NuCana (NCNA) 20-F 10 Mar 20 2019 FY Annual report (foreign) Financial data

Document and Entity Information

Document and Entity Information	12 Months Ended
	Dec. 31, 2019shares
Document Information [Line Items]
Document Type	20-F
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Trading Symbol	NCNA
Entity Registrant Name	NUCANA PLC
Entity Central Index Key	0001709626
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding	32,478,645
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Interactive Data Current	Yes
Title of 12(b) Security	American Depositary Shares, each representing one Ordinary Share, nominal value £0.04 per share
Security Exchange Name	NASDAQ
Entity File Number	001-38215
Entity Incorporation, State or Country Code	X0
Entity Address, Address Line One	NuCana plc
Entity Address, Address Line Two	3 Lochside Way
Entity Address, City or Town	Edinburgh
Entity Address, Country	GB
Document Annual Report	true
Document Transition Report	false
Entity Address, Postal Zip Code	EH12 9DT
Document Accounting Standard	International Financial Reporting Standards
Document Registration Statement	false
Document Shell Company Report	false
Business Contact
Document Information [Line Items]
Contact Personnel Name	Hugh S. Griffith
Entity Address, Address Line One	NuCana plc
Entity Address, Address Line Two	3 Lochside Way
Entity Address, City or Town	Edinburgh
Entity Address, Country	GB
Local Phone Number	3571111
City Area Code	131
Entity Address, Postal Zip Code	EH12 9DT

Consolidated Statements of Oper

Consolidated Statements of Operations - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Research and development expenses	£ (19,728)	£ (16,846)	£ (17,673)
Administrative expenses	(5,953)	(5,184)	(4,573)
Initial public offering related expenses			(1,794)
Net foreign exchange (losses) gains	(1,019)	2,902	(1,654)
Operating loss	(26,700)	(19,128)	(25,694)
Finance income	1,049	1,065	208
Loss before tax	(25,651)	(18,063)	(25,486)
Income tax credit	4,239	4,223	2,401
Loss for the year attributable to equity holders of the Company	£ (21,412)	£ (13,840)	£ (23,085)
Basic and diluted loss per share	£ (0.66)	£ (0.43)	£ (0.89)

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Loss - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement Of Comprehensive Income [Abstract]
Loss for the year	£ (21,412)	£ (13,840)	£ (23,085)
Items that may be reclassified subsequently to profit or loss:
Exchange differences on translation of foreign operations	(11)	12	(8)
Other comprehensive (expense) income for the year	(11)	12	(8)
Total comprehensive loss for the year	(21,423)	(13,828)	(23,093)
Attributable to:
Equity holders of the Company	£ (21,423)	£ (13,828)	£ (23,093)

Consolidated Statements of Fina

Consolidated Statements of Financial Position - GBP (£) £ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Non-current assets
Intangible assets	£ 3,960	£ 3,122
Property, plant and equipment	1,109	427
Deferred tax asset	46	47
Total non-current assets	5,115	3,596
Current assets
Prepayments, accrued income and other receivables	4,710	2,354
Current income tax receivable	8,481	4,263
Cash and cash equivalents	51,962	76,972
Total current assets	65,153	83,589
Total assets	70,268	87,185
Capital and reserves
Share capital and share premium	80,840	80,715
Other reserves	62,737	59,692
Accumulated deficit	(80,055)	(58,813)
Total equity attributable to equity holders of the Company	63,522	81,594
Non-current liabilities
Provisions	26	26
Lease liabilities	538
Total non-current liabilities	564	26
Current liabilities
Trade payables	2,412	2,455
Payroll taxes and social security	160	127
Accrued expenditure	3,342	2,983
Lease liabilities	268
Total current liabilities	6,182	5,565
Total liabilities	6,746	5,591
Total equity and liabilities	£ 70,268	£ 87,185

