assets (3) 195,480 17,055 2,057 71,022 285,614 Of which assets held for sale 608 — 2,037 — 2,645 Customer deposits 156,713 25,244 3,817 3,596 189,370 Total liabilities 157,268 25,263 3,912 82,463 268,906 Of which liabilities held for sale — — 3,912 — 3,912 (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. Half year to (4)(5) Retail Banking Corporate & Commercial Banking Corporate & Investment Banking Corporate Centre Total 30 June 2020 £m £m £m £m £m Net interest income/(expense) 1,369 181 — (22) 1,528 Non-interest income 227 49 — 24 300 Total operating income/(expense) 1,596 230 — 2 1,828 Operating expenses before credit impairment losses, provisions and charges (983) (161) — (68) (1,212) Credit impairment losses (214) (150) — — (364) Provisions for other liabilities and charges (56) 3 — (11) (64) Total operating credit impairment losses, provisions and charges (270) (147) — (11) (428) Profit/(loss) from continuing operations before tax 343 (78) — (77) 188 Revenue from external customers 1,973 278 — (424) 1,827 Inter-segment revenue (377) (48) — 426 1 Total operating income/(expense) 1,596 230 — 2 1,828 Revenue from external customers includes the following fee and commission income disaggregated by income type: (1) – Current account and debit card fees 245 21 — — 266 – Insurance, protection and investments 31 — — — 31 – Credit cards 35 — — — 35 – Non-banking and other fees (2) 20 24 — 4 48 Total fee and commission income 331 45 — 4 380 Fee and commission expense (164) (12) — (4) (180) Net fee and commission income 167 33 — — 200 31 December 2020 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 (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 (4) Restated, as reflected in Note 2 to the Consolidated Financial Statements in the 2020 Annual Report, following a management review of our structure. (5) Restated to reflect the presentation of discontinued operations, as set out in Note 33. Geographical information is not provided, as substantially all of Santander UK’s activities are in the UK. Sale of Retail Banking mortgage assets At 30 June 2021, Santander UK was in the process of selling a portfolio of Retail Banking residential mortgage assets and expected to complete the transaction in August 2021. Management considered the sale to be highly probable at 30 June 2021. As such, the Santander UK group reclassified as held for sale the mortgage assets. For more details, see Note 33.

Other Operating Income

Other Operating Income	6 Months Ended
	Jun. 30, 2021
Net Trading And Other Income [abstract]
Other Operating Income	3. OTHER OPERATING INCOME In H121, the Santander UK group repurchased certain debt securities as part of ongoing liability management exercises, resulting in a loss of £27m. In H120, the Santander UK group repurchased certain debt securities and subordinated liabilities as part of ongoing liability management exercises, resulting in a loss of £17m. In H121, the Santander UK group sold its UK head office site in Triton Square, London to a wholly owned subsidiary of its parent, Banco Santander SA, given the decision to invest in a new Milton Keynes campus. Other operating income includes a £71m gain on sale of the UK head office site. The net gain reflects a sale price based on independent valuations.

Operating Expenses Before Credi

Operating Expenses Before Credit Impairment Losses, Provisions and Charges	6 Months Ended
	Jun. 30, 2021
Expenses by nature [abstract]
Operating Expenses Before Credit Impairment Losses, Provisions and Charges	4. OPERATING EXPENSES BEFORE CREDIT IMPAIRMENT LOSSES, PROVISIONS AND CHARGES Half year to Half year to (1) 30 June 2021 30 June 2020 £m £m Staff costs 577 576 Other administration expenses 431 367 Depreciation, amortisation and impairment 320 269 Total 1,328 1,212 (1) Adjusted to reflect the presentation of discontinued operations as set out in Note 33. 'Other administration expenses' includes £10m (H120: £4m) expenses related to short-term leases.

Credit Impairment Losses and Pr

Credit Impairment Losses and Provisions	6 Months Ended
	Jun. 30, 2021
Disclosure Of Credit Impairment Loss And Provisions [Abstract]
Credit Impairment Losses and Provisions	5. CREDIT IMPAIRMENT LOSSES AND PROVISIONS Half year to Half year to (1) 30 June 2021 30 June 2020 £m £m Credit impairment loss/(release): Loans and advances to customers (33) 384 Recoveries of loans and advances, net of collection costs (13) (15) Off-balance sheet exposures (See Note 24) (24) (5) (70) 364 Provisions for other liabilities and charges (excluding off-balance sheet credit exposures) (See Note 24) 191 64 Provisions for residual value and voluntary termination (1) — 190 64 120 428 (1) Adjusted to reflect the presentation of discontinued operations as set out in Note 33. In H121 and H120 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	6 Months Ended
	Jun. 30, 2021
Major components of tax expense (income) [abstract]
Taxation	6. TAXATION The Santander UK group’s effective tax rate for H121 was 27.6% (H120: 25.5%). 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: Half year to Half year to (1) 30 June 2021 30 June 2020 £m £m Profit from continuing operations before tax 743 188 Tax calculated at a tax rate of 19% (H120: 19%) 141 36 Bank surcharge on profits 49 9 Non-deductible preference dividends paid 6 3 Non-deductible UK Bank Levy 14 6 Other non-deductible costs and non-taxable income 14 4 Effect of change in tax rate on deferred tax provision 13 7 Tax relief on dividends in respect of other equity instruments (32) (13) Adjustment to prior year provisions — (4) Tax on profit from continuing operations 205 48 (1) Adjusted to reflect the presentation of discontinued operations as set out in Note 33. The UK government announced in its budget on 3 March 2021 that it would increase the main rate of corporation tax by 6% to 25% with effect from 1 April 2023. This change was substantively enacted on 24 May 2021 and, as a result, the effect has been reflected in the closing deferred tax position included in these financial statements. The comparative 2020 results reflected an increase in tax rates by 2% following an announcement in the 2020 budget to reverse a previously planned rate reduction from April 2020.

Dividends on Ordinary Shares

Dividends on Ordinary Shares	6 Months Ended
	Jun. 30, 2021
Dividends On Ordinary Shares [Abstract]
Dividends on Ordinary Shares	7. DIVIDENDS ON ORDINARY SHARES No interim dividend was declared on the Company’s ordinary shares in issue (H120: £nil).

Derivative Financial Instrument

Derivative Financial Instruments	6 Months Ended
	Jun. 30, 2021
Disclosure of nature and extent of risks arising from financial instruments [abstract]
Derivative Financial Instruments	8. DERIVATIVE FINANCIAL INSTRUMENTS The table below includes the notional amounts of transactions outstanding at the balance sheet date; they do not represent actual exposures. 30 June 2021 31 December 2020 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 12,227 229 215 14,951 395 418 Interest rate contracts 31,912 545 440 40,160 888 542 Equity and credit contracts 1,106 141 53 1,140 123 55 Total derivatives held for trading 45,245 915 708 56,251 1,406 1,015 Derivatives held for hedging Designated as fair value hedges: Exchange rate contracts 598 52 1 789 84 6 Interest rate contracts 84,311 981 1,319 93,748 1,225 1,885 84,909 1,033 1,320 94,537 1,309 1,891 Designated as cash flow hedges: Exchange rate contracts 23,756 1,131 381 27,020 1,978 409 Interest rate contracts 26,609 307 38 19,407 467 23 50,365 1,438 419 46,427 2,445 432 Total derivatives held for hedging 135,274 2,471 1,739 140,964 3,754 2,323 Derivative netting (1) (1,384) (1,384) (1,754) (1,754) Total derivatives 180,519 2,002 1,063 197,215 3,406 1,584 (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 £401m (2020: £330m) and the amount of cash collateral paid that had been offset against the gross derivative liabilities was £245m (2020: £651m). IBOR Reform Note 32 includes details of the notional value of hedging instruments by benchmark interest rate impacted by IBOR reform and the notional amounts of assets, liabilities and off-balance sheet commitments affected by IBOR reform that have yet to transition to an alternative benchmark interest rate.

Other Financial Assets At Fair

Other Financial Assets At Fair Value Through Profit Or Loss	6 Months Ended
	Jun. 30, 2021
Financial assets at fair value through profit or loss [abstract]
Other Financial Assets At Fair Value Through Profit Or Loss	9. OTHER FINANCIAL ASSETS AT FAIR VALUE THROUGH PROFIT OR LOSS 30 June 2021 31 December 2020 £m £m Loans and advances to customers: Loans to housing associations 12 13 Other loans 62 86 74 99 Debt securities 105 109 179 208

Loans and Advances to Customers

Loans and Advances to Customers	6 Months Ended
	Jun. 30, 2021
Net Loans And Advances To Customers [Abstract]
Loans and Advances to Customers	10. LOANS AND ADVANCES TO CUSTOMERS 30 June 2021 31 December 2020 £m £m Loans and advances to customers 209,188 210,107 Credit impairment loss allowances on loans and advances to customers (1,141) (1,303) RV and voluntary termination provisions on finance leases (49) (54) Net loans and advances to customers 207,998 208,750 For movements in expected credit losses, see the 'Movement in total exposures and the corresponding ECL' table in the Santander UK group level - Credit risk review section of the Risk review.

Securitisations and Covered Bon

Securitisations and Covered Bonds	6 Months Ended
	Jun. 30, 2021
Disclosure Of Securitisations And Covered Bonds [Abstract]
Securitisations and Covered Bonds	11. 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. 30 June 2021 31 December 2020 £m £m Mortgage-backed master trust structures: – Holmes 2,566 3,073 – Fosse 2,170 2,258 – Langton — 2,782 4,736 8,113 Other asset-backed securitisation structures: – Motor 103 189 – Auto ABS UK Loans 1,325 1,460 1,428 1,649 Total securitisation programmes 6,164 9,762 Covered bond programmes – Euro 35bn Global Covered Bond Programme 19,000 23,670 Total securitisation and covered bond programmes 25,164 33,432 The following table sets out the internal and external issuances and redemptions in H121 and H120 for each securitisation and covered bond programme. Internal issuances External issuances Internal redemptions External redemptions H121 H120 H121 H120 H121 H120 H121 H120 £bn £bn £bn £bn £bn £bn £bn £bn Mortgage-backed master trust structures: – Holmes — — — — — 0.1 0.3 0.5 – Langton — — — — 2.4 — — — Other asset-backed securitisation structures: – Motor — — — — 0.1 0.1 — 0.1 – Auto ABS UK Loans — — — — 0.1 — 0.1 0.1 Covered bond programme — — — 3.0 — — 4.5 1.8 — — — 3.0 2.6 0.2 4.9 2.5 In H121, the remaining Langton bonds were redeemed and the associated mortgages were repurchased by Santander UK plc.

Reverse Repurchase Agreements -

Reverse Repurchase Agreements - Non Trading	6 Months Ended
	Jun. 30, 2021
Disclosure Of Reserve Repurchase Agreements Non-Trading [Abstract]
Reverse Repurchase Agreements - Non Trading	12. REVERSE REPURCHASE AGREEMENTS – NON TRADING 30 June 2021 31 December 2020 £m £m Agreements with banks 1,250 1,258 Agreements with customers 16,947 18,341 18,197 19,599

Other Financial Assets At Amort

Other Financial Assets At Amortised Cost	6 Months Ended
	Jun. 30, 2021
Financial assets at fair value through profit or loss [abstract]
Other Financial Assets At Amortised Cost	13. OTHER FINANCIAL ASSETS AT AMORTISED COST 30 June 2021 31 December 2020 £m £m Asset backed securities 467 491 Debt securities 63 672 530 1,163

Financial Assets At Fair Value

Financial Assets At Fair Value Through Other Comprehensive Income	6 Months Ended
	Jun. 30, 2021
Financial assets at fair value through other comprehensive income [abstract]
Financial Assets At Fair Value Through Other Comprehensive Income	14. FINANCIAL ASSETS AT FAIR VALUE THROUGH OTHER COMPREHENSIVE INCOME 30 June 2021 31 December 2020 £m £m Debt securities 6,348 8,929 Loans and advances to customers 20 21 6,368 8,950

Interests in Other Entities

Interests in Other Entities	6 Months Ended
	Jun. 30, 2021
Investments accounted for using equity method [abstract]
Interests in Other Entities	15. INTERESTS IN OTHER ENTITIES There have been no significant changes to the Santander UK group's interests in subsidiaries, associates, joint ventures and unconsolidated structured entities, as set out in Note 19 to the Consolidated Financial Statements in the 2020 Annual Report.

Intangible Assets

Intangible Assets	6 Months Ended
	Jun. 30, 2021
Disclosure of detailed information about intangible assets [abstract]
Intangible Assets	16. INTANGIBLE ASSETS Intangible assets comprise goodwill and computer software. Goodwill The following CGUs include in their carrying values goodwill that comprises the goodwill reported by Santander UK. The CGUs do not carry on their balance sheets any other intangible assets with indefinite useful lives. For key assumptions used in the value in use (VIU) calculation, as the basis of the recoverable amount of each CGU, see Note 20 to the Consolidated Financial Statements in the 2020 Annual Report. At 30 June 2021 a review was performed to identify any potential impairment indicators. No indicators of impairment were identified and so a full impairment test was not performed for the half year. The key assumptions are set out in the table below. Goodwill Discount rate Growth rate beyond initial cash flow projections 30 June 2021 31 December 2020 30 June 2021 31 December 2020 30 June 2021 31 December 2020 CGU £m £m % % % % Personal financial services 1,169 1,169 14.6 13.6 1.6 1.6 Private banking 30 30 11.0 8.9 1.6 1.6 Other 4 4 14.6 13.6 1.6 1.6 1,203 1,203

Property, Plant and Equipment

Property, Plant and Equipment	6 Months Ended
	Jun. 30, 2021
Property, plant and equipment [abstract]
Property, Plant and Equipment	17. PROPERTY, PLANT AND EQUIPMENT Property Office fixtures and equipment Computer software Operating lease assets Right-of-use assets Total (2) £m £m £m £m £m £m Cost: At 1 January 2021 1,272 1,375 436 720 218 4,021 Additions 23 12 — 194 59 288 Disposals (306) (267) — (122) (20) (715) At 30 June 2021 989 1,120 436 792 257 3,594 Accumulated depreciation: At 1 January 2021 489 1,068 434 178 118 2,287 Charge for the period (1) 17 46 — 48 10 121 Impairment during the period 57 29 — — 19 105 Disposals (210) (236) — (50) (19) (515) At 30 June 2021 353 907 434 176 128 1,998 Net book value 636 213 2 616 129 1,596 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 (1) 55 111 — 92 58 316 Impairment during the year 24 — — — — 24 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 (1) Following a review of the estimated useful lives of property as part of Santander UK's transformation program, the charge for the period includes accelerated property depreciation of £4m (2020: £9m). (2) Property, plant and equipment includes assets under construction of £72m (2020: £55m). Property, office fixtures and equipment and right-of-use assets were impaired in the period as a result of our multi-year transformation project. The impairment relates to leasehold properties within the scope of our branch network restructuring programme and head office sites which are either closing or consolidating.

Other Financial Liabilities at

Other Financial Liabilities at Fair Value Through Profit or Loss	6 Months Ended
	Jun. 30, 2021
Disclosure of financial liabilities [abstract]
Other Financial Liabilities at Fair Value Through Profit or Loss	18. OTHER FINANCIAL LIABILITIES AT FAIR VALUE THROUGH PROFIT OR LOSS 30 June 2021 31 December 2020 £m £m US$30bn Euro Medium Term Note Programme 96 102 Structured Notes Programmes 431 805 Eurobonds 144 150 Structured deposits 347 375 Collateral and associated financial guarantees 5 2 1,023 1,434

Deposits by Customers

Deposits by Customers	6 Months Ended
	Jun. 30, 2021
Deposits from customers [abstract]
Deposits by Customers	19. DEPOSITS BY CUSTOMERS 30 June 2021 31 December 2020 £m £m Demand and time deposits (1) 185,179 185,879 Amounts due to other Santander UK Group Holdings plc subsidiaries 67 59 Amounts due to Santander UK Group Holdings plc (2) 6,912 7,883 Amounts due to fellow Banco Santander subsidiaries and joint ventures 1,159 1,314 193,317 195,135 (1) Includes equity index-linked deposits of £578m (2020: £577m). The capital amount guaranteed/protected and the amount of return guaranteed in respect of the equity index-linked deposits were £578m and £2m (2020: £577m and £2m) respectively. (2) Includes downstreamed funding from our immediate parent company Santander UK Group Holdings plc.

Deposits by Banks

Deposits by Banks	6 Months Ended
	Jun. 30, 2021
Disclosure Of Deposits From Banks [Abstract]
Deposits by Banks	20. DEPOSITS BY BANKS 30 June 2021 31 December 2020 £m £m Items in the course of transmission 497 375 Deposits held as collateral 1,034 2,063 Other deposits (1) 18,951 18,519 Amounts due to Santander UK subsidiaries 20 1 20,502 20,958 (1) Includes drawdown from the TFS of £3.3bn (2020: £6.3bn) and drawdown from the TFSME of £15.2bn (2020: £11.7bn).