Consolidated Statements Of Chan

Consolidated Statements Of Changes In Equity - GBP (£) £ in Thousands	Total	Share Capital	Share CapitalSeries B Shareholders	Share Premium	Share PremiumSeries B Shareholders	Own Share Reserve	Share Option Reserve	Foreign Currency Translation Reserve	Capital Reserve	Accumulated Deficit
Beginning balance at Dec. 31, 2016	£ 25,241	£ 663		£ 42,770		£ (339)	£ 4,406	£ (3)		£ (22,256)
Loss for the year	(23,085)									(23,085)
Other comprehensive income (expense) for the year	(8)							(8)
Total comprehensive loss for the year	(23,093)							(8)		(23,085)
Share-based payments	11,731						11,731
Reduction in share premium				(42,466)					£ 42,466
Exercise of share options	120	1		119			(180)			180
Lapse of share options							(2)			2
Bonus issue to series B shareholders			£ 304		£ (304)
Issue of share capital	79,834	304		79,530
Initial public offering related expenses	(413)			(413)
Ending balance at Dec. 31, 2017	93,420	1,272		79,236		(339)	15,955	(11)	42,466	(45,159)
Loss for the year	(13,840)									(13,840)
Other comprehensive income (expense) for the year	12							12
Total comprehensive loss for the year	(13,828)							12		(13,840)
Share-based payments	1,795						1,795
Exercise of share options	207	17		190			(186)			186
Ending balance at Dec. 31, 2018	81,594	1,289		79,426		(339)	17,564	1	42,466	(58,813)
Loss for the year	(21,412)									(21,412)
Other comprehensive income (expense) for the year	(11)							(11)
Total comprehensive loss for the year	(21,423)							(11)		(21,412)
Share-based payments	3,226						3,226
Exercise of share options	125	10		115			(132)			132
Lapse of share options							(38)			38
Ending balance at Dec. 31, 2019	£ 63,522	£ 1,299		£ 79,541		£ (339)	£ 20,620	£ (10)	£ 42,466	£ (80,055)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Loss for the year	£ (21,412)	£ (13,840)	£ (23,085)
Adjustments for:
Income tax credit	(4,239)	(4,223)	(2,401)
Amortization and depreciation	718	371	194
Finance income	(1,049)	(1,065)	(208)
Share-based payments	3,226	1,795	11,731
Initial public offering (IPO) related expenses			1,794
Net foreign exchange losses (gains)	1,006	(2,959)	1,584
Cash flows from (used in) operations before changes in working capital	(21,750)	(19,921)	(10,391)
Movements in working capital:
(Increase) decrease in prepayments, accrued income and other receivables	(2,452)	817	458
(Decrease) increase in trade payables	(43)	1,335	392
Increase in payroll taxes, social security and accrued expenditure	393	1,321	551
Movements in working capital	(2,102)	3,473	1,401
Cash used in operations	(23,852)	(16,448)	(8,990)
Net income tax received	19	4,224	282
Net cash used in operating activities	(23,833)	(12,224)	(8,708)
Cash flows from investing activities
Interest received	1,116	973	162
Payments for property, plant and equipment	(46)	(210)	(370)
Payments for intangible assets	(1,215)	(1,414)	(725)
Net cash used in investing activities	(145)	(651)	(933)
Cash flows from financing activities
Payments of lease liabilities	(197)
Proceeds from lease incentives received	25
Proceeds from issue of share capital			79,834
Proceeds from issue of share capital - exercise of share options	125	207	120
IPO related expenses from issue of share capital - included in share premium			(413)
IPO related expenses included in statement of operations			(1,794)
Net cash (used in) from financing activities	(47)	207	77,747
Net (decrease) increase in cash and cash equivalents	(24,025)	(12,668)	68,106
Cash and cash equivalents at beginning of year	76,972	86,703	19,990
Effect of exchange rate changes on cash and cash equivalents	(985)	2,937	(1,393)
Cash and cash equivalents at end of year	£ 51,962	£ 76,972	£ 86,703

General Information

General Information	12 Months Ended
	Dec. 31, 2019
Disclosure Of General Information About Financial Statements [Abstract]
General information	1. General information NuCana plc (“NuCana” or the “Company”) is a clinical-stage biopharmaceutical company developing a portfolio of new medicines to treat cancer. We are harnessing the power of phosphoramidate chemistry to generate new medicines called ProTides. These compounds have the potential to improve cancer treatment by enhancing the efficacy and safety of several current standards of care. The Company has had American Depository Shares (“ADSs”) registered with the US Securities and Exchange Commission (“SEC”) and has been listed on Nasdaq since October 2, 2017. The Company is incorporated in England and Wales and domiciled in the United Kingdom. The Company’s registered office is located at 77/78 Cannon Street, London EC4N 6AF, United Kingdom and its principal place of business is located at 3 Lochside Way, Edinburgh, EH12 9DT, United Kingdom. The Company has two wholly owned subsidiaries, NuCana, Inc. and NuCana BioMed Trustee Company Limited. The financial statements for the Company were authorized for issue by the board of directors on March 10, 2020.

Significant Accounting Policies

Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
Disclosure Of Summary Of Significant Accounting Policies [Abstract]
Significant accounting policies	2. Significant accounting policies Basis of preparation The financial statements have been prepared in accordance with International Financial Reporting Standards (“IFRS”) as issued by the International Accounting Standards Board (“IASB”). The Company’s financial statements comprise the financial statements of the Company and its subsidiaries at December 31, 2019. The financial statements are presented in pounds sterling, which is also the Company’s functional currency. All values are rounded to the nearest thousand, except where otherwise indicated. Going concern In common with many companies in the biopharmaceutical sector, the Company incurs significant expenditure in its early years as it researches and develops its potential products for market. The board of directors, having reviewed the operating budgets and development plans, considers that the Company has adequate resources to continue in operation for the foreseeable future. The board of directors is therefore satisfied that it is appropriate to adopt the going concern basis of accounting in preparing the financial statements. The Company believes that its cash and cash equivalents of £52.0 million at December 31, 2019, will be sufficient to fund its current operating plan for at least the next 12 months. As the Company continues to incur losses, the transition to profitability is dependent upon the successful development, approval and commercialization of its product candidates and achieving a level of revenues adequate to support its cost structure. The Company may never achieve profitability, and unless and until it does, it will continue to need to raise additional capital. There can be no assurances, however, that additional funding will be available on acceptable terms. Judgements and estimates The preparation of the financial statements requires management to make judgements, estimates and assumptions that affect the amounts reported for assets and liabilities at the balance sheet dates and the amounts reported for revenue and expenses during the year. The nature of estimations means that actual outcomes could differ from those estimates. The following judgements have had the most significant effect on the amounts recognized in the financial statements: Research and development expenses The Company recognizes research and development expenses in the statement of operations in the period in which they are incurred. When development activities reach the advanced stage, as set out in the specific criteria of International Accounting Standard (“IAS”) 38, Intangible Assets 2. Significant accounting policies (continued) Taxation Management judgement is required to determine the amount of deferred tax assets that should be recognized, based upon the likely timing and level of future taxable profits. Further details are contained in Note 7. The following estimates have had the most significant effect on the amounts recognized in the financial statements: Recognition of Clinical Trial Expenses As part of the process of preparing our consolidated financial statements, we may be required to estimate accrued or prepaid expenses related to our clinical trials. In order to obtain reasonable estimates, we review open contracts and master service agreements • fees paid to CROs for services performed on clinical trials; and • pass-through costs for activities at clinical trial investigator sites. Accruals for clinical trial expenses, including estimated amounts recognized consistent with the above policy, were £1.7 million at December 31, 2019 as compared to £1.5 million at December 31, 2018. Recognition of Contracted Manufacturing Expenses As part of the process of preparing our consolidated financial statements, we may be required to estimate accrued or prepaid expenses related to our contracted manufacturing expenses. In order to obtain reasonable estimates, we review open contracts and master service agreements. In addition, we consult with applicable personnel in order to identify services that have been performed and which have not yet been invoiced, and services not yet performed for which we have been invoiced in advance. Accruals