Repurchase Agreements - Non Tra

Repurchase Agreements - Non Trading	6 Months Ended
	Jun. 30, 2021
Disclosure Of Repurchase Agreement [Abstract]
Repurchase Agreements - Non Trading	21. REPURCHASE AGREEMENTS – NON TRADING 30 June 2021 31 December 2020 £m £m Agreements with banks 2,898 6,358 Agreements with customers 11,191 9,490 14,089 15,848

Debt Securities in Issue

Debt Securities in Issue	6 Months Ended
	Jun. 30, 2021
Debt Securities In Issue [Abstract]
Debt Securities in Issue	22. DEBT SECURITIES IN ISSUE 30 June 2021 31 December 2020 £m £m Medium-term notes 6,908 8,212 Euro 35bn Global Covered Bond Programme 14,677 19,285 US$20bn Commercial Paper Programmes 3,052 2,824 Certificates of deposit 2,466 2,858 Credit linked notes 58 57 Securitisation programmes 1,979 2,330 29,140 35,566

Subordinated Liabilities

Subordinated Liabilities	6 Months Ended
	Jun. 30, 2021
Disclosure Of Subordinated Liabilities [Abstract]
Subordinated Liabilities	23. SUBORDINATED LIABILITIES 30 June 2021 31 December 2020 £m £m 2,513 2,556

Provisions

Provisions	6 Months Ended
	Jun. 30, 2021
Disclosure of other provisions [abstract]
Provisions	24. PROVISIONS 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 2021 76 8 34 45 75 226 464 Additional provisions (See Note 5) — — — 43 — 148 191 Provisions released (See Note 5) — — — — (24) — (24) Utilisation and other (39) — (34) (9) — (105) (187) At 30 June 2021 37 8 — 79 51 269 444 Conduct remediation Payment Protection Insurance (PPI) At 30 June 2021, the remaining provision for PPI redress and related costs was £37m (2020: £76m). There was no additional provision in H121. At 30 June 2021, the outstanding activities which drive the ongoing provision requirement relate to the final aspects of complaints close down activity and the commercial settlement with the Official Receiver. Customers continue to 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. 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.

Retirement Benefit Plans

Retirement Benefit Plans	6 Months Ended
	Jun. 30, 2021
Disclosure of net defined benefit liability (asset) [abstract]
Retirement Benefit Plans	25. RETIREMENT BENEFIT PLANS The amounts recognised in the balance sheet were as follows: 30 June 2021 31 December 2020 £m £m Assets/(liabilities) Funded defined benefit pension scheme - surplus 1,083 495 Funded defined benefit pension scheme - deficit (113) (361) Unfunded pension and post retirement medical benefits (39) (42) Total net assets 931 92 An expense of £33m (H120: £33m) was recognised for defined contribution plans in the period and is included in staff costs classified within operating expenses (see Note 4). b) Defined benefit pension schemes The amounts recognised in other comprehensive income were as follows: 30 June 2021 30 June 2020 £m £m Return on plan assets (excluding amounts included in net interest expense) 383 (1,115) Actuarial losses arising from changes in demographic assumptions — 38 Actuarial gains arising from experience adjustments (30) (10) Actuarial (gains)/losses arising from changes in financial assumptions (1,098) 1,463 Pension remeasurement (745) 376 The net assets recognised in the balance sheet were determined as follows: 30 June 2021 31 December 2020 £m £m Present value of defined benefit obligations (12,675) (13,887) Fair value of scheme assets 13,606 13,979 Net defined benefit assets 931 92 Actuarial assumptions The principal actuarial assumptions used for the defined benefit schemes were: 30 June 2021 31 December 2020 % % To determine benefit obligations: – Discount rate for scheme liabilities 1.9 1.3 – General price inflation 3.2 3.0 – General salary increase 1.0 1.0 – Expected rate of pension increase 3.1 2.9 Years Years Longevity at 60 for current pensioners, on the valuation date: – Males 27.5 27.5 – Females 30.1 30.0 Longevity at 60 for future pensioners currently aged 40, on the valuation date: – Males 29.1 29.0 – Females 31.6 31.5 In March 2021, the Trustee entered into a longevity swap. Approximately 85% of pensioner liabilities were covered by the longevity swap at inception. The value of the swap at 30 June 2021 was nil. 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 30 June 2021 31 December 2020 Assumption Change in pension obligation at period end from £m £m Discount rate 25 bps increase (573) (662) General price inflation (1) 25 bps increase 411 480 Mortality Each additional year of longevity assumed 453 515 (1) The comparative figure for general price inflation sensitivity of £365m as at 31 December 2020 has been restated. This was due to an error and does not impact the actual reported assumption value.

Contingent Liabilities and Comm

Contingent Liabilities and Commitments	6 Months Ended
	Jun. 30, 2021
Disclosure of contingent liabilities [abstract]
Contingent Liabilities and Commitments	26. CONTINGENT LIABILITIES AND COMMITMENTS 30 June 2021 31 December 2020 £m £m Guarantees given to third parties 739 939 Formal standby facilities, credit lines and other commitments 40,957 42,221 41,696 43,160 Where the items set out below can be reliably estimated, they are disclosed in the table above. 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 SA in 2008 and subsequently transferred to Santander UK plc) and a Banco Santander SA 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. During H121, AXA France commenced litigation against the Santander Entities seeking recovery of £636m and any further losses relating to pre-2005 PPI. The Santander Entities acknowledged service indicating their intention to defend the claim in full and have now issued an application for AXA France’s claim to be struck out/summarily dismissed. 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 24 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 24 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 H121 we 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. Other provisions include an amount of £41m (2020: £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 24 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 have been 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. During H121, an outline agreement in principle in respect of a number of these lease arrangements was reached directly between the lessee and HMRC. Certain other lease arrangements, where the tax liabilities are considered immaterial, remain under review. 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. These matters moved to formal litigation in 2020, as required under the terms of the leases, and it is currently anticipated that hearings will be held at the First Tier Tax Tribunal in 2022. Other 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 judgement 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.

Other Equity Instruments

Other Equity Instruments	6 Months Ended
	Jun. 30, 2021
Disclosure of classes of share capital [abstract]
Other Equity Instruments	27. OTHER EQUITY INSTRUMENTS Interest rate 30 June 2021 31 December 2020 % 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 n/a — 210 - £500m Fixed Rate Reset Perpetual AT1 Capital Securities 6.30 March 2025 500 500 - £210m Fixed Rate Reset Perpetual AT1 Capital Securities 4.25 March 2026 210 — 2,191 2,191 In March 2021, Santander UK plc purchased and redeemed the remaining £210m of the original £500m 5.18%Fixed Rate Reset Perpetual AT1 Capital Securities and issued £210m 4.25% Fixed Rate Reset Perpetual AT1 Capital Securities, which were fully subscribed by the Company’s immediate parent company, Santander UK Group Holdings plc.

Non-Controlling Interests

Non-Controlling Interests	6 Months Ended
	Jun. 30, 2021
Disclosure Of Non Controlling Interests [Abstract]
Non-Controlling Interests	28. NON-CONTROLLING INTERESTS 30 June 2021 31 December 2020 £m £m PSA Finance UK Limited 180 162 180 162

Assets Charged as Security for

Assets Charged as Security for Liabilities and Collateral Accepted as Security for Assets	6 Months Ended
	Jun. 30, 2021
Disclosure Of Assets Pledged As Security [Abstract]
Assets Charged as Security for Liabilities and Collateral Accepted as Security for Assets	29. ASSETS CHARGED AS SECURITY FOR LIABILITIES AND COLLATERAL ACCEPTED AS SECURITY FOR ASSETS Securitisations and covered bonds As described in Note 14 to the Consolidated Financial Statements in the 2020 Annual Report, Santander UK plc and certain of its subsidiaries issue securitisations and covered bonds. At 30 June 2021, there were £25,164m (2020: £33,432m) of gross assets in these secured programmes, of which £130m (2020: £2,294m) related to internally retained issuances that were available for use as collateral for liquidity purposes in the future. At 30 June 2021, a total of £2,011m (2020: £4,530m) 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 £500m at 30 June 2021 (2020: £1,114m), or for use as collateral for liquidity purposes in the future.

Related Party Disclosures

Related Party Disclosures	6 Months Ended
	Jun. 30, 2021
Disclosure of transactions between related parties [abstract]
Related Party Disclosures	30. RELATED PARTY DISCLOSURES The financial position and performance of the Santander UK group have not been materially affected in H121 by any related party transactions, or changes to related party transactions, except for the classification of the assets and liabilities of the CIB business as held for sale and its results as discontinued operations, given the transfer of the CIB business to SLB, as set out in Note 33. These transactions were made in the ordinary course of business, 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, transactions with pension schemes operated by the Santander UK group are described in Note 30 to the Consolidated Financial Statements in the 2020 Annual Report.

Financial Instruments

Financial Instruments	6 Months Ended
	Jun. 30, 2021
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Abstract]
Financial Instruments	31. 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 to the Consolidated Financial Statements in the 2020 Annual Report describes how the classes of financial instruments are measured, and how income and expenses, including fair value gains and losses, are recognised. b) Fair values of financial instruments carried at amortised cost The following tables analyse the fair value of the financial instruments carried at amortised cost at 30 June 2021 and 31 December 2020. It does not include fair value information for financial assets and financial liabilities carried at amortised cost if the carrying amount is a reasonable approximation of fair value. Details of the valuation methodology of the financial assets and financial liabilities carried at amortised cost can be found in Note 40(e) to the Consolidated Financial Statements in the 2020 Annual Report. 30 June 2021 31 December 2020 Carrying Carrying Fair value value Fair value value £m £m £m £m Assets Loans and advances to customers 210,815 207,998 211,279 208,750 Loans and advances to banks 1,312 1,312 1,682 1,682 Reverse repurchase agreements - non trading 18,198 18,197 19,608 19,599 Other financial assets at amortised cost 539 530 1,192 1,163 Assets held for sale (1) 2,493 2,486 — — 233,357 230,523 233,761 231,194 Liabilities Deposits by customers 193,389 193,317 195,242 195,135 Deposits by banks 20,503 20,502 20,967 20,958 Repurchase agreements - non trading 14,088 14,089 15,847 15,848 Debt securities in issue 29,890 29,140 36,397 35,566 Subordinated liabilities 3,080 2,513 3,069 2,556 Liabilities held for sale (1) 3,790 3,790 — — 264,740 263,351 271,522 270,063 (1) Assets and liabilities classified as held for sale are measured in accordance with IFRS 9. Any assets and liabilities held for sale which are measured at fair value are included in the fair value table in section (c) below. c) 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 30 June 2021 and 31 December 2020, analysed by their levels in the fair value hierarchy - Level 1, Level 2 and Level 3. 30 June 2021 31 December 2020 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 — 1,403 1 1,404 — 2,455 2 2,457 A Interest rate contracts — 1,841 — 1,841 — 2,566 14 2,580 A & C Equity and credit contracts — 93 48 141 — 71 52 123 B & D Netting — (1,384) — (1,384) — (1,754) — (1,754) — 1,953 49 2,002 — 3,338 68 3,406 Other financial assets at FVTPL Loans and advances to customers — — 74 74 — — 99 99 A Debt securities — — 105 105 — — 109 109 A, B & D — — 179 179 — — 208 208 Financial assets at FVOCI Debt securities 6,160 188 — 6,348 8,501 428 — 8,929 D Loans and advances to customers — — 20 20 — — 21 21 D 6,160 188 20 6,368 8,501 428 21 8,950 Assets held for sale (1) — 143 16 159 — — — — Total assets at fair value 6,160 2,284 264 8,708 8,501 3,766 297 12,564 Liabilities Derivative financial instruments Exchange rate contracts — 565 — 565 — 833 — 833 A Interest rate contracts — 1,827 2 1,829 — 2,447 3 2,450 A & C Equity and credit contracts — 23 30 53 — 26 29 55 B & D Netting — (1,384) — (1,384) — (1,754) — (1,754) — 1,031 32 1,063 — 1,552 32 1,584 Other financial liabilities at FVTPL Debt securities in issue — 666 5 671 — 1,051 6 1,057 A Structured deposits — 347 — 347 — 375 — 375 A Collateral and associated financial guarantees — 3 2 5 — — 2 2 D — 1,016 7 1,023 — 1,426 8 1,434 Liabilities held for sale (1) — 122 — 122 — — — — Total liabilities at fair value — 2,169 39 2,208 — 2,978 40 3,018 (1) Includes assets and liabilities held for sale which are measured at fair value. Transfers between levels of the fair value hierarchy In H121 there were no significant (H120: no significant) transfers of financial instruments between levels of the fair value hierarchy. d) Valuation techniques The main valuation techniques employed in internal models to measure the fair value of the financial instruments are disclosed in Note 40(c) to the Consolidated Financial Statements in the 2020 Annual Report. The Santander UK group did not make any material changes to the valuation techniques and internal models it used during H121. e) 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: 30 June 2021 31 December 2020 £m £m Risk-related: - Bid-offer and trade specific adjustments (7) (8) - Uncertainty 21 23 - Credit risk adjustment 7 11 - Funding fair value adjustment 2 3 23 29 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. For more details, see ‘Risk-related adjustments’ in Note 40(g) to the Consolidated Financial Statements in the 2020 Annual Report. f) Internal models based on information other than market data (Level 3) Valuation techniques There have been no significant changes to the valuation techniques as set out in Note 40(h) to the Consolidated Financial Statements in the 2020 Annual Report. 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 H121 and H120: Assets Liabilities Derivatives Other financial assets at FVTPL Financial assets at FVOCI Assets held for sale Total Derivatives Other financial liabilities at FVTPL Liabilities held for sale Total £m £m £m £m £m £m £m £m £m At 1 January 2021 68 208 21 — 297 (32) (8) — (40) Total (losses)/gains recognised: - Fair value movements (1) (7) (1) — (9) — 3 — 3 Transfers to held for sale — (16) — 16 — — (2) — (2) Netting (1) — 5 — — 5 — — — — Settlements (18) (11) — — (29) — — — — At 30 June 2021 49 179 20 16 264 (32) (7) — (39) Gains/(losses) recognised in profit or loss/other comprehensive income relating to assets and liabilities held at the end of the period (1) (7) (1) — (9) — 3 — 3 At 1 January 2020 75 386 56 — 517 (32) (61) — (93) Total gains/(losses) recognised: - Fair value movements 6 (2) 1 — 5 (1) 15 — 14 - Foreign exchange and other movements — (2) — — (2) — 2 — 2 Transfers out — — — — — — 28 — 28 Netting (1) — (37) — — (37) — — — — Additions — — — — — — (1) — (1) Sales — (11) (19) — (30) — — — — Settlements (17) (110) (8) — (135) 3 1 — 4 At 30 June 2020 64 224 30 — 318 (30) (16) — (46) Gains/(losses) recognised in profit or loss/other comprehensive income relating to assets and liabilities held at the end of the period 6 (4) 1 — 3 (1) 17 — 16 (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 to the Consolidated Financial Statements in the 2020 Annual Report. Effect of changes in significant unobservable assumptions to reasonably possible alternatives (Level 3)

Interest Rate Benchmark Reform

Interest Rate Benchmark Reform	6 Months Ended
	Jun. 30, 2021
Interest Rate Benchmark Reform [Abstract]
Interest Rate Benchmark Reform	32. INTEREST RATE BENCHMARK REFORM The following tables show the notional amounts of assets, liabilities and off-balance sheet commitments at 30 June 2021 and 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. 30 June 2021 GBP (2) LIBOR USD (2) LIBOR Other (2) Total £m £m £m £m Assets Derivatives (1) 21,893 2,580 653 25,126 Other financial assets at fair value through profit and loss 808 20 — 828 Financial assets at amortised cost 12,706 157 83 12,946 Financial assets at fair value through comprehensive income 189 — — 189 Assets held for sale 1,602 161 — 1,763 37,198 2,918 736 40,852 Liabilities Derivatives (1) 26,227 2,558 56 28,841 Other financial liabilities at fair value through profit and loss 967 41 — 1,008 Financial liabilities at amortised cost 895 567 — 1,462 Liabilities held for sale — 27 — 27 28,089 3,193 56 31,338 Off-balance sheet commitments given 10,277 803 198 11,278 31 December 2020 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. Following the announcement by the FCA on 5 March 2021 that certain LIBOR settings will permanently cease immediately after 31 December 2021 (and for overnight and 12-month US dollar LIBOR after 30 June 2023), the ISDA fallback spread adjustment is fixed as of the date of the FCA announcement. (2) Cessation dates are: GBP, JPY, NOK LIBOR 31/12/21, USD LIBOR 30/06/23, EONIA 3/1/22 IBOR Reform The table below shows the notional amount of derivatives in hedging relationships directly affected by uncertainties related to IBOR reform. 30 June 2021 31 December 2020 GBP USD Other Total GBP USD Other Total £m £m £m £m £m £m £m £m Total notional value of hedging instruments: – Cash flow hedges 11,610 2,894 — 14,504 15,198 5,119 — 20,317 – Fair value hedges 20,758 160 696 21,614 32,223 1,077 778 34,078 32,368 3,054 696 36,118 47,421 6,196 778 54,395 Maturing after cessation date (1) – Cash flow hedges 9,788 2,533 — 12,321 10,553 2,562 — 13,115 – Fair value hedges 16,232 160 696 17,088 12,477 162 720 13,359 26,020 2,693 696 29,409 23,030 2,724 720 26,474 (1) Cessation dates are: GBP, JPY, NOK LIBOR 31/12/21, USD LIBOR 30/06/23, EONIA 3/1/22

Discontinued Operations and Ass

Discontinued Operations and Assets and Liabilities Held for Sale	6 Months Ended
	Jun. 30, 2021
Discontinued Operations and Assets and Liabilities Held for Sale [Abstract]
Discontinued Operations and Assets and Liabilities Held for Sale	33. DISCONTINUED OPERATIONS AND ASSETS AND LIABILITIES HELD FOR SALE Discontinued operations Proposed transfer of the CIB business to SLB Following a hearing held on 23 June 2021, the High Court of England and Wales has sanctioned the transfer of substantially all the CIB business of Santander UK to SLB by way of a Part VII banking business transfer scheme. The transfers of CIB business to SLB under the scheme are scheduled to take place before the end of 2021. At 30 June 2021, the CIB business met the requirements for presentation as discontinued operations. The financial performance and cash flow information relating to the discontinued operations were as follows: Half year to Half year to 30 June 2021 30 June 2020 £m £m Net interest income 24 25 Net fee and commission income 27 22 Total operating income 51 47 Operating expenses before credit impairment losses, provisions and charges (22) (33) Credit impairment losses 7 (12) Provisions for other liabilities and charges (3) (4) Total operating credit impairment losses, provisions and charges 4 (16) Profit/(loss) from discontinued operations before tax 33 (2) Tax on profit/(loss) from discontinued operations (9) 1 Profit/(loss) from discontinued operations after tax 24 (1) There were no gains or losses recognised on the measurement to fair value less costs to sell or on the disposal of the asset groups constituting the discontinued operations. In H121, the net cash flows attributable to the operating activities, investing activities and financing activities in respect of discontinued operations were £1,754m outflow (H120: £524m inflow), £nil (H120: £nil) and £nil (H120: £nil), respectively Assets and liabilities held for sale Proposed transfer of the CIB business to SLB Management considered the related business transfers to be highly probable at 30 June 2021. As such, the Santander UK group reclassified as held for sale the assets and liabilities relating to the CIB business. These business transfers will be made for a cash consideration equivalent to the book value of the associated assets and liabilities, which represents a fair value for the Santander UK group. Costs to sell are expected to be immaterial. Sale of Retail Banking mortgage assets At 30 June 2021, Santander UK was in the process of selling a portfolio of Retail Banking residential mortgage assets and expected to complete the transaction in August 2021. Management considered the sale to be highly probable at 30 June 2021. As such, the Santander UK group reclassified as held for sale the mortgage assets. The sale is expected to be made for a cash consideration exceeding the book value of the mortgage assets, resulting in a gain on disposal. Costs to sell are expected to be immaterial. Assets and liabilities held for sale At 30 June 2021, the assets and liabilities held for sale comprised of: Net assets/liabilities held for sale CIB business transfer to SLB Sale of residential mortgages Total £m £m £m Assets Derivative financial instruments 143 — 143 Other financial assets at fair value through profit or loss 16 — 16 Loans and advances to customers 1,878 608 2,486 Total assets held for sale 2,037 608 2,645 Liabilities Derivative financial instruments 95 — 95 Other financial liabilities at fair value through profit or loss 27 — 27 Deposits by customers 3,769 — 3,769 Deposits by banks 21 — 21 Total liabilities held for sale 3,912 — 3,912

Events After the Balance Sheet

Events After the Balance Sheet Date	6 Months Ended
	Jun. 30, 2021
Disclosure of non-adjusting events after reporting period [abstract]
Events After the Balance Sheet Date	34. EVENTS AFTER THE BALANCE SHEET DATE There have been no significant events between 30 June 2021 and the date of approval of these financial statements which would require a change to or additional disclosure in the financial statements, except for the following non-adjusting event:

Accounting Policies (Policies)

Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2021
Disclosure Of Significant Accounting Policies [Abstract]
Basis of preparation	The financial information in these Condensed Consolidated Interim Financial Statements is unaudited and does not constitute statutory accounts as defined in section 434 of the UK Companies Act 2006. Statutory accounts for the year ended 31 December 2020 have been delivered to the Registrar of Companies. The Condensed Consolidated Interim Financial Statements reflect all adjustments that, in the opinion of management, are necessary for a fair statement of the results of operations for the interim period. All such adjustments to the financial information are of a normal, recurring nature. Because the results from common banking activities are so closely related and responsive to changes in market conditions, the results for any interim period are not necessarily indicative of the results that can be expected for the year. On 31 December 2020, International Financial Reporting Standards (IFRSs) as adopted by the European Union at that date was brought into UK law and became UK-adopted International Accounting Standards (IAS), with future changes being subject to endorsement by the UK Endorsement Board. Santander UK plc (the Company) and its subsidiaries (collectively Santander UK or the Santander UK group) transitioned to UK-adopted International Accounting Standards in its consolidated financial statements on 1 January 2021. This change constitutes a change in accounting framework. However, there is no impact on recognition, measurement or disclosure in the period reported as a result of the change in framework. The Condensed Consolidated Interim Financial Statements have been prepared in accordance with UK adopted IAS 34 ‘Interim Financial Reporting', IAS 34 ‘Interim Financial Reporting’ as issued by the International Accounting Standards Board (IASB), IAS 34 ‘Interim Financial Reporting’ as adopted by the EU, and the Disclosure Guidance and Transparency Rules sourcebook of the UK's Financial Conduct Authority (FCA). They do not include all the information and disclosures normally required for full annual financial statements and should be read in conjunction with the Consolidated Financial Statements of Santander UK for the year ended 31 December 2020 which were prepared in accordance with international accounting standards in conformity with the requirements of the Companies Act 2006 and IFRSs adopted pursuant to Regulation (EC) No 1606/2002 as it applies in the European Union. Those consolidated financial statements were also prepared in accordance with IFRSs as issued by the IASB, including interpretations issued by the IFRS Interpretations Committee, as there were no applicable differences from IFRSs as issued by the IASB for the periods presented. At 30 June 2021, there were no differences between IFRSs adopted by the UK, IFRSs issued by the IASB, and IFRSs adopted by the EU in terms of their application to Santander UK. The financial statements for Santander UK for the year ended 31 December 2021 will be prepared in accordance with IFRS as adopted by the UK, IFRSs as issued by the IASB, including interpretations issued by the IFRS Interpretations Committee, and IFRSs as adopted by the EU. Except as noted below, the same accounting policies, presentation and methods of computation are followed in these Condensed Consolidated Interim Financial Statements as were applied in the presentation of Santander UK’s 2020 Annual Report. Future accounting developments At 30 June 2021, for the Santander UK group, there were no significant new or revised standards and interpretations, and amendments thereto, which have been issued, including those which are not yet effective or which have otherwise not been early adopted where permitted.
Non-current assets held for sale	Non-current assets held for sale Non-current assets (or disposal groups) are classified as held for sale when their carrying amount is to be recovered principally through a sale transaction rather than continuing use. In order to be classified as held for sale, the asset must be available for immediate sale in its present condition subject only to terms that are usual and customary and the sale must be highly probable. Non-current assets (or disposal groups) held for sale are measured at the lower of carrying amount and fair value less cost to sell, with the exception of financial instruments which remain governed by the requirements of IFRS 9, and are held at their IFRS 9 carrying value. For details of the disposal of assets and liabilities in connection with the transfer of the Corporate & Investment Banking (CIB) business to the London Branch of Banco Santander SA (SLB), and for details on the sale of Retail Banking residential mortgage assets, see Note 33.
Discontinued operations	Discontinued operations A discontinued operation is a component of the Santander UK group that has been disposed of or is classified as held for sale and that represents a separate major line of business or geographical area of operations, is part of a single co-ordinated plan to dispose of such a line of business or area of operations, or is a subsidiary acquired exclusively with a view to resale. The results of discontinued operations are presented separately in the Condensed Consolidated Income Statement. Prior periods have been restated accordingly.
Going concern	Going concern In light of the continuing economic uncertainty, the Directors updated their going concern assessment in preparing these Condensed Consolidated Interim Financial Statements. After making enquiries, the Directors have a reasonable expectation that Santander UK has adequate resources to continue in operational existence for at least twelve months from the date of this report and, therefore, having reassessed the principal risks and uncertainties, the Directors consider it appropriate for the Condensed Consolidated Interim Financial Statements to be prepared on a going concern basis. In making their going concern assessment in connection with preparing the Condensed Consolidated Interim Financial Statements, the Directors considered a wide range of information that included Santander UK’s long-term business and strategic plans, forecasts and projections, estimated capital, funding and liquidity requirements, contingent liabilities and the reasonably possible changes in trading performance arising from potential economic, market and product developments. The Directors' assessment specifically considered the impacts of Covid-19, including for the following areas: – Economic scenarios and probability weights: These underpin our ECL impairment allowances and are discussed in detail in the Credit risk section of the Risk review. The Directors reviewed the economic scenarios to ensure that they captured the wide range of potential outcomes for the UK economy. Whereas the outlook for 2021 is more positive following the success of the vaccination programme and cautious optimism that the worst of the health crisis has passed, the downside scenarios continue to encapsulate different potential outcomes including higher-for-longer unemployment, and emergence of Covid-19 variants that are more resistant to existing vaccines leading to further lockdowns and a slower recovery that is more akin to the ‘U’ shape of past recessions. – Liquidity: Santander UK remains in a strong liquidity position and Covid-19 did not trigger a liquidity stress. – Capital: Santander UK remains strongly capitalised and Covid-19 did not trigger a capital stress. – Customers: Santander UK has supported many thousands of individuals and businesses affected by Covid-19 with a range of measures, including payment holidays on mortgages, personal loans and credit cards as well as taking an active part in UK Government loan schemes to help businesses. The majority of customers who took payment holidays have resumed payments. We continue to work with those customers who remain affected to understand their individual situations and, where necessary, help them resume payments. – Operations: All our operations continue to operate effectively with a significant majority of staff working from home and the majority of branches remaining open and returning to more normal hours. The current operating model could be sustained indefinitely with additional resilience being continuously implemented and is not affecting our ability to operate all our services, or raising concerns about our Business Continuity Planning, and – Key suppliers: Suppliers continue to be closely monitored in line with our Third Party Risk Management Framework. No significant service issues have been reported across our cohort of critical suppliers, and no material issues have been reported across our broader (non-critical) supply chain. Isolated supplier impacts have been seen, but these are being managed.
Critical judgements and accounting estimates	CRITICAL JUDGEMENTS AND ACCOUNTING ESTIMATES The preparation of the Condensed Consolidated Interim Financial Statements requires management to make judgements and accounting estimates that affect the reported amount of assets and liabilities at the date of the Condensed Consolidated Interim 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 the course of preparing the Condensed Consolidated Interim 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 losses Key judgements – Determining an appropriate definition of default – Establishing the criteria for a significant increase in credit risk (SICR) and, for corporate borrowers, internal credit risk rating – Applying post model adjustments – Assessing individual corporate Stage 3 exposures Key estimates – Forward-looking multiple economic scenario assumptions – Probability weights assigned to multiple economic scenarios For more on each of these key judgements and estimates, including the impact of Covid-19 on them, see 'Management judgements and estimates 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 Key judgements – Determining whether a present obligation exists – Assessing the likely outcome of future legal decisions Key estimates – Probability, timing, nature and amount of any outflows that may arise from past events Note 26 'Contingent liabilities and commitments' 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. These judgements 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, or to calculate any resulting sensitivities. For more on each of these key judgements and estimates, see Notes 24 and 26. c) Pensions Key judgements – Setting the criteria for constructing the corporate bond yield curve used to determine the discount rate – Determining the methodology for setting the inflation assumption Key estimates – Discount rate applied to future cash flows – Rate of price inflation – Expected lifetime of the schemes' members For more on each of these key judgements and estimates, including the impact of Covid-19 on them, see Note 25. Sensitivity of defined benefit pension scheme estimates For detailed disclosures see ‘Actuarial assumption sensitivities’ in Note 25. d) Goodwill Key judgements: – Determining the basis of goodwill impairment calculation assumptions, including management's planning assumptions considering internal capital allocations needed to support Santander UK's strategy, current market conditions and the macro-economic outlook. Key estimates: – Forecast cash flows for cash generating units, including estimated allocations of regulatory capital – Growth rate beyond initial cash flow projections – Discount rates which factor in risk-free rates and applicable risk premiums These are variables subject to fluctuations in external market rates and economic conditions beyond management’s control For more on each of these key judgements and estimates, see Note 16. Sensitivity of goodwill
Profit before tax re-segmentation of short term markets business in Corporate Centre rather than Corporate & Investment Banking	As reflected in the 2020 Annual Report, the segmental basis of presentation in these Condensed Consolidated Interim Financial Statements changed following a management review of our structure. At 31 December 2020 this resulted in customer assets of £2.0bn and customer deposits of £3.1bn being transferred from Business Banking (in Retail Banking) to CCB, non-core corporate mortgages of £0.4bn transferring from Corporate Centre to CCB, and a number of smaller business lines transferring from CIB to Corporate Centre. For H120, this resulted in an increase in profit before tax in Retail Banking of £28m, a decrease in CCB of £31m, a decrease in CIB of £4m, and an increase of £7m in Corporate Centre. The net impact for Santander UK was nil. Proposed transfer of the CIB business to SLB Following a hearing held on 23 June 2021, the High Court of England and Wales has sanctioned the transfer of substantially all the CIB business of Santander UK to SLB by way of a Part VII banking business transfer scheme. The transfers of CIB business to SLB under the scheme are scheduled to take place before the end of 2021. Management considered the related business transfers to be highly probable at 30 June 2021. As such, the Santander UK group reclassified as held for sale the assets and liabilities relating to the CIB business. For more details, see Note 33. At 30 June 2021, the CIB business met the requirements for presentation as discontinued operations. For more details, see Note 33.

Accounting Policies (Tables)

Accounting Policies (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure Of Significant Accounting Policies [Abstract]
Disclosure of Information about Key Judgements and Key Estimates	Key judgements – Determining an appropriate definition of default – Establishing the criteria for a significant increase in credit risk (SICR) and, for corporate borrowers, internal credit risk rating – Applying post model adjustments – Assessing individual corporate Stage 3 exposures Key estimates – Forward-looking multiple economic scenario assumptions – Probability weights assigned to multiple economic scenarios Key judgements – Determining whether a present obligation exists – Assessing the likely outcome of future legal decisions Key estimates – Probability, timing, nature and amount of any outflows that may arise from past events Key judgements – Setting the criteria for constructing the corporate bond yield curve used to determine the discount rate – Determining the methodology for setting the inflation assumption Key estimates – Discount rate applied to future cash flows – Rate of price inflation – Expected lifetime of the schemes' members Key judgements: – Determining the basis of goodwill impairment calculation assumptions, including management's planning assumptions considering internal capital allocations needed to support Santander UK's strategy, current market conditions and the macro-economic outlook. Key estimates: – Forecast cash flows for cash generating units, including estimated allocations of regulatory capital – Growth rate beyond initial cash flow projections – Discount rates which factor in risk-free rates and applicable risk premiums These are variables subject to fluctuations in external market rates and economic conditions beyond management’s control

Segments (Tables)

Segments (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure of operating segments [abstract]
Summary of Results by Segment	Results by segment Half year to Retail Banking Corporate & Commercial Banking Corporate & Investment Banking Corporate Centre Total 30 June 2021 £m £m £m £m £m Net interest income/(expense) 1,741 203 — (39) 1,905 Non-interest income 189 51 — 46 286 Total operating income 1,930 254 — 7 2,191 Operating expenses before credit impairment losses, provisions and charges (922) (177) — (229) (1,328) Credit impairment losses 48 22 — — 70 Provisions for other liabilities and charges (67) (5) — (118) (190) Total operating credit impairment losses, provisions and charges (19) 17 — (118) (120) Profit/(loss) from continuing operations before tax 989 94 — (340) 743 Revenue from external customers 2,197 273 — (279) 2,191 Inter-segment revenue (267) (19) — 286 — Total operating income 1,930 254 — 7 2,191 Revenue from external customers includes the following fee and commission income disaggregated by income type: (1) – Current account and debit card fees 185 23 — — 208 – Insurance, protection and investments 33 — — — 33 – Credit cards 29 — — — 29 – Non-banking and other fees (2) 6 29 — — 35 Total fee and commission income 253 52 — — 305 Fee and commission expense (140) (10) — (4) (154) Net fee and commission income/(expense) 113 42 — (4) 151 30 June 2021 Customer loans 186,706 17,055 1,913 2,673 208,347 Total assets (3) 195,480 17,055 2,057 71,022 285,614 Of which assets held for sale 608 — 2,037 — 2,645 Customer deposits 156,713 25,244 3,817 3,596 189,370 Total liabilities 157,268 25,263 3,912 82,463 268,906 Of which liabilities held for sale — — 3,912 — 3,912 (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. Half year to (4)(5) Retail Banking Corporate & Commercial Banking Corporate & Investment Banking Corporate Centre Total 30 June 2020 £m £m £m £m £m Net interest income/(expense) 1,369 181 — (22) 1,528 Non-interest income 227 49 — 24 300 Total operating income/(expense) 1,596 230 — 2 1,828 Operating expenses before credit impairment losses, provisions and charges (983) (161) — (68) (1,212) Credit impairment losses (214) (150) — — (364) Provisions for other liabilities and charges (56) 3 — (11) (64) Total operating credit impairment losses, provisions and charges (270) (147) — (11) (428) Profit/(loss) from continuing operations before tax 343 (78) — (77) 188 Revenue from external customers 1,973 278 — (424) 1,827 Inter-segment revenue (377) (48) — 426 1 Total operating income/(expense) 1,596 230 — 2 1,828 Revenue from external customers includes the following fee and commission income disaggregated by income type: (1) – Current account and debit card fees 245 21 — — 266 – Insurance, protection and investments 31 — — — 31 – Credit cards 35 — — — 35 – Non-banking and other fees (2) 20 24 — 4 48 Total fee and commission income 331 45 — 4 380 Fee and commission expense (164) (12) — (4) (180) Net fee and commission income 167 33 — — 200 31 December 2020 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 (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 (4) Restated, as reflected in Note 2 to the Consolidated Financial Statements in the 2020 Annual Report, following a management review of our structure.

Operating Expenses Before Cre_2

Operating Expenses Before Credit Impairment Losses, Provisions and Charges (Tables)	6 Months Ended
	Jun. 30, 2021
Expenses by nature [abstract]
Summary of Operating Expenses Before Credit Impairment Losses, Provisions and Charges	Half year to Half year to (1) 30 June 2021 30 June 2020 £m £m Staff costs 577 576 Other administration expenses 431 367 Depreciation, amortisation and impairment 320 269 Total 1,328 1,212 (1) Adjusted to reflect the presentation of discontinued operations as set out in Note 33.