for contracted manufacturing expenses, including estimated amounts recognized consistent with the above policy, were £0.2 million at December 31, 2019 as compared to £0.2 million at December 31, 2018. Share-based payments Estimating fair value for share-based payment transactions requires determination of the most appropriate valuation model, which depends on the terms and conditions of the grant. This estimate also requires determination of the most appropriate inputs to the valuation model, including the expected life of the share option, volatility, dividend yield and assumptions about them and, in the case of the Company, the value of an ordinary share. For the measurement of the fair value of equity-settled transactions with employees at the grant date, the Company uses the Black-Scholes model. The assumptions and models used for estimating fair value for share-based payment transactions are detailed in Note 13. Basis of consolidation The consolidated financial statements comprise the financial statements of the Company and its subsidiaries. Subsidiaries are consolidated from the date of acquisition, being the date on which the Company obtains control, and continue to be consolidated until the date when such control ceases. The financial statements of the subsidiaries are prepared for the same reporting period as the parent company, using consistent accounting policies. All intra-group balances, transactions, unrealized gains and losses resulting from intra-group transactions and dividends are eliminated in full. Assets, liabilities, income and expenses of a subsidiary acquired or disposed of during the year are included in the consolidated financial statements from the date the Company gains control until the date the Company ceases to control the subsidiary. 2. Significant accounting policies (continued) Foreign currencies The Company’s consolidated financial statements are presented in pounds sterling, which is also the parent company’s functional currency. For each group entity, the Company determines the functional currency and items included in the financial statements of each entity are measured using that functional currency. Transactions and balances Transactions in foreign currencies are initially recorded by the Company’s entities at their respective functional currency spot rates at the date the transaction first qualifies for recognition. Monetary assets and liabilities denominated in foreign currencies are translated at the functional currency spot rates of exchange at the reporting date. Differences arising on settlement or translation of monetary items are recognized in the consolidated statements of operations. Non-monetary items that are measured in terms of historical cost in a foreign currency are translated using the exchange rates at the dates of the initial transactions. Group companies On consolidation, the assets and liabilities of foreign operations are translated into pounds sterling at the rate of exchange prevailing at the reporting date and their statements of operations are translated at the average exchange rate for the financial period in which those transactions occur. The exchange differences arising on translation for consolidation are recognized in the consolidated statements of comprehensive income or loss. Segment reporting The Company operates in one operating segment. Operating segments are reported in a manner consistent with the internal reporting provided to the Company’s chief operating decision maker (“the CODM”). The Company’s CODM, its Chief Executive Officer, views the Company’s operations and manages its business as a single operating segment, which is the business of developing and commercializing for use in Oncology. The Company’s principal operations and decision-making functions are located in the United Kingdom from where global decisions are made. Initial public offering (IPO) related expenses Incremental costs incurred and directly attributable to the offering of securities in 2017 were deducted from the related proceeds of the offering. The net amount is recorded as contributed shareholders’ equity in the period when such shares were issued. Costs that relate to the stock market listing or are otherwise not incremental and directly attributable to issuing new shares, are recorded as an expense in the statement of comprehensive income. Costs that relate to both share issuance and listing are allocated between those functions on a rational and consistent basis. In the absence of a more specific basis for apportionment, an allocation of common costs based on the proportion of new shares issued to the total number of (new and existing) shares listed has been used. Property, plant and equipment Property, plant and equipment is stated at cost, net of accumulated depreciation and accumulated impairment losses, if any. There are no restrictions on title nor equipment pledged as security for liabilities. 