Credit Impairment Losses and _2

Credit Impairment Losses and Provisions (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure Of Credit Impairment Loss And Provisions [Abstract]
Summary of Impairment Losses and Provisions	Half year to Half year to (1) 30 June 2021 30 June 2020 £m £m Credit impairment loss/(release): Loans and advances to customers (33) 384 Recoveries of loans and advances, net of collection costs (13) (15) Off-balance sheet exposures (See Note 24) (24) (5) (70) 364 Provisions for other liabilities and charges (excluding off-balance sheet credit exposures) (See Note 24) 191 64 Provisions for residual value and voluntary termination (1) — 190 64 120 428 (1) Adjusted to reflect the presentation of discontinued operations as set out in Note 33.

Taxation (Tables)

Taxation (Tables)	6 Months Ended
	Jun. 30, 2021
Major components of tax expense (income) [abstract]
Schedule of Tax on Profit Before Tax Differs from Theoretical Amount that Arise Using Basic Corporation Tax Rate	The Santander UK group’s effective tax rate for H121 was 27.6% (H120: 25.5%). 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: Half year to Half year to (1) 30 June 2021 30 June 2020 £m £m Profit from continuing operations before tax 743 188 Tax calculated at a tax rate of 19% (H120: 19%) 141 36 Bank surcharge on profits 49 9 Non-deductible preference dividends paid 6 3 Non-deductible UK Bank Levy 14 6 Other non-deductible costs and non-taxable income 14 4 Effect of change in tax rate on deferred tax provision 13 7 Tax relief on dividends in respect of other equity instruments (32) (13) Adjustment to prior year provisions — (4) Tax on profit from continuing operations 205 48 (1) Adjusted to reflect the presentation of discontinued operations as set out in Note 33.

Derivative Financial Instrume_2

Derivative Financial Instruments (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure of nature and extent of risks arising from financial instruments [abstract]
Notional Amounts and Fair Values of Derivatives	The table below includes the notional amounts of transactions outstanding at the balance sheet date; they do not represent actual exposures. 30 June 2021 31 December 2020 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 12,227 229 215 14,951 395 418 Interest rate contracts 31,912 545 440 40,160 888 542 Equity and credit contracts 1,106 141 53 1,140 123 55 Total derivatives held for trading 45,245 915 708 56,251 1,406 1,015 Derivatives held for hedging Designated as fair value hedges: Exchange rate contracts 598 52 1 789 84 6 Interest rate contracts 84,311 981 1,319 93,748 1,225 1,885 84,909 1,033 1,320 94,537 1,309 1,891 Designated as cash flow hedges: Exchange rate contracts 23,756 1,131 381 27,020 1,978 409 Interest rate contracts 26,609 307 38 19,407 467 23 50,365 1,438 419 46,427 2,445 432 Total derivatives held for hedging 135,274 2,471 1,739 140,964 3,754 2,323 Derivative netting (1) (1,384) (1,384) (1,754) (1,754) Total derivatives 180,519 2,002 1,063 197,215 3,406 1,584 (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 £401m (2020: £330m) and the amount of cash collateral paid that had been offset against the gross derivative liabilities was £245m (2020: £651m).

Other Financial Assets At Fai_2

Other Financial Assets At Fair Value Through Profit Or Loss (Tables)	6 Months Ended
	Jun. 30, 2021
Financial assets at fair value through profit or loss [abstract]
Summary of Other Financial Assets at Fair Value Through Profit or Loss	30 June 2021 31 December 2020 £m £m Loans and advances to customers: Loans to housing associations 12 13 Other loans 62 86 74 99 Debt securities 105 109 179 208

Loans and Advances to Custome_2

Loans and Advances to Customers (Tables)	6 Months Ended
	Jun. 30, 2021
Net Loans And Advances To Customers [Abstract]
Summary of Net Loans and Advances to Customers	30 June 2021 31 December 2020 £m £m Loans and advances to customers 209,188 210,107 Credit impairment loss allowances on loans and advances to customers (1,141) (1,303) RV and voluntary termination provisions on finance leases (49) (54) Net loans and advances to customers 207,998 208,750

Securitisations and Covered B_2

Securitisations and Covered Bonds (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure Of Securitisations And Covered Bonds [Abstract]
Analysis of Securitisations and Covered Bonds	The gross assets securitised, or for the covered bond programme assigned, at 30 June 2021 and 31 December 2020 were: 30 June 2021 31 December 2020 £m £m Mortgage-backed master trust structures: – Holmes 2,566 3,073 – Fosse 2,170 2,258 – Langton — 2,782 4,736 8,113 Other asset-backed securitisation structures: – Motor 103 189 – Auto ABS UK Loans 1,325 1,460 1,428 1,649 Total securitisation programmes 6,164 9,762 Covered bond programmes – Euro 35bn Global Covered Bond Programme 19,000 23,670 Total securitisation and covered bond programmes 25,164 33,432
Summary of Issuances and Redemptions of Securitisation and Covered Bond Programme	The following table sets out the internal and external issuances and redemptions in H121 and H120 for each securitisation and covered bond programme. Internal issuances External issuances Internal redemptions External redemptions H121 H120 H121 H120 H121 H120 H121 H120 £bn £bn £bn £bn £bn £bn £bn £bn Mortgage-backed master trust structures: – Holmes — — — — — 0.1 0.3 0.5 – Langton — — — — 2.4 — — — Other asset-backed securitisation structures: – Motor — — — — 0.1 0.1 — 0.1 – Auto ABS UK Loans — — — — 0.1 — 0.1 0.1 Covered bond programme — — — 3.0 — — 4.5 1.8 — — — 3.0 2.6 0.2 4.9 2.5

Reverse Repurchase Agreements_2

Reverse Repurchase Agreements - Non Trading (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure Of Reserve Repurchase Agreements Non-Trading [Abstract]
Summary of Reverse Repurchase Agreements - Non Trading	30 June 2021 31 December 2020 £m £m Agreements with banks 1,250 1,258 Agreements with customers 16,947 18,341 18,197 19,599

Other Financial Assets At Amo_2

Other Financial Assets At Amortised Cost (Tables)	6 Months Ended
	Jun. 30, 2021
Financial assets at fair value through profit or loss [abstract]
Summary of Other Financial Assets at Amortised Cost	30 June 2021 31 December 2020 £m £m Asset backed securities 467 491 Debt securities 63 672 530 1,163

Financial Assets At Fair Valu_2

Financial Assets At Fair Value Through Other Comprehensive Income (Tables)	6 Months Ended
	Jun. 30, 2021
Financial assets at fair value through other comprehensive income [abstract]
Summary of Financial Assets at Fair Value Through Other Comprehensive Income	30 June 2021 31 December 2020 £m £m Debt securities 6,348 8,929 Loans and advances to customers 20 21 6,368 8,950

Intangible Assets (Tables)

Intangible Assets (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure of detailed information about intangible assets [abstract]
Schedule of Goodwill for Cash Generating Units	The following CGUs include in their carrying values goodwill that comprises the goodwill reported by Santander UK. The CGUs do not carry on their balance sheets any other intangible assets with indefinite useful lives. For key assumptions used in the value in use (VIU) calculation, as the basis of the recoverable amount of each CGU, see Note 20 to the Consolidated Financial Statements in the 2020 Annual Report. At 30 June 2021 a review was performed to identify any potential impairment indicators. No indicators of impairment were identified and so a full impairment test was not performed for the half year. The key assumptions are set out in the table below. Goodwill Discount rate Growth rate beyond initial cash flow projections 30 June 2021 31 December 2020 30 June 2021 31 December 2020 30 June 2021 31 December 2020 CGU £m £m % % % % Personal financial services 1,169 1,169 14.6 13.6 1.6 1.6 Private banking 30 30 11.0 8.9 1.6 1.6 Other 4 4 14.6 13.6 1.6 1.6 1,203 1,203

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	6 Months Ended
	Jun. 30, 2021
Property, plant and equipment [abstract]
Disclosure of detailed information about investment property	Property Office fixtures and equipment Computer software Operating lease assets Right-of-use assets Total (2) £m £m £m £m £m £m Cost: At 1 January 2021 1,272 1,375 436 720 218 4,021 Additions 23 12 — 194 59 288 Disposals (306) (267) — (122) (20) (715) At 30 June 2021 989 1,120 436 792 257 3,594 Accumulated depreciation: At 1 January 2021 489 1,068 434 178 118 2,287 Charge for the period (1) 17 46 — 48 10 121 Impairment during the period 57 29 — — 19 105 Disposals (210) (236) — (50) (19) (515) At 30 June 2021 353 907 434 176 128 1,998 Net book value 636 213 2 616 129 1,596 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 (1) 55 111 — 92 58 316 Impairment during the year 24 — — — — 24 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 (1) Following a review of the estimated useful lives of property as part of Santander UK's transformation program, the charge for the period includes accelerated property depreciation of £4m (2020: £9m). (2) Property, plant and equipment includes assets under construction of £72m (2020: £55m).

Other Financial Liabilities a_2

Other Financial Liabilities at Fair Value Through Profit or Loss (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure of financial liabilities [abstract]
Summary of Other Financial Liabilities at Fair Value Through Profit or Loss	30 June 2021 31 December 2020 £m £m US$30bn Euro Medium Term Note Programme 96 102 Structured Notes Programmes 431 805 Eurobonds 144 150 Structured deposits 347 375 Collateral and associated financial guarantees 5 2 1,023 1,434

Deposits by Customers (Tables)

Deposits by Customers (Tables)	6 Months Ended
	Jun. 30, 2021
Deposits from customers [abstract]
Schedule of Deposits by Customers	30 June 2021 31 December 2020 £m £m Demand and time deposits (1) 185,179 185,879 Amounts due to other Santander UK Group Holdings plc subsidiaries 67 59 Amounts due to Santander UK Group Holdings plc (2) 6,912 7,883 Amounts due to fellow Banco Santander subsidiaries and joint ventures 1,159 1,314 193,317 195,135 (1) Includes equity index-linked deposits of £578m (2020: £577m). The capital amount guaranteed/protected and the amount of return guaranteed in respect of the equity index-linked deposits were £578m and £2m (2020: £577m and £2m) respectively. (2) Includes downstreamed funding from our immediate parent company Santander UK Group Holdings plc.

Deposits by Banks (Tables)

Deposits by Banks (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure Of Deposits From Banks [Abstract]
Schedule of Deposits by Banks	30 June 2021 31 December 2020 £m £m Items in the course of transmission 497 375 Deposits held as collateral 1,034 2,063 Other deposits (1) 18,951 18,519 Amounts due to Santander UK subsidiaries 20 1 20,502 20,958 (1) Includes drawdown from the TFS of £3.3bn (2020: £6.3bn) and drawdown from the TFSME of £15.2bn (2020: £11.7bn).

Repurchase Agreements - Non T_2

Repurchase Agreements - Non Trading (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure Of Repurchase Agreement [Abstract]
Summary of Repurchase Agreements - Non Trading	30 June 2021 31 December 2020 £m £m Agreements with banks 2,898 6,358 Agreements with customers 11,191 9,490 14,089 15,848

Debt Securities in Issue (Table

Debt Securities in Issue (Tables)	6 Months Ended
	Jun. 30, 2021
Debt Securities In Issue [Abstract]
Summary of Debt Securities in Issue	30 June 2021 31 December 2020 £m £m Medium-term notes 6,908 8,212 Euro 35bn Global Covered Bond Programme 14,677 19,285 US$20bn Commercial Paper Programmes 3,052 2,824 Certificates of deposit 2,466 2,858 Credit linked notes 58 57 Securitisation programmes 1,979 2,330 29,140 35,566

Subordinated Liabilities (Table

Subordinated Liabilities (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure Of Subordinated Liabilities [Abstract]
Schedule of Subordinated Liabilities	30 June 2021 31 December 2020 £m £m 2,513 2,556

Provisions (Tables)

Provisions (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure of other provisions [abstract]
Summary of Provisions Reconciliation	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 2021 76 8 34 45 75 226 464 Additional provisions (See Note 5) — — — 43 — 148 191 Provisions released (See Note 5) — — — — (24) — (24) Utilisation and other (39) — (34) (9) — (105) (187) At 30 June 2021 37 8 — 79 51 269 444

Retirement Benefit Plans (Table

Retirement Benefit Plans (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure of net defined benefit liability (asset) [abstract]
Summary of Retirement Benefit Plans	The amounts recognised in the balance sheet were as follows: 30 June 2021 31 December 2020 £m £m Assets/(liabilities) Funded defined benefit pension scheme - surplus 1,083 495 Funded defined benefit pension scheme - deficit (113) (361) Unfunded pension and post retirement medical benefits (39) (42) Total net assets 931 92
Disclosure of Pension Remeasurements Included in Other Comprehensive income	The amounts recognised in other comprehensive income were as follows: 30 June 2021 30 June 2020 £m £m Return on plan assets (excluding amounts included in net interest expense) 383 (1,115) Actuarial losses arising from changes in demographic assumptions — 38 Actuarial gains arising from experience adjustments (30) (10) Actuarial (gains)/losses arising from changes in financial assumptions (1,098) 1,463 Pension remeasurement (745) 376
Summary of Net Assets Recognised	The net assets recognised in the balance sheet were determined as follows: 30 June 2021 31 December 2020 £m £m Present value of defined benefit obligations (12,675) (13,887) Fair value of scheme assets 13,606 13,979 Net defined benefit assets 931 92
Summary of Principal Actuarial Assumptions Used for Defined Benefit Schemes	The principal actuarial assumptions used for the defined benefit schemes were: 30 June 2021 31 December 2020 % % To determine benefit obligations: – Discount rate for scheme liabilities 1.9 1.3 – General price inflation 3.2 3.0 – General salary increase 1.0 1.0 – Expected rate of pension increase 3.1 2.9 Years Years Longevity at 60 for current pensioners, on the valuation date: – Males 27.5 27.5 – Females 30.1 30.0 Longevity at 60 for future pensioners currently aged 40, on the valuation date: – Males 29.1 29.0 – Females 31.6 31.5
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 30 June 2021 31 December 2020 Assumption Change in pension obligation at period end from £m £m Discount rate 25 bps increase (573) (662) General price inflation (1) 25 bps increase 411 480 Mortality Each additional year of longevity assumed 453 515 (1) The comparative figure for general price inflation sensitivity of £365m as at 31 December 2020 has been restated. This was due to an error and does not impact the actual reported assumption value.

Contingent Liabilities and Co_2

Contingent Liabilities and Commitments (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure of contingent liabilities [abstract]
Summary of Contingent Liabilities and Commitments	30 June 2021 31 December 2020 £m £m Guarantees given to third parties 739 939 Formal standby facilities, credit lines and other commitments 40,957 42,221 41,696 43,160

Other Equity Instruments (Table

Other Equity Instruments (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure of classes of share capital [abstract]
Summary of other equity instruments	Interest rate 30 June 2021 31 December 2020 % 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 n/a — 210 - £500m Fixed Rate Reset Perpetual AT1 Capital Securities 6.30 March 2025 500 500 - £210m Fixed Rate Reset Perpetual AT1 Capital Securities 4.25 March 2026 210 — 2,191 2,191

Non-Controlling Interests (Tabl

Non-Controlling Interests (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure Of Non Controlling Interests [Abstract]
Schedule of Amounts Attributable to Non-Controlling Interests	30 June 2021 31 December 2020 £m £m PSA Finance UK Limited 180 162 180 162

Financial Instruments (Tables)