2. Significant accounting policies (continued) Depreciation is provided on property, plant and equipment over their expected useful economic life as follows: Asset class Depreciation method and period Office and computer equipment Straight-line over 3 years Fixtures and fittings Straight-line over 5 years, or, for non-removable items, the remaining term of an associated lease, whichever is shorter Right of use assets Straight-line over the lease term, which are between two and five years, or the estimated useful lives of the assets, whichever is shorter Intangible assets Intangible assets are stated at cost, net of accumulated amortization and accumulated impairment losses, if any. Cost in relation to patents includes registration, documentation and other legal fees associated with obtaining the patent. Computer software cost represents the initial purchase price of the asset. The amortization method and amortization period for the principal categories of intangible assets are follows: Asset class Amortization method and period Patents Straight-line over 20 years Computer software Straight-line between 3 and 5 years The Company’s primary patents each have a life of 20 years. Further patents are granted in various jurisdictions to extend the territorial coverage of the primary patent. These patents are granted up to the period of the related primary patent. Costs are thus amortized over the remaining life of the relevant primary patent. The amortization expense on intangible assets with finite lives is recognized in the statements of operations as an administrative expense. The amortization method and the amortization period for an intangible asset with a finite useful life are reviewed at least at each financial year end. Changes in the expected useful economic life or the expected pattern of consumption of future economic benefits embodied in the asset are accounted for by changing the amortization period or method, as appropriate. Intangible assets are tested for impairment when there is an indicator of impairment. Cash and cash equivalents Cash and cash equivalents in the statement of financial position include cash at banks with a maturity of less than three months, which is subject to an insignificant risk of changes in value. Research and development Research and development expenses are currently recognized in the statement of operations in the year in which they are incurred. Development expenditures on an individual project will be recognized as an intangible asset when the Company can demonstrate: • the technical feasibility of completing the intangible asset so that the asset will be available for use or sale; • its intention to complete and its ability and intention to use or sell the asset; • how the asset will generate future economic benefits; • the availability of resources to complete the asset; and • the ability to measure reliably the expenditure during development. 2. Significant accounting policies (continued) Income Taxes Current income tax Current income tax assets and liabilities are measured at the amount expected to be recovered from or paid to the taxation authorities. The tax rates and tax laws used to compute the amount are those that are enacted or substantively enacted at the reporting date in the countries where the Company operates and generates taxable income. Deferred income tax Deferred income tax is provided in full, using the liability method, on temporary differences arising between the tax bases of assets and liabilities and their carrying amounts in the Company’s financial statements. However, the deferred income tax is not accounted for if it arises from initial recognition of an asset or liability in a transaction other than a business combination that at the time of the transaction affects neither accounting nor taxable profit or loss. Deferred income tax is determined using tax rates and laws that have been enacted or substantially enacted by the year end date and are expected to apply when the related deferred income tax asset is realized or the deferred tax liability is settled. Deferred tax assets are recognized to the extent that it is probable that future taxable profit will be available against which the temporary differences can be utilized. Income tax credit The Company benefits from the U.K. and U.S. research and development tax credit regimes. In the U.K. a portion of the Company’s losses can be surrendered for a cash rebate of up to 33.35% of eligible expenditures. In the U.S. the Company is able to offset the research and development credits against corporation tax payable. Such credits are accounted for within the tax provision, in the year in which the expenditures are incurred. Operating leases (prior to January 1, 2019) Leases where the lessor retains substantially all the risks and benefits of ownership of the asset are classified as operating leases and rentals payable are charged in the consolidated statements of operations on a straight-line basis over the lease term. Leases (from January 1, 2019) The Company assesses, at contract inception, whether a contract is, or contains, a lease. That is, if the contract conveys the right to control the use of an identified asset for a period of time in exchange for consideration. The Company applies a single recognition and measurement approach for all leases, except for short-term leases and leases of low-value assets. The Company recognizes lease liabilities to make lease payments and right of use assets representing the right to use the underlying assets. Right of use assets The Company recognizes right of use assets at the commencement date of the lease (i.e. the date the underlying asset is available for use). Right of use assets are measured at cost, less any accumulated depreciation and impairment losses, and adjusted for any remeasurement of lease liabilities. The cost of right of use assets includes the amount of lease liabilities recognized, initial direct costs incurred, and lease payments made at or before the commencement date less any lease incentives received. Right of use assets, which relate solely to office space, are depreciated on a straight-line basis over the shorter of the lease terms, which are between two and five years, or the estimated useful lives of the assets. Lease At the commencement date of the lease, the Company recognizes a lease liability measured at the present value of lease payments to be made over the lease term. The lease payments include fixed payments less any lease incentives receivable, and any variable lease payments that depend on an index. 