Financial Instruments (Tables)	6 Months Ended
	Jun. 30, 2021
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Abstract]
Fair Values of Financial Instruments Carried at Amortised Cost	The following tables analyse the fair value of the financial instruments carried at amortised cost at 30 June 2021 and 31 December 2020. It does not include fair value information for financial assets and financial liabilities carried at amortised cost if the carrying amount is a reasonable approximation of fair value. Details of the valuation methodology of the financial assets and financial liabilities carried at amortised cost can be found in Note 40(e) to the Consolidated Financial Statements in the 2020 Annual Report. 30 June 2021 31 December 2020 Carrying Carrying Fair value value Fair value value £m £m £m £m Assets Loans and advances to customers 210,815 207,998 211,279 208,750 Loans and advances to banks 1,312 1,312 1,682 1,682 Reverse repurchase agreements - non trading 18,198 18,197 19,608 19,599 Other financial assets at amortised cost 539 530 1,192 1,163 Assets held for sale (1) 2,493 2,486 — — 233,357 230,523 233,761 231,194 Liabilities Deposits by customers 193,389 193,317 195,242 195,135 Deposits by banks 20,503 20,502 20,967 20,958 Repurchase agreements - non trading 14,088 14,089 15,847 15,848 Debt securities in issue 29,890 29,140 36,397 35,566 Subordinated liabilities 3,080 2,513 3,069 2,556 Liabilities held for sale (1) 3,790 3,790 — — 264,740 263,351 271,522 270,063 (1) Assets and liabilities classified as held for sale are measured in accordance with IFRS 9. Any assets and liabilities held for sale which are measured at fair value are included in the fair value table in section (c) below.
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 30 June 2021 and 31 December 2020, analysed by their levels in the fair value hierarchy - Level 1, Level 2 and Level 3. 30 June 2021 31 December 2020 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 — 1,403 1 1,404 — 2,455 2 2,457 A Interest rate contracts — 1,841 — 1,841 — 2,566 14 2,580 A & C Equity and credit contracts — 93 48 141 — 71 52 123 B & D Netting — (1,384) — (1,384) — (1,754) — (1,754) — 1,953 49 2,002 — 3,338 68 3,406 Other financial assets at FVTPL Loans and advances to customers — — 74 74 — — 99 99 A Debt securities — — 105 105 — — 109 109 A, B & D — — 179 179 — — 208 208 Financial assets at FVOCI Debt securities 6,160 188 — 6,348 8,501 428 — 8,929 D Loans and advances to customers — — 20 20 — — 21 21 D 6,160 188 20 6,368 8,501 428 21 8,950 Assets held for sale (1) — 143 16 159 — — — — Total assets at fair value 6,160 2,284 264 8,708 8,501 3,766 297 12,564 Liabilities Derivative financial instruments Exchange rate contracts — 565 — 565 — 833 — 833 A Interest rate contracts — 1,827 2 1,829 — 2,447 3 2,450 A & C Equity and credit contracts — 23 30 53 — 26 29 55 B & D Netting — (1,384) — (1,384) — (1,754) — (1,754) — 1,031 32 1,063 — 1,552 32 1,584 Other financial liabilities at FVTPL Debt securities in issue — 666 5 671 — 1,051 6 1,057 A Structured deposits — 347 — 347 — 375 — 375 A Collateral and associated financial guarantees — 3 2 5 — — 2 2 D — 1,016 7 1,023 — 1,426 8 1,434 Liabilities held for sale (1) — 122 — 122 — — — — Total liabilities at fair value — 2,169 39 2,208 — 2,978 40 3,018 (1) Includes assets and liabilities held for sale which are measured at fair value.
Summary of Fair Value Adjustment	The magnitude and types of fair value adjustment are listed in the following table: 30 June 2021 31 December 2020 £m £m Risk-related: - Bid-offer and trade specific adjustments (7) (8) - Uncertainty 21 23 - Credit risk adjustment 7 11 - Funding fair value adjustment 2 3 23 29
Summary of 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 H121 and H120: Assets Liabilities Derivatives Other financial assets at FVTPL Financial assets at FVOCI Assets held for sale Total Derivatives Other financial liabilities at FVTPL Liabilities held for sale Total £m £m £m £m £m £m £m £m £m At 1 January 2021 68 208 21 — 297 (32) (8) — (40) Total (losses)/gains recognised: - Fair value movements (1) (7) (1) — (9) — 3 — 3 Transfers to held for sale — (16) — 16 — — (2) — (2) Netting (1) — 5 — — 5 — — — — Settlements (18) (11) — — (29) — — — — At 30 June 2021 49 179 20 16 264 (32) (7) — (39) Gains/(losses) recognised in profit or loss/other comprehensive income relating to assets and liabilities held at the end of the period (1) (7) (1) — (9) — 3 — 3 At 1 January 2020 75 386 56 — 517 (32) (61) — (93) Total gains/(losses) recognised: - Fair value movements 6 (2) 1 — 5 (1) 15 — 14 - Foreign exchange and other movements — (2) — — (2) — 2 — 2 Transfers out — — — — — — 28 — 28 Netting (1) — (37) — — (37) — — — — Additions — — — — — — (1) — (1) Sales — (11) (19) — (30) — — — — Settlements (17) (110) (8) — (135) 3 1 — 4 At 30 June 2020 64 224 30 — 318 (30) (16) — (46) Gains/(losses) recognised in profit or loss/other comprehensive income relating to assets and liabilities held at the end of the period 6 (4) 1 — 3 (1) 17 — 16 (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 to the Consolidated Financial Statements in the 2020 Annual Report.

Interest Rate Benchmark Reform

Interest Rate Benchmark Reform (Tables)	6 Months Ended
	Jun. 30, 2021
Interest Rate Benchmark Reform [Abstract]
Schedule of Amounts Affected by IBOR Reform	The following tables show the notional amounts of assets, liabilities and off-balance sheet commitments at 30 June 2021 and 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. 30 June 2021 GBP (2) LIBOR USD (2) LIBOR Other (2) Total £m £m £m £m Assets Derivatives (1) 21,893 2,580 653 25,126 Other financial assets at fair value through profit and loss 808 20 — 828 Financial assets at amortised cost 12,706 157 83 12,946 Financial assets at fair value through comprehensive income 189 — — 189 Assets held for sale 1,602 161 — 1,763 37,198 2,918 736 40,852 Liabilities Derivatives (1) 26,227 2,558 56 28,841 Other financial liabilities at fair value through profit and loss 967 41 — 1,008 Financial liabilities at amortised cost 895 567 — 1,462 Liabilities held for sale — 27 — 27 28,089 3,193 56 31,338 Off-balance sheet commitments given 10,277 803 198 11,278 31 December 2020 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. Following the announcement by the FCA on 5 March 2021 that certain LIBOR settings will permanently cease immediately after 31 December 2021 (and for overnight and 12-month US dollar LIBOR after 30 June 2023), the ISDA fallback spread adjustment is fixed as of the date of the FCA announcement. (2) Cessation dates are: GBP, JPY, NOK LIBOR 31/12/21, USD LIBOR 30/06/23, EONIA 3/1/22
Schedule of Derivatives Directly Affected by IBOR Reform Uncertainties	The table below shows the notional amount of derivatives in hedging relationships directly affected by uncertainties related to IBOR reform. 30 June 2021 31 December 2020 GBP USD Other Total GBP USD Other Total £m £m £m £m £m £m £m £m Total notional value of hedging instruments: – Cash flow hedges 11,610 2,894 — 14,504 15,198 5,119 — 20,317 – Fair value hedges 20,758 160 696 21,614 32,223 1,077 778 34,078 32,368 3,054 696 36,118 47,421 6,196 778 54,395 Maturing after cessation date (1) – Cash flow hedges 9,788 2,533 — 12,321 10,553 2,562 — 13,115 – Fair value hedges 16,232 160 696 17,088 12,477 162 720 13,359 26,020 2,693 696 29,409 23,030 2,724 720 26,474 (1) Cessation dates are: GBP, JPY, NOK LIBOR 31/12/21, USD LIBOR 30/06/23, EONIA 3/1/22

Discontinued Operations and A_2

Discontinued Operations and Assets and Liabilities Held for Sale (Tables)	6 Months Ended
	Jun. 30, 2021
Discontinued Operations and Assets and Liabilities Held for Sale [Abstract]
Disclosure of financial performance relating to discontinued operations	At 30 June 2021, the CIB business met the requirements for presentation as discontinued operations. The financial performance and cash flow information relating to the discontinued operations were as follows: Half year to Half year to 30 June 2021 30 June 2020 £m £m Net interest income 24 25 Net fee and commission income 27 22 Total operating income 51 47 Operating expenses before credit impairment losses, provisions and charges (22) (33) Credit impairment losses 7 (12) Provisions for other liabilities and charges (3) (4) Total operating credit impairment losses, provisions and charges 4 (16) Profit/(loss) from discontinued operations before tax 33 (2) Tax on profit/(loss) from discontinued operations (9) 1 Profit/(loss) from discontinued operations after tax 24 (1)
Disclosure of Assets and Liabilities Classified as Held for Sale	At 30 June 2021, the assets and liabilities held for sale comprised of: Net assets/liabilities held for sale CIB business transfer to SLB Sale of residential mortgages Total £m £m £m Assets Derivative financial instruments 143 — 143 Other financial assets at fair value through profit or loss 16 — 16 Loans and advances to customers 1,878 608 2,486 Total assets held for sale 2,037 608 2,645 Liabilities Derivative financial instruments 95 — 95 Other financial liabilities at fair value through profit or loss 27 — 27 Deposits by customers 3,769 — 3,769 Deposits by banks 21 — 21 Total liabilities held for sale 3,912 — 3,912

Segments - Additional Informati

Segments - Additional Information (Detail) £ in Millions	6 Months Ended
	Jun. 30, 2020GBP (£)	Jun. 30, 2021GBP (£)segment	Dec. 31, 2020GBP (£)
Disclosure of operating segments [line items]
Number of segments | segment		4
Total assets		£ 285,614	£ 292,332
Customer deposits		189,370	185,707
Retail Banking
Disclosure of operating segments [line items]
Total assets		195,480	192,070
Customer deposits		156,713	152,167
Retail Banking | Reclassified Amounts
Disclosure of operating segments [line items]
Total assets			(2,000)
Customer deposits			(3,100)
Profit (loss) before tax	£ 28
Corporate & Commercial Banking
Disclosure of operating segments [line items]
Total assets		17,055	17,626
Customer deposits		25,244	24,985
Corporate & Commercial Banking | Reclassified Amounts
Disclosure of operating segments [line items]
Total assets			2,000
Customer deposits			3,100
Corporate loans			400
Profit (loss) before tax	(31)
Corporate Centre
Disclosure of operating segments [line items]
Total assets		71,022	79,852
Customer deposits		3,596	2,049
Corporate Centre | Reclassified Amounts
Disclosure of operating segments [line items]
Corporate loans			(400)
Profit (loss) before tax	7
Corporate & Investment Banking
Disclosure of operating segments [line items]
Total assets		2,057	2,784
Customer deposits		£ 3,817	£ 6,506
Corporate & Investment Banking | Reclassified Amounts
Disclosure of operating segments [line items]
Profit (loss) before tax	£ (4)

Segments - Summary of Results b

Segments - Summary of Results by Segment (Detail) - GBP (£) £ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020		Dec. 31, 2020
Disclosure of operating segments [line items]
Net interest income/(expense)	£ 1,905	£ 1,528	[1]
Non-interest income	286	300
Total operating income/(expense)	2,191	1,828
Operating expenses before credit impairment losses, provisions and charges	(1,328)	(1,212)	[1]
Credit impairment losses	70	(364)	[1]
Provisions for other liabilities and charges	(190)	(64)	[1]
Total operating credit impairment losses, provisions and charges	(120)	(428)	[1]
Profit from continuing operations before tax	743	188	[1]
Revenue from external customers	2,191	1,827
Inter-segment revenue	2,191	1,828	[1]
Revenue from external customers includes the following fee and commission income disaggregated by income type:
Current account and debit card fees	208	266
Insurance, protection and investments	33	31
Credit cards	29	35
Non-banking and other fees	35	48
Total fee and commission income	305	380	[1]
Fee and commission expense	(154)	(180)	[1]
Net fee and commission income/(expense)	151	200
Customer loans	208,347			£ 207,010
Total assets	285,614			292,332
Assets held for sale	2,645			0
Customer deposits	189,370			185,707
Total liabilities	268,906			276,396
Liabilities held for sale	3,912			0
Elimination of intersegment amounts
Disclosure of operating segments [line items]
Inter-segment revenue	0	1
Retail Banking
Disclosure of operating segments [line items]
Net interest income/(expense)	1,741	1,369
Non-interest income	189	227
Total operating income/(expense)	1,930	1,596
Operating expenses before credit impairment losses, provisions and charges	(922)	(983)
Credit impairment losses	48	(214)
Provisions for other liabilities and charges	(67)	(56)
Total operating credit impairment losses, provisions and charges	(19)	(270)
Profit from continuing operations before tax	989	343
Revenue from external customers	2,197	1,973
Revenue from external customers includes the following fee and commission income disaggregated by income type:
Current account and debit card fees	185	245
Insurance, protection and investments	33	31
Credit cards	29	35
Non-banking and other fees	6	20
Total fee and commission income	253	331
Fee and commission expense	(140)	(164)
Net fee and commission income/(expense)	113	167
Customer loans	186,706			183,404
Total assets	195,480			192,070
Assets held for sale	608
Customer deposits	156,713			152,167
Total liabilities	157,268			152,715
Liabilities held for sale	0
Retail Banking | Elimination of intersegment amounts
Disclosure of operating segments [line items]
Inter-segment revenue	(267)	(377)
Corporate & Commercial Banking
Disclosure of operating segments [line items]
Net interest income/(expense)	203	181
Non-interest income	51	49
Total operating income/(expense)	254	230
Operating expenses before credit impairment losses, provisions and charges	(177)	(161)
Credit impairment losses	22	(150)
Provisions for other liabilities and charges	(5)	3
Total operating credit impairment losses, provisions and charges	17	(147)
Profit from continuing operations before tax	94	(78)
Revenue from external customers	273	278
Revenue from external customers includes the following fee and commission income disaggregated by income type:
Current account and debit card fees	23	21
Insurance, protection and investments	0	0
Credit cards	0	0
Non-banking and other fees	29	24
Total fee and commission income	52	45
Fee and commission expense	(10)	(12)
Net fee and commission income/(expense)	42	33
Customer loans	17,055			17,626
Total assets	17,055			17,626
Assets held for sale	0
Customer deposits	25,244			24,985
Total liabilities	25,263			25,011
Liabilities held for sale	0
Corporate & Commercial Banking | Elimination of intersegment amounts
Disclosure of operating segments [line items]
Inter-segment revenue	(19)	(48)
Corporate & Investment Banking
Disclosure of operating segments [line items]
Net interest income/(expense)	0	0
Non-interest income	0	0
Total operating income/(expense)	0	0
Operating expenses before credit impairment losses, provisions and charges	0	0
Credit impairment losses	0	0
Provisions for other liabilities and charges	0	0
Total operating credit impairment losses, provisions and charges	0	0
Profit from continuing operations before tax	0	0
Revenue from external customers	0	0
Revenue from external customers includes the following fee and commission income disaggregated by income type:
Current account and debit card fees	0	0
Insurance, protection and investments	0	0
Credit cards	0	0
Non-banking and other fees	0	0
Total fee and commission income	0	0
Fee and commission expense	0	0
Net fee and commission income/(expense)	0	0
Customer loans	1,913			2,784
Total assets	2,057			2,784
Assets held for sale	2,037
Customer deposits	3,817			6,506
Total liabilities	3,912			6,517
Liabilities held for sale	3,912
Corporate & Investment Banking | Elimination of intersegment amounts
Disclosure of operating segments [line items]
Inter-segment revenue	0	0
Corporate Centre
Disclosure of operating segments [line items]
Net interest income/(expense)	(39)	(22)
Non-interest income	46	24
Total operating income/(expense)	7	2
Operating expenses before credit impairment losses, provisions and charges	(229)	(68)
Credit impairment losses	0	0
Provisions for other liabilities and charges	(118)	(11)
Total operating credit impairment losses, provisions and charges	(118)	(11)
Profit from continuing operations before tax	(340)	(77)
Revenue from external customers	(279)	(424)
Revenue from external customers includes the following fee and commission income disaggregated by income type:
Current account and debit card fees	0	0
Insurance, protection and investments	0	0
Credit cards	0	0
Non-banking and other fees	0	4
Total fee and commission income	0	4
Fee and commission expense	(4)	(4)
Net fee and commission income/(expense)	(4)	0
Customer loans	2,673			3,196
Total assets	71,022			79,852
Assets held for sale	0
Customer deposits	3,596			2,049
Total liabilities	82,463			£ 92,153
Liabilities held for sale	0
Corporate Centre | Elimination of intersegment amounts
Disclosure of operating segments [line items]
Inter-segment revenue	£ 286	£ 426
[1]	Adjusted to reflect the presentation of discontinued operations as set out in Note 33.

Other Operating Income (Detail)

Other Operating Income (Detail) - GBP (£) £ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Net Trading And Other Income [abstract]
Loss on repurchase of debt securities and subordinated liabilities	£ 27	£ 17
Gains on disposals of UK head office site	£ 71

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	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Expenses by nature [abstract]
Staff costs	£ 577	£ 576
Other administration expenses	431	367
Depreciation, amortisation and impairment	320	269
Total operating expenses before impairment losses, provisions and charges	£ 1,328	£ 1,212	[1]
[1]	Adjusted to reflect the presentation of discontinued operations as set out in Note 33.

Operating Expenses Before Cre_4

Operating Expenses Before Credit Impairment Losses, Provisions and Charges - Additional Information (Detail) - GBP (£) £ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Operating Expenses Before Credit Impairment Losses Provision And Charges [Line Items]
Expense relating to short-term leases	£ 10	£ 4
Accumulated depreciation
Operating Expenses Before Credit Impairment Losses Provision And Charges [Line Items]
Impairment during the period	£ 105	£ 24

Credit Impairment Losses and _3

Credit Impairment Losses and Provisions - Summary of Impairment Losses and Provisions (Detail) - GBP (£) £ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Disclosure Of Credit Impairment Loss And Provisions [Abstract]
Loans and advances to customers	£ (33)	£ 384
Recoveries of loans and advances, net of collection costs	(13)	(15)
Off-balance sheet exposures	(24)	(5)
Impairment loss on financial assets	(70)	364	[1]
Provisions for other liabilities and charges (excluding off-balance sheet credit exposures)	191	64
Provisions for residual value and voluntary termination	(1)	0
Provisions for other liabilities and charges	190	64	[1]
Total operating credit impairment loss/(release), provisions and charges	£ 120	£ 428	[1]
[1]	Adjusted to reflect the presentation of discontinued operations as set out in Note 33.

Credit Impairment Losses and _4

Credit Impairment Losses and Provisions - Additional Information (Detail) - GBP (£) £ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
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 - Additional Informati

Taxation - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Income Tax Expense Benefits [Line Items]
Effective tax rate (percentage)	27.60%	25.50%
Increase in applicable tax rate		2.00%
Applicable tax rate (percentage)	19.00%	19.00%
At 1 April 2023
Income Tax Expense Benefits [Line Items]
Increase in applicable tax rate	6.00%
Applicable tax rate (percentage)	25.00%

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	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Major components of tax expense (income) [abstract]
Profit from continuing operations before tax	£ 743	£ 188	[1]
Tax calculated at a tax rate of 19% (H120: 19%)	141	36
Bank surcharge on profits	49	9
Non-deductible preference dividends paid	6	3
Non-deductible UK Bank Levy	14	6
Other non-deductible costs and non-taxable income	14	4
Effect of change in tax rate on deferred tax provision	13	7
Tax relief on dividends in respect of other equity instruments	(32)	(13)
Adjustment to prior year provisions	0	(4)
Tax on profit from continuing operations	£ 205	£ 48	[1]
[1]	Adjusted to reflect the presentation of discontinued operations as set out in Note 33.