2. Significant accounting policies (continued) In calculating the present value of lease payments, the Company uses its incremental borrowing rate at the lease commencement date because the interest rate implicit in the lease is not readily determinable. After the commencement date, the amount of the lease liability is increased to reflect the accretion of interest and reduced for the lease payments made. In addition, the carrying amount of the lease liability is remeasured if there is a modification, a change in the lease term or a change in the lease payments. The Company determines the lease term as the non-cancellable term of the lease, together with any periods covered by an option to extend the lease if it is reasonably certain to be exercised, or any periods covered by an option to terminate the lease, if it is reasonably certain not to be exercised. The Company has a number of lease contracts that include extension and termination options. The Company applies judgement in evaluating whether it is reasonably certain whether or not to exercise the option to renew or terminate the lease. That is, it considers all relevant factors that create an economic incentive for it to exercise either the renewal or termination. After the commencement date, the Company reassesses the lease term if there is a significant event or change in circumstances that is within its control and affects its ability to exercise or not to exercise the option to renew or to terminate, such as the construction of significant leasehold improvements. Refer to Note 14 for information on potential future rental payments relating to periods following the exercise date of extension options that are not included in the lease liability. Impairment of non-financial assets The Company assesses, at each reporting date, whether there is an indication that an asset may be impaired. If any indication exists, the Company estimates the recoverable amount of the asset. An impairment loss is recognized whenever the carrying amount of an asset or its cash-generating unit exceeds its recoverable amount. Impairment losses are recognized in the consolidated statements of operations. A cash-generating unit is the smallest identifiable group of assets that generates cash inflows that are largely independent of the cash inflows from other assets or groups of assets. Calculation of recoverable amount The recoverable amount of assets and cash-generating units is the higher of their fair value less costs to sell and value in use. In assessing value in use, the estimated future cash flows are discounted to their present value using a pre-tax discount rate that reflects current market assessments of the time value of money and the risks specific to the asset. For an asset that does not generate largely independent cash inflows, the recoverable amount is determined for the cash-generating unit to which the asset belongs. Reversal of impairment An assessment is made at each reporting date as to whether there is an indication that a previously recognized impairment loss may no longer exist or may have decreased. If such an indication exists, the recoverable amount is estimated. A previously recognized impairment loss is reversed only if there has been a change in the estimates used to determine the recoverable amount since the last impairment loss was recognized. If that is the case, the carrying value is increased to its recoverable amount. An impairment loss is reversed only to the extent that the asset’s carrying amount does not exceed the carrying amount that would have been determined, net of depreciation or amortization, if no impairment loss had been recognized. Share-based payments Employees, directors and consultants of the Company receive remuneration in the form of share options, whereby individuals render services as consideration for equity instruments and the cost is recognized as share-based payments under IFRS 2. 2. Significant accounting policies (continued) Under IFRS 2 Share-based Payment The fair value determined at the grant date of equity settled share-based payments is expensed on a straight line basis over the vesting period, with a corresponding increase in equity to the share option reserve. Fair value measurement The fair value of the financial assets and liabilities is included at the amount at which an instrument could be exchanged in a current transaction between willing parties, other than in a forced liquidation or sale. Fair value is based on the price that would be received from the sale of an asset or that would be paid to transfer a liability in an orderly transaction between market participants at the measurement date. In order to increase consistency and comparability in fair value measurements, IFRS 13 establishes a fair value hierarchy that prioritizes observable and unobservable inputs used to measure fair value into three broad levels, which are described as follows: Level 1: Quoted (unadjusted) prices in active markets for identical assets or liabilities. Level 2: Other techniques for which all inputs that have a significant effect on the recorded fair value are observable, either directly or indirectly. Level 3: Techniques that use inputs