Dividends on Ordinary Shares (D

Dividends on Ordinary Shares (Detail) - GBP (£)	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Dividends On Ordinary Shares [Abstract]
Dividends declared and paid	£ 0	£ 0

Derivative Financial Instrume_3

Derivative Financial Instruments - Notional Amounts and Fair Values of Derivatives (Detail) - GBP (£) £ in Millions	Jun. 30, 2021	Dec. 31, 2020
Disclosure of detailed information about hedges [line items]
Notional amount	£ 180,519	£ 197,215
Fair value assets	2,002	3,406
Fair value liabilities	1,063	1,584
Derivatives held for trading:
Disclosure of detailed information about hedges [line items]
Notional amount	45,245	56,251
Derivatives held for hedging
Disclosure of detailed information about hedges [line items]
Notional amount	135,274	140,964
Fair value hedges | Derivatives held for hedging
Disclosure of detailed information about hedges [line items]
Notional amount	84,909	94,537
Cash flow hedges | Derivatives held for hedging
Disclosure of detailed information about hedges [line items]
Notional amount	50,365	46,427
Exchange rate contracts | Derivatives held for trading:
Disclosure of detailed information about hedges [line items]
Notional amount	12,227	14,951
Exchange rate contracts | Fair value hedges | Derivatives held for hedging
Disclosure of detailed information about hedges [line items]
Notional amount	598	789
Exchange rate contracts | Cash flow hedges | Derivatives held for hedging
Disclosure of detailed information about hedges [line items]
Notional amount	23,756	27,020
Interest rate contracts | Derivatives held for trading:
Disclosure of detailed information about hedges [line items]
Notional amount	31,912	40,160
Interest rate contracts | Fair value hedges | Derivatives held for hedging
Disclosure of detailed information about hedges [line items]
Notional amount	84,311	93,748
Interest rate contracts | Cash flow hedges | Derivatives held for hedging
Disclosure of detailed information about hedges [line items]
Notional amount	26,609	19,407
Equity and credit contracts | Derivatives held for trading:
Disclosure of detailed information about hedges [line items]
Notional amount	1,106	1,140
Derivative assets
Disclosure of detailed information about hedges [line items]
Cash collateral received subject to enforceable master netting arrangement or similar agreement not set off against financial assets	401	330
Derivatives
Disclosure of detailed information about hedges [line items]
Cash collateral pledged subject to enforceable master netting arrangement or similar agreement not set off against financial liabilities	245	651
Fair value
Disclosure of detailed information about hedges [line items]
Fair value assets	2,002	3,406
Fair value liabilities	1,063	1,584
Fair value | Derivatives held for trading:
Disclosure of detailed information about hedges [line items]
Fair value assets	915	1,406
Fair value liabilities	708	1,015
Fair value | Derivatives held for hedging
Disclosure of detailed information about hedges [line items]
Fair value assets	2,471	3,754
Fair value liabilities	1,739	2,323
Fair value | Fair value hedges | Derivatives held for hedging
Disclosure of detailed information about hedges [line items]
Fair value assets	1,033	1,309
Fair value liabilities	1,320	1,891
Fair value | Cash flow hedges | Derivatives held for hedging
Disclosure of detailed information about hedges [line items]
Fair value assets	1,438	2,445
Fair value liabilities	419	432
Fair value | Exchange rate contracts | Derivatives held for trading:
Disclosure of detailed information about hedges [line items]
Fair value assets	229	395
Fair value liabilities	215	418
Fair value | Exchange rate contracts | Fair value hedges | Derivatives held for hedging
Disclosure of detailed information about hedges [line items]
Fair value assets	52	84
Fair value liabilities	1	6
Fair value | Exchange rate contracts | Cash flow hedges | Derivatives held for hedging
Disclosure of detailed information about hedges [line items]
Fair value assets	1,131	1,978
Fair value liabilities	381	409
Fair value | Interest rate contracts | Derivatives held for trading:
Disclosure of detailed information about hedges [line items]
Fair value assets	545	888
Fair value liabilities	440	542
Fair value | Interest rate contracts | Fair value hedges | Derivatives held for hedging
Disclosure of detailed information about hedges [line items]
Fair value assets	981	1,225
Fair value liabilities	1,319	1,885
Fair value | Interest rate contracts | Cash flow hedges | Derivatives held for hedging
Disclosure of detailed information about hedges [line items]
Fair value assets	307	467
Fair value liabilities	38	23
Fair value | Equity and credit contracts | Derivatives held for trading:
Disclosure of detailed information about hedges [line items]
Fair value assets	141	123
Fair value liabilities	53	55
Fair value | Derivative netting
Disclosure of detailed information about hedges [line items]
Fair value assets	(1,384)	(1,754)
Fair value liabilities	£ (1,384)	£ (1,754)

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	Jun. 30, 2021	Dec. 31, 2020
Disclosure of other financial assets at fair value through profit or loss [Line Items]
Financial assets at fair value through profit or loss	£ 179	£ 208
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	74	99
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	105	109
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	12	13
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	£ 62	£ 86

Loans and Advances to Custome_3

Loans and Advances to Customers - Summary of Net Loans and Advances to Customers (Detail) - GBP (£) £ in Millions	Jun. 30, 2021	Dec. 31, 2020
Net Loans And Advances To Customers [Abstract]
Loans and advances to customers	£ 209,188	£ 210,107
Credit impairment loss allowances on loans and advances to customers	(1,141)	(1,303)
RV and voluntary termination provisions on finance leases	(49)	(54)
Net loans and advances to customers	£ 207,998	£ 208,750

Securitisations and Covered B_3

Securitisations and Covered Bonds - Analysis of Securitisations and Covered Bonds (Detail) £ in Millions	Jun. 30, 2021GBP (£)	Jun. 30, 2021EUR (€)	Dec. 31, 2020GBP (£)	Dec. 31, 2020EUR (€)
Disclosure of securitisations and covered bonds [line items]
Gross assets	£ 25,164		£ 33,432
Notional amount	180,519		197,215
Euro 35bn Global Covered Bond Programme
Disclosure of securitisations and covered bonds [line items]
Notional amount | €		€ 35,000,000,000		€ 35,000,000,000
Securitisation programmes
Disclosure of securitisations and covered bonds [line items]
Gross assets	6,164		9,762
Securitisation programmes | Mortgage-backed master trust structures:
Disclosure of securitisations and covered bonds [line items]
Gross assets	4,736		8,113
Securitisation programmes | Other asset-backed securitisation structures:
Disclosure of securitisations and covered bonds [line items]
Gross assets	1,428		1,649
Holmes | Securitisation programmes | Mortgage-backed master trust structures:
Disclosure of securitisations and covered bonds [line items]
Gross assets	2,566		3,073
Fosse | Securitisation programmes | Mortgage-backed master trust structures:
Disclosure of securitisations and covered bonds [line items]
Gross assets	2,170		2,258
Langton | Securitisation programmes | Mortgage-backed master trust structures:
Disclosure of securitisations and covered bonds [line items]
Gross assets	0		2,782
Motor | Securitisation programmes | Other asset-backed securitisation structures:
Disclosure of securitisations and covered bonds [line items]
Gross assets	103		189
Auto ABS UK Loans | Securitisation programmes | Other asset-backed securitisation structures:
Disclosure of securitisations and covered bonds [line items]
Gross assets	1,325		1,460
Euro 35bn Global Covered Bond Programme | Covered bond programmes
Disclosure of securitisations and covered bonds [line items]
Gross assets	£ 19,000		£ 23,670

Securitisations and Covered B_4

Securitisations and Covered Bonds - Summary of Issuances and Redemptions of Securitisation and Covered Bond Programme (Detail) - GBP (£) £ in Billions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Internal issuances
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	£ 0	£ 0
Internal and external redemptions	2.6	0.2
Internal issuances | Covered bond programmes
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0
Internal and external redemptions	0	0
External issuances
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	3
Internal and external redemptions	4.9	2.5
External issuances | Covered bond programmes
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	3
Internal and external redemptions	4.5	1.8
Holmes | Internal issuances | Mortgage-backed master trust structures:
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0
Internal and external redemptions	0	0.1
Holmes | External issuances | Mortgage-backed master trust structures:
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0
Internal and external redemptions	0.3	0.5
Langton | Internal issuances | Mortgage-backed master trust structures:
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0
Internal and external redemptions	2.4	0
Langton | External issuances | Mortgage-backed master trust structures:
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0
Internal and external redemptions	0	0
Motor | Internal issuances | Other asset-backed securitisation structures:
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0
Internal and external redemptions	0.1	0.1
Motor | External issuances | Other asset-backed securitisation structures:
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0
Internal and external redemptions	0	0.1
Auto ABS UK Loans | Internal issuances | Other asset-backed securitisation structures:
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0
Internal and external redemptions	0.1	0
Auto ABS UK Loans | External issuances | Other asset-backed securitisation structures:
Issuances And Redemptions Of Securitisation And Covered Bond [Line items]
Internal and external issuances	0	0
Internal and external redemptions	£ 0.1	£ 0.1

Reverse Repurchase Agreements_3

Reverse Repurchase Agreements - Non Trading - Summary of Reverse Repurchase Agreements - Non Trading (Detail) - GBP (£) £ in Millions	Jun. 30, 2021	Dec. 31, 2020
Disclosure of reserve repurchase agreements non-trading [line items]
Reverse repurchase agreements – non trading	£ 18,197	£ 19,599
Agreements with banks
Disclosure of reserve repurchase agreements non-trading [line items]
Reverse repurchase agreements – non trading	1,250	1,258
Agreements with customers
Disclosure of reserve repurchase agreements non-trading [line items]
Reverse repurchase agreements – non trading	£ 16,947	£ 18,341

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	Jun. 30, 2021	Dec. 31, 2020
Financial assets at amortised cost [line items]
Financial assets at amortised cost	£ 530	£ 1,163
Asset backed securities
Financial assets at amortised cost [line items]
Financial assets at amortised cost	467	491
Debt securities
Financial assets at amortised cost [line items]
Financial assets at amortised cost	£ 63	£ 672

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	Jun. 30, 2021	Dec. 31, 2020
Disclosure of financial assets at fair value through other comprehensive income [line items]
Financial assets at fair value through other comprehensive income	£ 6,368	£ 8,950
Debt securities
Disclosure of financial assets at fair value through other comprehensive income [line items]
Financial assets at fair value through other comprehensive income	6,348	8,929
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	£ 20	£ 21

Intangible Assets - Schedule of

Intangible Assets - Schedule of Goodwill for Cash Generating Units (Detail) - GBP (£) £ in Millions	Jun. 30, 2021	Dec. 31, 2020
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	14.60%	13.60%
Growth rate beyond initial cash flow projections	1.60%	1.60%
Private banking
Disclosure of information for cash-generating units [line items]
Goodwill	£ 30	£ 30
Discount rate	11.00%	8.90%
Growth rate beyond initial cash flow projections	1.60%	1.60%
Other
Disclosure of information for cash-generating units [line items]
Goodwill	£ 4	£ 4
Discount rate	14.60%	13.60%
Growth rate beyond initial cash flow projections	1.60%	1.60%

Property, Plant and Equipment_2

Property, Plant and Equipment (Detail) - GBP (£) £ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	£ 1,734
Property, plant and equipment at end of period	1,596		£ 1,734
Accelerated depreciation expense	4		9
Construction in progress	72		55
Property
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	783
Property, plant and equipment at end of period	636		783
Office fixtures and equipment
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	307
Property, plant and equipment at end of period	213		307
Computer software
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	2
Property, plant and equipment at end of period	2		2
Operating lease assets
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	542
Property, plant and equipment at end of period	616		542
Right-of-use assets
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	100
Property, plant and equipment at end of period	129		100
Cost
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	4,021	£ 4,095	4,095
Additions	288	299
Disposals	(715)	(373)
Property, plant and equipment at end of period	3,594		4,021
Cost | Property
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	1,272	1,270	1,270
Additions	23	61
Disposals	(306)	(59)
Property, plant and equipment at end of period	989		1,272
Cost | Office fixtures and equipment
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	1,375	1,436	1,436
Additions	12	43
Disposals	(267)	(104)
Property, plant and equipment at end of period	1,120		1,375
Cost | Computer software
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	436	439	439
Additions	0	2
Disposals	0	(5)
Property, plant and equipment at end of period	436		436
Cost | Operating lease assets
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	720	738	738
Additions	194	185
Disposals	(122)	(203)
Property, plant and equipment at end of period	792		720
Cost | Right-of-use assets
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	218	212	212
Additions	59	8
Disposals	(20)	(2)
Property, plant and equipment at end of period	257		218
Accumulated depreciation
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	(2,287)	(2,128)	(2,128)
Additions	(121)	(316)
Disposals	515	181
Impairment during the period	105	24
Property, plant and equipment at end of period	(1,998)		(2,287)
Accumulated depreciation | Property
Disclosure of detailed information about property, plant and equipment [line items]
Property, plant and equipment at beginning of period	(489)	(454)	(454)
Additions	(17)	(55)
Disposals	210	44
Impairment during the period	57	24
Property, plant and equipment at end of period	(353)		(489)
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	(1,068)	(1,016)	(1,016)
Additions	(46)	(111)
Disposals	236	59
Impairment during the period	29	0
Property, plant and equipment at end of period	(907)		(1,068)
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)	(434)
Additions	0	0
Disposals	0	0
Impairment during the period	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	(178)	(164)	(164)
Additions	(48)	(92)
Disposals	50	78
Impairment during the period	0	0
Property, plant and equipment at end of period	(176)		(178)
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	(118)	(60)	(60)
Additions	(10)	(58)
Disposals	19	0
Impairment during the period	19	£ 0
Property, plant and equipment at end of period	£ (128)		£ (118)

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) £ in Millions	Jun. 30, 2021GBP (£)	Jun. 30, 2021USD ($)	Dec. 31, 2020GBP (£)	Dec. 31, 2020USD ($)
Disclosure of other financial liabilities at fair value through profit or loss [line items]
Other financial liabilities at fair value through profit or loss	£ 1,023		£ 1,434
Notional amount	180,519		197,215
US$30bn Euro Medium Term Note Programme
Disclosure of other financial liabilities at fair value through profit or loss [line items]
Other financial liabilities at fair value through profit or loss	96		102
Notional amount | $		$ 30,000,000,000		$ 30,000,000,000
Structured Notes Programmes
Disclosure of other financial liabilities at fair value through profit or loss [line items]
Other financial liabilities at fair value through profit or loss	431		805
Eurobonds
Disclosure of other financial liabilities at fair value through profit or loss [line items]
Other financial liabilities at fair value through profit or loss	144		150
Structured deposits
Disclosure of other financial liabilities at fair value through profit or loss [line items]
Other financial liabilities at fair value through profit or loss	347		375
Collateral and associated financial guarantees
Disclosure of other financial liabilities at fair value through profit or loss [line items]
Other financial liabilities at fair value through profit or loss	£ 5		£ 2

Deposits by Customers - Schedul

Deposits by Customers - Schedule of Deposits by Customers (Detail) - GBP (£) £ in Millions	Jun. 30, 2021	Dec. 31, 2020
Disclosure of deposits from customers [line items]
Deposits by customers	£ 193,317	£ 195,135
Demand and time deposits
Disclosure of deposits from customers [line items]
Deposits by customers	185,179	185,879
Amounts due to other Santander UK Group Holdings plc subsidiaries
Disclosure of deposits from customers [line items]
Deposits by customers	67	59
Amounts due to Santander UK Group Holdings plc
Disclosure of deposits from customers [line items]
Deposits by customers	6,912	7,883
Amounts due to fellow Banco Santander subsidiaries and joint ventures
Disclosure of deposits from customers [line items]
Deposits by customers	1,159	1,314
Equity Index Linked Deposits
Disclosure of deposits from customers [line items]
Deposits by customers	578	577
Capital amount guaranteed or protected	578	577
Capital amount of return guaranteed	£ 2	£ 2

Deposits by Banks - Schedule of

Deposits by Banks - Schedule of Deposits by Banks (Detail) - GBP (£) £ in Millions	Jun. 30, 2021	Dec. 31, 2020
Disclosure of deposits from banks [line items]
Deposits by banks	£ 20,502	£ 20,958
Drawdown from TFS	3,300	6,300
Drawdown from TFSME	15,200	11,700
Items in the course of transmission
Disclosure of deposits from banks [line items]
Deposits by banks	497	375
Deposits held as collateral
Disclosure of deposits from banks [line items]
Deposits by banks	1,034	2,063
Other deposits
Disclosure of deposits from banks [line items]
Deposits by banks	18,951	18,519
Amounts due to Santander UK subsidiaries
Disclosure of deposits from banks [line items]
Deposits by banks	£ 20	£ 1

Repurchase Agreements - Non T_3

Repurchase Agreements - Non Trading - Summary of Repurchase Agreements Non Trading (Detail) - GBP (£) £ in Millions	Jun. 30, 2021	Dec. 31, 2020
Disclosure Of Repurchase Agreements [line items]
Repurchase agreements - non trading	£ 14,089	£ 15,848
Agreements with banks
Disclosure Of Repurchase Agreements [line items]
Repurchase agreements - non trading	2,898	6,358
Agreements with customers
Disclosure Of Repurchase Agreements [line items]
Repurchase agreements - non trading	£ 11,191	£ 9,490

Debt Securities In Issue - Summ

Debt Securities In Issue - Summary of Debt Securities in Issue (Detail) £ in Millions	Jun. 30, 2021GBP (£)	Jun. 30, 2021EUR (€)	Jun. 30, 2021USD ($)	Dec. 31, 2020GBP (£)	Dec. 31, 2020EUR (€)	Dec. 31, 2020USD ($)
Debt securities in issue [line items]
Debt securities	£ 29,140			£ 35,566
Notional amount	180,519			197,215
Medium-term notes
Debt securities in issue [line items]
Debt securities	6,908			8,212
Euro 35bn Global Covered Bond Programme
Debt securities in issue [line items]
Debt securities	14,677			19,285
Notional amount | €		€ 35,000,000,000			€ 35,000,000,000
US$20bn Commercial Paper Programmes
Debt securities in issue [line items]
Debt securities	3,052			2,824
Notional amount | $			$ 20,000,000,000			$ 20,000,000,000
Certificates of deposit
Debt securities in issue [line items]
Debt securities	2,466			2,858
Credit linked notes
Debt securities in issue [line items]
Debt securities	58			57
Securitisation programmes
Debt securities in issue [line items]
Debt securities	£ 1,979			£ 2,330