that have a significant effect on the recorded fair value that are not based on observable market data. The fair values of cash, cash equivalents, short-term deposits, other receivables, trade payables and other payables approximate their carrying amounts largely due to the short-term maturities of these instruments. Accounting Standards In preparing these financial statements, the Company has applied all relevant IAS, IFRS and International Financial Reporting Interpretations Committee (“IFRIC”) Interpretations as of the date of approval of these financial statements and which are mandatory for the financial year ended December 31, 2019. The following accounting standards, interpretations and amendments have been adopted as of January 1, 2019 in these financial statements and, with the exception of IFRS 16 Leases • IFRS 16 Leases • IFRIC 23 Uncertainty over Income Tax Treatments • Amendments to IFRS 3 – Annual Improvements to IFRSs 2015–2017 Cycle • Amendments to IFRS 9 – Prepayment Features with Negative Compensation • Amendments to IFRS 11 – Annual Improvements to IFRSs 2015–2017 Cycle • Amendments to IAS 12 – Annual Improvements to IFRSs 2015–2017 Cycle • Amendments to IAS 19 – Plan Amendment, Curtailment or Settlement • Amendments to IAS 23 – Annual Improvements to IFRSs 2015–2017 Cycle • Amendments to IAS 28 – Long-term Interests in Associates and Joint Ventures 2. Significant accounting policies (continued) The IASB and IFRIC have issued the following standards and amendments with an effective date after the date of these financial statements: • Amendments to References to the Conceptual Framework in IFRS Standards (effective from January 1, 2020) • Amendments to IFRS 3 – Definition of a Business (effective from January 1, 2020) • Amendments to IFRS 9, IAS 39, and IFRS 7 – Interest Rate Benchmark Reform (effective from January 1, 2020) • Amendments to IAS 1 and IAS 8 – Definition of Material (effective from January 1, 2020) • IFRS 17 Insurance Contracts (effective from January 1, 2021) • Amendments to IFRS 4 – Applying IFRS 9 Financial Instruments with IFRS 4 Insurance Contracts (effective from January 1, 2021) • Amendments to IAS 1 Presentation of Financial Statements – Classification of Liabilities as Current or Non-Current (effective from January 1, 2022) The Company has reviewed and considered that the above standards and amendments either do not apply to the Company or will not have a material impact in future periods. IFRS 16 Leases IFRS 16 supersedes IAS 17 Leases Determining whether an Arrangement contains a Lease Operating Leases-Incentives Evaluating the Substance of Transactions Involving the Legal Form of a Lease The Company adopted IFRS 16 using the modified retrospective method of adoption with the date of initial application of January 1, 2019. Under this method, the standard is applied retrospectively with the cumulative effect of initially applying the standard recognized at the date of initial application. The Company elected to use the transition practical expedient to not reassess whether a contract is, or contains a lease at January 1, 2019. Instead, the Company applied the standard only to contracts that were previously identified as leases applying IAS 17 and IFRIC 4 at the date of initial application. The effect of adoption of IFRS 16 as at January 1, 2019 (increase/(decrease)) is, as follows: 2019 (in thousands) £ Assets Right of use assets (note 9) 455 Prepayments (21 ) Total assets 434 Liabilities Lease liabilities - non-current 287 Lease liabilities - current 147 Total liabilities 434 The Company has lease contracts solely for office space. Before the adoption of IFRS 16, the Company classified each of its leases (as lessee) as an operating lease. Refer to Note 2 Operating leases 2. Significant accounting policies (continued) Upon adoption of IFRS 16, the Company applied a single recognition and measurement approach for all leases except for short-term leases and leases of low-value assets. Refer to Note 2 Leases The Company recognized right of use assets and lease liabilities for the leases previously classified as operating leases. The right of use assets were recognized based on the amount equal to the lease liabilities, adjusted for any related prepaid lease payments previously recognized, and are included in property, plant and equipment on the balance sheet. Lease liabilities were recognized based on the present value of the remaining lease payments, discounted using the incremental borrowing rate at the date of initial application. The Company also applied the following available practical expedients wherein it: • Used a single discount rate to a portfolio of leases with reasonably similar characteristics. • Excluded the initial direct costs from the measurement of the right of use asset at the date of initial application. The lease liabilities as at January 1, 2019 can be reconciled to the operating lease commitments as of December 31, 2018, as follows: 2019 (in thousands, except borrowing rate) £ Operating lease obligations as at December 31, 2018 433 Weighted average incremental borrowing rate as at January 1, 2019 4.17 % Discounted operating lease obligations as at January 1, 2019 413 Less: Lease not commenced as at December 31, 2018 (138 ) Lease obligations prepaid as at December 31, 2018 (21 ) Service component included in operating lease obligations as at December 31, 2018 (3 ) Add: Lease payments relating to renewal periods not included in operating lease obligations as at December 31, 2018 183 Lease liabilities as at January 1, 2019 434