Subordinated Liabilities - Sche

Subordinated Liabilities - Schedule of Subordinated Liabilities (Detail) - GBP (£) £ in Millions	Jun. 30, 2021	Dec. 31, 2020
Disclosure Of Subordinated Liabilities [Abstract]
Subordinated liabilities	£ 2,513	£ 2,556

Subordinated Liabilities - Addi

Subordinated Liabilities - Additional Information (Detail) - GBP (£) £ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Disclosure Of Subordinated Liabilities [Abstract]
Loss on repurchase of debt securities and subordinated liabilities	£ 27	£ 17

Provisions - Summary of Provisi

Provisions - Summary of Provisions Reconciliation (Detail) £ in Millions	6 Months Ended
	Jun. 30, 2021GBP (£)
Disclosure of other provisions [line items]
Beginning balance	£ 464
Additional provisions	191
Provisions released	(24)
Utilisation and other	(187)
Ending balance	444
PPI
Disclosure of other provisions [line items]
Beginning balance	76
Additional provisions	0
Provisions released	0
Utilisation and other	(39)
Ending balance	37
Other products
Disclosure of other provisions [line items]
Beginning balance	8
Additional provisions	0
Provisions released	0
Utilisation and other	0
Ending balance	8
Bank Levy
Disclosure of other provisions [line items]
Beginning balance	34
Additional provisions	0
Provisions released	0
Utilisation and other	(34)
Ending balance	0
Property
Disclosure of other provisions [line items]
Beginning balance	45
Additional provisions	43
Provisions released	0
Utilisation and other	(9)
Ending balance	79
Off balance sheet ECL
Disclosure of other provisions [line items]
Beginning balance	75
Additional provisions	0
Provisions released	(24)
Utilisation and other	0
Ending balance	51
Regulatory and other
Disclosure of other provisions [line items]
Beginning balance	226
Additional provisions	148
Provisions released	0
Utilisation and other	(105)
Ending balance	£ 269

Provisions - Additional Informa

Provisions - Additional Information (Detail) - GBP (£) £ in Millions	6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Future expected provision number
Provisions	£ 444	£ 464
Additional provisions	191
PPI
Future expected provision number
Provisions	37	76
Additional provisions	0
Property
Future expected provision number
Provisions	79	£ 45
Additional provisions	43
Regulatory and other provisions, multi-year transformation programme
Future expected provision number
Additional provisions	69
Regulatory and other provisions
Future expected provision number
Provisions for redundancy costs related to transformation programs	57
Operational risk provisions
Future expected provision number
Additional provisions	50
Legal proceedings provision
Future expected provision number
Additional provisions	£ 13

Retirement Benefit Plans - Summ

Retirement Benefit Plans - Summary of Retirement Benefit Plans (Detail) - GBP (£) £ in Millions	Jun. 30, 2021	Dec. 31, 2020
Assets/(liabilities)
Net defined benefit assets	£ 931	£ 92
Unfunded pension and post retirement medical benefits
Assets/(liabilities)
Surplus (deficit) in plan	(39)	(42)
Surplus | Funded defined benefit pension scheme
Assets/(liabilities)
Surplus (deficit) in plan	1,083	495
Deficit | Funded defined benefit pension scheme
Assets/(liabilities)
Surplus (deficit) in plan	£ (113)	£ (361)

Retirement Benefit Plans - Addi

Retirement Benefit Plans - Additional Information (Detail) - GBP (£) £ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020
Disclosure of net defined benefit liability (asset) [line items]
Defined contribution expense	£ 33	£ 33
Defined benefit expense	23	£ 17
Assets at fair value	8,708			£ 12,564
Longevity swap
Disclosure of net defined benefit liability (asset) [line items]
Percentage of pensioner liabilities covered by longevity swap			85.00%
Assets at fair value	£ 0

Retirement Benefit Plans - Su_2

Retirement Benefit Plans - Summary of Amounts Recognised in Other Comprehensive Income (Detail) - GBP (£) £ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Disclosure of net defined benefit liability (asset) [abstract]
Return on plan assets (excluding amounts included in net interest expense)	£ 383	£ (1,115)
Actuarial losses arising from changes in demographic assumptions	0	38
Actuarial gains arising from experience adjustments	(30)	(10)
Actuarial (gains)/losses arising from changes in financial assumptions	(1,098)	1,463
Pension remeasurement	£ (745)	£ 376

Retirement Benefit Plans - Su_3

Retirement Benefit Plans - Summary of Net Assets Recognised (Detail) - GBP (£) £ in Millions	Jun. 30, 2021	Dec. 31, 2020
Disclosure of net defined benefit liability (asset) [abstract]
Present value of defined benefit obligations	£ (12,675)	£ (13,887)
Fair value of scheme assets	13,606	13,979
Net defined benefit assets	£ 931	£ 92

Retirement Benefit Plans - Su_4

Retirement Benefit Plans - Summary of Principal Actuarial Assumptions Used for Defined Benefit Schemes (Detail)	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Disclosure of net defined benefit liability (asset) [abstract]
Discount rate for scheme liabilities	1.90%	1.30%
General price inflation	3.20%	3.00%
General salary increase	1.00%	1.00%
Expected rate of pension increase	3.10%	2.90%
Longevity at 60 for current pensioners, on the valuation date:
Males	27 years 6 months	27 years 6 months
Females	30 years 1 month 6 days	30 years
Longevity at 60 for future pensioners currently aged 40, on the valuation date:
Males	29 years 1 month 6 days	29 years
Females	31 years 7 months 6 days	31 years 6 months

Retirement Benefit Plans - Su_5

Retirement Benefit Plans - Summary of Actuarial Assumption Sensitivities (Detail) - GBP (£) £ in Millions	6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
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	£ (573)	£ (662)
Percentage of reasonably possible increase in actuarial assumption	0.25%
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	£ 411	480
Percentage of reasonably possible increase in actuarial assumption	0.25%
Actuarial assumption of expected rates of inflation | Previously stated
Disclosure of sensitivity analysis for actuarial assumptions [line items]
Increase (decrease) in defined benefit obligation		365
Actuarial assumption of mortality rates
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	£ 453	£ 515

Contingent Liabilities and Co_3

Contingent Liabilities and Commitments - Summary of Contingent Liabilities and Commitments (Detail) - GBP (£) £ in Millions	Jun. 30, 2021	Dec. 31, 2020
Disclosure of contingent liabilities and commitments [line items]
Guarantees and formal standby facilities, credit lines and other commitments	£ 41,696	£ 43,160
Contingent liability for guarantees
Disclosure of contingent liabilities and commitments [line items]
Guarantees given to third parties	739	939
Loan commitments
Disclosure of contingent liabilities and commitments [line items]
Formal standby facilities, credit lines and other commitments	£ 40,957	£ 42,221

Contingent Liabilities and Co_4

Contingent Liabilities and Commitments - Additional Information (Detail) £ in Millions	Nov. 02, 2015	Jun. 30, 2021EUR (€)	Jun. 30, 2021GBP (£)	Jun. 30, 2021EUR (€)	Dec. 31, 2020GBP (£)	Jul. 31, 2020GBP (£)
Disclosure of contingent liabilities and commitments [line items]
Provisions			£ 444		£ 464
Top of range
Disclosure of contingent liabilities and commitments [line items]
Santander UK liability under indemnity cap | €				€ 39,850,000
Bottom of range
Disclosure of contingent liabilities and commitments [line items]
Losses on litigation settlements | €		€ 1,000,000,000
AXA France
Disclosure of contingent liabilities and commitments [line items]
Amount claimed by other party			636
UK&I Banks
Disclosure of contingent liabilities and commitments [line items]
Preferred stock issued, threshold amount | €		€ 0
Genworth Financial International Holdings Inc
Disclosure of contingent liabilities and commitments [line items]
Amount awarded to other party						£ 624
Visa Europe Ltd
Disclosure of contingent liabilities and commitments [line items]
Visa Europe Ltd sale to Visa Inc	100.00%
Compliance with Consumer Credit Act
Disclosure of contingent liabilities and commitments [line items]
Provisions			£ 41		£ 47

Other Equity Instruments - Summ

Other Equity Instruments - Summary of Other Equity Instruments (Detail)	1 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2021GBP (£)	Jun. 30, 2021GBP (£)	Dec. 31, 2020GBP (£)
Disclosure of other equity instruments [line items]
Other equity instruments		£ 2,191,000,000	£ 2,191,000,000
£300m Step-up Callable Perpetual Reserve Capital Instruments
Disclosure of other equity instruments [line items]
Capital securities issued		£ 300,000,000	300,000,000
Interest rate		0.07037
Other equity interest maturity period		2026-02
Other equity instruments		£ 235,000,000	235,000,000
£500m Fixed Rate Reset Perpetual AT1 Capital Securities
Disclosure of other equity instruments [line items]
Capital securities issued		£ 500,000,000	500,000,000
Interest rate		0.0675
Other equity interest maturity period		2024-06
Other equity instruments		£ 496,000,000	496,000,000
£750m Fixed Rate Reset Perpetual AT1 Capital Securities
Disclosure of other equity instruments [line items]
Capital securities issued		£ 750,000,000	750,000,000
Interest rate		0.07375
Other equity interest maturity period		2022-06
Other equity instruments		£ 750,000,000	750,000,000
£500m Fixed Rate Reset Perpetual AT1 Capital Securities
Disclosure of other equity instruments [line items]
Capital securities issued		£ 500,000,000	500,000,000
Interest rate	0.0518	0.0518
Other equity instruments		£ 0	210,000,000
£500m Fixed Rate Reset Perpetual AT1 Capital Securities
Disclosure of other equity instruments [line items]
Capital securities issued		£ 500,000,000	500,000,000
Interest rate		0.0630
Other equity interest maturity period		2025-03
Other equity instruments		£ 500,000,000	500,000,000
£210m Fixed Rate Reset Perpetual AT1 Capital Securities
Disclosure of other equity instruments [line items]
Capital securities issued	£ 210,000,000	£ 210,000,000	210,000,000
Interest rate	0.0425	0.0425
Other equity interest maturity period		2026-03
Other equity instruments		£ 210,000,000	£ 0

Other Equity Instruments - Addi

Other Equity Instruments - Additional Information (Detail)	1 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2021GBP (£)	Jun. 30, 2021GBP (£)	Dec. 31, 2020GBP (£)
£500m Fixed Rate Reset Perpetual AT1 Capital Securities
Disclosure of other equity instruments [line items]
Capital securities purchased and redeemed, remaining amount	£ 210,000,000
Capital securities purchased and redeemed	£ 500,000,000
Interest rate	0.0518	0.0518
Capital securities issued		£ 500,000,000	£ 500,000,000
£210m Fixed Rate Reset Perpetual AT1 Capital Securities
Disclosure of other equity instruments [line items]
Interest rate	0.0425	0.0425
Capital securities issued	£ 210,000,000	£ 210,000,000	£ 210,000,000

Non-Controlling Interests - Sch

Non-Controlling Interests - Schedule of Amounts Attributable to Non-Controlling Interests (Detail) - GBP (£) £ in Millions	Jun. 30, 2021	Dec. 31, 2020
Disclosure of noncontrolling interests [line items]
Non-controlling interests	£ 180	£ 162
PSA Finance UK Limited
Disclosure of noncontrolling interests [line items]
Non-controlling interests	£ 180	£ 162

Assets Charged as Security fo_2

Assets Charged as Security for Liabilities and Collateral Accepted as Security for Assets (Detail) - GBP (£) £ in Millions	Jun. 30, 2021	Dec. 31, 2020
Disclosure of assets pledged as security [line items]
Gross assets charged as security for liabilities	£ 25,164	£ 33,432
Internally Retained | Securitisations and covered bonds
Disclosure of assets pledged as security [line items]
Notes issued under securitisation and covered bond programmes retained internally	2,011	4,530
Third Party Bilateral Secured Funding Transactions | Securitisations and covered bonds
Disclosure of assets pledged as security [line items]
Notes issued to Santander UK plc/subsidiaries as collateral	500	1,114
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	£ 130	£ 2,294

Financial Instruments - Analysi

Financial Instruments - Analysis of Fair Value of Financial Instruments Carried at Amortised Cost (Detail) - GBP (£) £ in Millions	Jun. 30, 2021	Dec. 31, 2020
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Loans and advances to customers	£ 207,998	£ 208,750
Loans and advances to banks	1,312	1,682
Reverse repurchase agreements - non trading	18,197	19,599
Assets held for sale	2,645	0
Deposits by customers	193,317	195,135
Deposits by banks	20,502	20,958
Repurchase agreements - non trading	14,089	15,848
Debt securities in issue	29,140	35,566
Subordinated liabilities	2,513	2,556
Liabilities held for sale	3,912	0
Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Financial assets	233,357	233,761
Financial liabilities at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Financial liabilities	264,740	271,522
Deposits by customers | Financial liabilities at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Deposits by customers	193,389	195,242
Deposits by banks | Financial liabilities at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Deposits by banks	20,503	20,967
Repurchase agreements - non trading | Financial liabilities at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Repurchase agreements - non trading	14,088	15,847
Debt securities in issue | Financial liabilities at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Debt securities in issue	29,890	36,397
Subordinated liabilities | Financial liabilities at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Subordinated liabilities	3,080	3,069
Liabilities held for sale | Financial liabilities at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Liabilities held for sale	3,790	0
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	210,815	211,279
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	1,312	1,682
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	18,198	19,608
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	539	1,192
Assets held for sale | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Assets held for sale	2,493	0
Fair value | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Financial assets	230,523	231,194
Fair value | Financial liabilities at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Financial liabilities	263,351	270,063
Fair value | Deposits by customers | Financial liabilities at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Deposits by customers	193,317	195,135
Fair value | Deposits by banks | Financial liabilities at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Deposits by banks	20,502	20,958
Fair value | Repurchase agreements - non trading | Financial liabilities at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Repurchase agreements - non trading	14,089	15,848
Fair value | Debt securities in issue | Financial liabilities at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Debt securities in issue	29,140	35,566
Fair value | Subordinated liabilities | Financial liabilities at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Subordinated liabilities	2,513	2,556
Fair value | Liabilities held for sale | Financial liabilities at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Liabilities held for sale	3,790	0
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	207,998	208,750
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	1,312	1,682
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	18,197	19,599
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	530	1,163
Fair value | Assets held for sale | Financial assets at amortised cost
Disclosure Of Fair Values Of Financial Instruments Carried At Amortised Cost [line items]
Assets held for sale	£ 2,486	£ 0

Financial Instruments - Fair Va

Financial Instruments - Fair Values of Financial Instruments Measured at Fair Value on a Recurring Basis (Detail) - GBP (£) £ in Millions	6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 8,708	£ 12,564
Liabilities at fair value	2,208	3,018
Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	1,063	1,584
Other financial liabilities at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	1,023	1,434
Liabilities held for sale
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	122	0
Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	2,002	3,406
Other financial assets at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	179	208
Financial assets at FVOCI
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	6,368	8,950
Assets held for sale
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	159	0
Level 1
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	6,160	8,501
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 | Other financial liabilities at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	0	0
Level 1 | Liabilities held for sale
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 FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	0	0
Level 1 | Financial assets at FVOCI
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	6,160	8,501
Level 1 | Assets held for sale
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	0	0
Level 2
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	2,284	3,766
Liabilities at fair value	2,169	2,978
Level 2 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	1,031	1,552
Level 2 | Other financial liabilities at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	1,016	1,426
Level 2 | Liabilities held for sale
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	122	0
Level 2 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	1,953	3,338
Level 2 | Other financial assets at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	0	0
Level 2 | Financial assets at FVOCI
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	188	428
Level 2 | Assets held for sale
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	143	0
Level 3
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	264	297
Liabilities at fair value	39	40
Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	32	32
Level 3 | Other financial liabilities at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	7	8
Level 3 | Liabilities held for sale
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	0	0
Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	49	68
Level 3 | Other financial assets at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	179	208
Level 3 | Financial assets at FVOCI
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	20	21
Level 3 | Assets held for sale
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 16	0
Exchange rate contracts | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Valuation technique	A
Liabilities at fair value	£ 565	833
Exchange rate contracts | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 1,404	2,457
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	565	833
Exchange rate contracts | Level 2 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	1,403	2,455
Exchange rate contracts | Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	0	0
Exchange rate contracts | Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 1	2
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	£ 1,829	2,450
Interest rate contracts | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 1,841	2,580
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	1,827	2,447
Interest rate contracts | Level 2 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	1,841	2,566
Interest rate contracts | Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	2	3
Interest rate contracts | Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 0	14
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	£ 53	55
Equity and credit contracts | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 141	123
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	23	26
Equity and credit contracts | Level 2 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	93	71
Equity and credit contracts | Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	30	29
Equity and credit contracts | Level 3 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	48	52
Netting | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	(1,384)	(1,754)
Netting | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	(1,384)	(1,754)
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	(1,384)	(1,754)
Netting | Level 2 | Derivative financial instruments
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	(1,384)	(1,754)
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
Loans and advances to customers | Other financial assets at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 74	99
Valuation technique	A
Loans and advances to customers | Financial assets at FVOCI
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 20	21
Valuation technique	D
Loans and advances to customers | Level 1 | Other financial assets at FVTPL
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 FVOCI
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 FVTPL
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 FVOCI
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 FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	74	99
Loans and advances to customers | Level 3 | Financial assets at FVOCI
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	20	21
Debt securities | Other financial assets at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 105	109
Valuation technique	A, B & D
Debt securities | Financial assets at FVOCI
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 6,348	8,929
Valuation technique	D
Debt securities | Level 1 | Other financial assets at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 0	0
Debt securities | Level 1 | Financial assets at FVOCI
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	6,160	8,501
Debt securities | Level 2 | Other financial assets at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	0	0
Debt securities | Level 2 | Financial assets at FVOCI
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	188	428
Debt securities | Level 3 | Other financial assets at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	105	109
Debt securities | Level 3 | Financial assets at FVOCI
Disclosure of fair value measurements of assets and liabilities [line items]
Assets at fair value	£ 0	0
Debt securities in issue | Other financial liabilities at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Valuation technique	A
Liabilities at fair value	£ 671	1,057
Debt securities in issue | Level 1 | Other financial liabilities at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	0	0
Debt securities in issue | Level 2 | Other financial liabilities at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	666	1,051
Debt securities in issue | Level 3 | Other financial liabilities at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	£ 5	6
Structured deposits | Other financial liabilities at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Valuation technique	A
Liabilities at fair value	£ 347	375
Structured deposits | Level 1 | Other financial liabilities at FVTPL
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 FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	347	375
Structured deposits | Level 3 | Other financial liabilities at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	£ 0	0
Collateral and associated financial guarantees | Other financial liabilities at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Valuation technique	D
Liabilities at fair value	£ 5	2
Collateral and associated financial guarantees | Level 1 | Other financial liabilities at FVTPL
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 FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	3	0
Collateral and associated financial guarantees | Level 3 | Other financial liabilities at FVTPL
Disclosure of fair value measurements of assets and liabilities [line items]
Liabilities at fair value	£ 2	£ 2

Financial Instruments - Additio

Financial Instruments - Additional Information (Detail) - GBP (£) £ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Disclosure Of Fair Value Measurement Of Assets And Liabilities [Abstract]
Transfers out of Level 1 into Level 2 of fair value hierarchy, assets held at end of reporting period	£ 0	£ 0
Transfers out of Level 2 into Level 1 of fair value hierarchy, assets held at end of reporting period	0	0
Transfers into Level 3 of fair value hierarchy, assets	0	0
Transfers out of Level 3 of fair value hierarchy, assets	0	0
Transfers out of Level 1 into Level 2 of fair value hierarchy, liabilities held at end of reporting period	0	0
Transfers out of Level 2 into Level 1 of fair value hierarchy, liabilities held at end of reporting period	0	0
Transfers into Level 3 of fair value hierarchy, liabilities	0	0
Transfers out of Level 3 of fair value hierarchy, liabilities	£ 0	£ 0

Financial Instruments - Summary

Financial Instruments - Summary of Fair Value Adjustment (Detail) - GBP (£) £ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Bid-offer and trade specific adjustments
Disclosure of financial assets [line items]
Fair value adjustments	£ (7)	£ (8)
Uncertainty
Disclosure of financial assets [line items]
Fair value adjustments	21	23
Credit risk adjustment
Disclosure of financial assets [line items]
Fair value adjustments	7	11
Funding fair value adjustment
Disclosure of financial assets [line items]
Fair value adjustments	2	3
Risk related adjustments
Disclosure of financial assets [line items]
Fair value adjustments	£ 23	£ 29

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	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Disclosure of fair value measurement of assets [line items]
Beginning balance	£ 292,332
Beginning balance	(276,396)
Transfers out	0	£ 0
Transfers out	0	0
Ending Balance	285,614
Ending Balance	(268,906)
Level 3 | Other financial assets at FVTPL
Disclosure of fair value measurement of assets [line items]
Beginning balance	208	386
Fair value movements	(7)	(2)
Transfers to held for sale	(16)
Netting	5	(37)
Settlements	(11)	(110)
Foreign exchange and other movements		(2)
Transfers out		0
Additions		0
Sales		(11)
Ending Balance	179	224
Gains/(losses) recognised in profit or loss/other comprehensive income relating to assets and liabilities held at the end of the period	(7)	(4)
Level 3 | Financial assets at FVOCI
Disclosure of fair value measurement of assets [line items]
Beginning balance	21	56
Fair value movements	(1)	1
Transfers to held for sale	0
Netting	0	0
Settlements	0	(8)
Foreign exchange and other movements		0
Transfers out		0
Additions		0
Sales		(19)
Ending Balance	20	30
Gains/(losses) recognised in profit or loss/other comprehensive income relating to assets and liabilities held at the end of the period	(1)	1
Level 3 | Assets held for sale
Disclosure of fair value measurement of assets [line items]
Beginning balance	0	0
Fair value movements	0	0
Transfers to held for sale	16
Netting	0	0
Settlements	0	0
Foreign exchange and other movements		0
Transfers out		0
Additions		0
Sales		0
Ending Balance	16	0
Gains/(losses) recognised in profit or loss/other comprehensive income relating to assets and liabilities held at the end of the period	0	0
Level 3 | Financial assets at fair value
Disclosure of fair value measurement of assets [line items]
Beginning balance	297	517
Fair value movements	(9)	5
Transfers to held for sale	0
Netting	5	(37)
Settlements	(29)	(135)
Foreign exchange and other movements		(2)
Transfers out		0
Additions		0
Sales		(30)
Ending Balance	264	318
Gains/(losses) recognised in profit or loss/other comprehensive income relating to assets and liabilities held at the end of the period	(9)	3
Level 3 | Other financial liabilities at FVTPL
Disclosure of fair value measurement of assets [line items]
Beginning balance	(8)	(61)
Fair value movements	3	15
Transfers to held for sale	(2)
Netting	0	0
Settlements	0	1
Foreign exchange and other movements		2
Transfers out		28
Additions		(1)
Sales		0
Ending Balance	(7)	(16)
Gains/(losses) recognised in profit or loss/other comprehensive income relating to assets and liabilities held at the end of the period	3	17
Level 3 | Liabilities held for sale
Disclosure of fair value measurement of assets [line items]
Beginning balance	0	0
Fair value movements	0	0
Transfers to held for sale	0
Netting	0	0
Settlements	0	0
Foreign exchange and other movements		0
Transfers out		0
Additions		0
Sales		0
Ending Balance	0	0
Gains/(losses) recognised in profit or loss/other comprehensive income relating to assets and liabilities held at the end of the period	0	0
Level 3 | Financial liabilities at fair value
Disclosure of fair value measurement of assets [line items]
Beginning balance	(40)	(93)
Fair value movements	3	14
Transfers to held for sale	(2)
Netting	0	0
Settlements	0	4
Foreign exchange and other movements		2
Transfers out		28
Additions		(1)
Sales		0
Ending Balance	(39)	(46)
Gains/(losses) recognised in profit or loss/other comprehensive income relating to assets and liabilities held at the end of the period	3	16
Level 3 | Derivatives
Disclosure of fair value measurement of assets [line items]
Beginning balance	(32)	(32)
Fair value movements	0	(1)
Transfers to held for sale	0
Netting	0	0
Settlements	0	3
Foreign exchange and other movements		0
Transfers out		0
Additions		0
Sales		0
Ending Balance	(32)	(30)
Gains/(losses) recognised in profit or loss/other comprehensive income relating to assets and liabilities held at the end of the period	0	(1)
Level 3 | Derivatives
Disclosure of fair value measurement of assets [line items]
Beginning balance	68	75
Fair value movements	(1)	6
Transfers to held for sale	0
Netting	0	0
Settlements	(18)	(17)
Foreign exchange and other movements		0
Transfers out		0
Additions		0
Sales		0
Ending Balance	49	64
Gains/(losses) recognised in profit or loss/other comprehensive income relating to assets and liabilities held at the end of the period	£ (1)	£ 6

Interest Rate Benchmark Refor_2

Interest Rate Benchmark Reform - Amounts Impacted by IBOR Reform (Detail) - GBP (£) £ in Millions	Jun. 30, 2021	Dec. 31, 2020
Assets
Derivative financial instruments	£ 2,002	£ 3,406
Other financial assets at fair value through profit or loss	179	208
Financial assets at amortised cost	530	1,163
Financial assets at fair value through other comprehensive income	6,368	8,950
Assets held for sale	2,645	0
Total assets	285,614	292,332
Liabilities
Derivative financial instruments	1,063	1,584
Other financial liabilities at fair value through profit or loss	1,023	1,434
Liabilities held for sale	3,912	0
Total liabilities	268,906	276,396
GBP LIBOR
Assets
Derivative financial instruments	21,893	33,486
Other financial assets at fair value through profit or loss	808	968
Financial assets at amortised cost	12,706	15,062
Financial assets at fair value through other comprehensive income	189	428
Assets held for sale	1,602
Total assets	37,198	49,944
Liabilities
Derivative financial instruments	26,227	35,217
Other financial liabilities at fair value through profit or loss	967	1,129
Financial liabilities at amortised cost	895	2,354
Liabilities held for sale	0
Total liabilities	28,089	38,700
Off-balance sheet commitments given	10,277	11,400
USD LIBOR
Assets
Derivative financial instruments	2,580	4,514
Other financial assets at fair value through profit or loss	20	22
Financial assets at amortised cost	157	1,191
Financial assets at fair value through other comprehensive income	0	0
Assets held for sale	161
Total assets	2,918	5,727
Liabilities
Derivative financial instruments	2,558	5,205
Other financial liabilities at fair value through profit or loss	41	69
Financial liabilities at amortised cost	567	1,319
Liabilities held for sale	27
Total liabilities	3,193	6,593
Off-balance sheet commitments given	803	2,126
Other
Assets
Derivative financial instruments	653	2,149
Other financial assets at fair value through profit or loss	0	0
Financial assets at amortised cost	83	90
Financial assets at fair value through other comprehensive income	0	0
Assets held for sale	0
Total assets	736	2,239
Liabilities
Derivative financial instruments	56	88
Other financial liabilities at fair value through profit or loss	0	0
Financial liabilities at amortised cost	0	0
Liabilities held for sale	0
Total liabilities	56	88
Off-balance sheet commitments given	198	573
Total
Assets
Derivative financial instruments	25,126	40,149
Other financial assets at fair value through profit or loss	828	990
Financial assets at amortised cost	12,946	16,343
Financial assets at fair value through other comprehensive income	189	428
Assets held for sale	1,763
Total assets	40,852	57,910
Liabilities
Derivative financial instruments	28,841	40,510
Other financial liabilities at fair value through profit or loss	1,008	1,198
Financial liabilities at amortised cost	1,462	3,673
Liabilities held for sale	27
Total liabilities	31,338	45,381
Off-balance sheet commitments given	£ 11,278	£ 14,099

Interest Rate Benchmark Refor_3

Interest Rate Benchmark Reform - Derivatives Directly Affected (Detail) number in Millions	Jun. 30, 2021	Dec. 31, 2020
GBP LIBOR
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	32,368	47,421
GBP LIBOR | Maturing after cessation date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	26,020	23,030
USD LIBOR
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	3,054	6,196
USD LIBOR | Maturing after cessation date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	2,693	2,724
Other
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	696	778
Other | Maturing after cessation date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	696	720
Total
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	36,118	54,395
Total | Maturing after cessation date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	29,409	26,474
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	11,610	15,198
Cash flow hedges | GBP LIBOR | Maturing after cessation date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	9,788	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	2,894	5,119
Cash flow hedges | USD LIBOR | Maturing after cessation date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	2,533	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	0
Cash flow hedges | Other | Maturing after cessation date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	0	0
Cash flow hedges | Total
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	14,504	20,317
Cash flow hedges | Total | Maturing after cessation date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	12,321	13,115
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	20,758	32,223
Fair value hedges | GBP LIBOR | Maturing after cessation date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	16,232	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	160	1,077
Fair value hedges | USD LIBOR | Maturing after cessation date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	160	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	696	778
Fair value hedges | Other | Maturing after cessation date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	696	720
Fair value hedges | Total
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	21,614	34,078
Fair value hedges | Total | Maturing after cessation date
Analysis of notional and fair values of derivatives by trading and settlement method [line item]
Notional value of hedging instruments	17,088	13,359

Discontinued Operations and A_3

Discontinued Operations and Assets and Liabilities Classified as Held for Sale - Disclosure of Financial Performance Relating to Discontinued Operations (Detail) - GBP (£) £ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Disclosure of analysis of single amount of discontinued operations [line items]
Net interest income	£ 1,905	£ 1,528	[1]
Net fee and commission income	151	200	[1]
Total operating income	2,191	1,828	[1]
Operating expenses before credit impairment losses, provisions and charges	(1,328)	(1,212)	[1]
Credit impairment losses	70	(364)	[1]
Provisions for other liabilities and charges	(190)	(64)	[1]
Total operating credit impairment losses, provisions and charges	(120)	(428)	[1]
Profit/(loss) from discontinued operations after tax	24	(1)	[1]
Discontinued operations
Disclosure of analysis of single amount of discontinued operations [line items]
Net interest income	24	25
Net fee and commission income	27	22
Total operating income	51	47
Operating expenses before credit impairment losses, provisions and charges	(22)	(33)
Credit impairment losses	7	(12)
Provisions for other liabilities and charges	(3)	(4)
Total operating credit impairment losses, provisions and charges	4	(16)
Profit/(loss) from discontinued operations before tax	33	(2)
Tax on profit/(loss) from discontinued operations	(9)	1
Profit/(loss) from discontinued operations after tax	£ 24	£ (1)
[1]	Adjusted to reflect the presentation of discontinued operations as set out in Note 33.

Discontinued Operations and A_4

Discontinued Operations and Assets and Liabilities Classified as Held for Sale - Additional Information (Detail) - GBP (£) £ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Discontinued Operations and Assets and Liabilities Held for Sale [Abstract]
Gain (loss) recognised on measurement to fair value less costs to sell or on disposal of assets or disposal groups constituting discontinued operation	£ 0	£ 0
Cash flows from (used in) operating activities, discontinued operations	1,754	524
Cash flows from (used in) investing activities, discontinued operations	0	0
Cash flows from (used in) financing activities, discontinued operations	£ 0	£ 0

Discontinued Operations and A_5

Discontinued Operations and Assets and Liabilities Classified as Held for Sale - Disclosure of Assets and Liabilities Held for Sale (Detail) - GBP (£) £ in Millions	Jun. 30, 2021	Dec. 31, 2020
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Assets held for sale	£ 2,645	£ 0
Liabilities held for sale	3,912	£ 0
Derivative financial instruments
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Liabilities held for sale	95
Other financial liabilities at FVTPL
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Liabilities held for sale	27
Deposits by customers
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Liabilities held for sale	3,769
Deposits by banks
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Liabilities held for sale	21
Derivative financial instruments
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Assets held for sale	143
Other financial assets at fair value through profit or loss
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Assets held for sale	16
Loans and advances to customers
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Assets held for sale	2,486
CIB business transfer to SLB
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Assets held for sale	2,037
Liabilities held for sale	3,912
CIB business transfer to SLB | Derivative financial instruments
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Liabilities held for sale	95
CIB business transfer to SLB | Other financial liabilities at FVTPL
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Liabilities held for sale	27
CIB business transfer to SLB | Deposits by customers
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Liabilities held for sale	3,769
CIB business transfer to SLB | Deposits by banks
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Liabilities held for sale	21
CIB business transfer to SLB | Derivative financial instruments
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Assets held for sale	143
CIB business transfer to SLB | Other financial assets at fair value through profit or loss
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Assets held for sale	16
CIB business transfer to SLB | Loans and advances to customers
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Assets held for sale	1,878
Sale of residential mortgages
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Assets held for sale	608
Liabilities held for sale	0
Sale of residential mortgages | Derivative financial instruments
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Liabilities held for sale	0
Sale of residential mortgages | Other financial liabilities at FVTPL
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Liabilities held for sale	0
Sale of residential mortgages | Deposits by customers
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Liabilities held for sale	0
Sale of residential mortgages | Deposits by banks
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Liabilities held for sale	0
Sale of residential mortgages | Derivative financial instruments
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Assets held for sale	0
Sale of residential mortgages | Other financial assets at fair value through profit or loss
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Assets held for sale	0
Sale of residential mortgages | Loans and advances to customers
Disclosure of Assets and Liabilities Classified as Held for Sale [Line Items]
Assets held for sale	£ 608

Events After the Balance Shee_2

Events After the Balance Sheet Date (Detail) - GBP (£) £ in Millions	Jul. 30, 2021	Jun. 30, 2021
Disclosure of non-adjusting events after reporting period [line items]
Description of nature of non-adjusting event after reporting period		On 30 July 2021, the Santander UK group through Santander Consumer (UK) plc sold its entire 50% shareholding in PSA Finance UK Limited (PSA) to PSA Financial Services Spain EFC SA, a joint venture between Santander Consumer Finance SA, a fellow subsidiary of Banco Santander SA, and Banque PSA Finance SA, the auto finance arm of Group PSA Peugeot Citroën. The impact of the sale was to derecognise total assets of £3.2bn, total liabilities of £2.9bn and a non-controlling interest of £0.15bn. No material gain or loss arose on sale. For more information on PSA, including summary financial information, see Note 19 to the Consolidated Financial Statements in the 2020 Annual Report.
Events After the Balance Sheet Date		34. EVENTS AFTER THE BALANCE SHEET DATE There have been no significant events between 30 June 2021 and the date of approval of these financial statements which would require a change to or additional disclosure in the financial statements, except for the following non-adjusting event:
Disposal of major subsidiary | Santander Consumer (UK) | PSA Finance UK Limited
Disclosure of non-adjusting events after reporting period [line items]
Proportion of ownership interest in subsidiary sold	50.00%
Assets derecognised at disposal	£ 3,200
Liabilities derecognised at disposal	2,900
Disposal of major subsidiary | Santander Consumer (UK) | PSA Finance UK Limited | Non-controlling interests
Disclosure of non-adjusting events after reporting period [line items]
Non-controlling interest derecognised at disposal	£ 150