Staff Costs

Staff Costs	12 Months Ended
	Dec. 31, 2019
Classes Of Employee Benefits Expense [Abstract]
Staff costs	3. Staff costs Included in research and development expenses: 2019 2018 2017 (in thousands) £ £ £ Wages and salaries 3,304 2,761 2,391 Social security costs 395 314 272 Pension costs 140 139 119 Share-based payments 1,878 835 8,868 5,717 4,049 11,650 3. Staff costs (continued) Included in administrative expenses: 2019 2018 2017 (in thousands) £ £ £ Wages and salaries 1,190 790 408 Social security costs 115 75 37 Pension costs 37 37 21 Share-based payments 1,348 960 2,863 2,690 1,862 3,329 Total employee benefit expense 8,407 5,911 14,979 The average number of staff employed under contracts of service were: 2019 2018 2017 (number) Research and development activities 23 18 18 Administrative activities 5 4 2 28 22 20 Directors’ remuneration & other benefits 2019 2018 2017 (in thousands) £ £ £ Directors’ remuneration 1,222 1,143 793 Pension and other benefits 52 51 37 1,274 1,194 830 The number of directors who exercised share options in 2019 was 1 (2018: 1; 2017: nil). The gain on exercise of these options was £1.4 million (2018: £4.2 million; 2017: nil). The above amounts for remuneration include the following in respect of the highest paid director: 2019 2018 2017 (in thousands) £ £ £ Director’s remuneration 780 739 601 Pension and other benefits 48 47 33 828 786 634

Basic and Diluted Loss Per Shar

Basic and Diluted Loss Per Share	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Basic and diluted loss per share	4. Basic and diluted loss per share 2019 2018 2017 (in thousands, except per share data) £ £ £ Loss for the year (21,412 ) (13,840 ) (23,085 ) Basic and diluted weighted average number of shares 32,327 31,972 26,069 £ £ £ Basic and diluted loss per share (0.66 ) (0.43 ) (0.89 ) Basic loss per share is calculated by dividing the loss for the year attributable to the equity holders of the Company by the weighted average number of shares outstanding during the year. The equity-settled transactions were considered to be anti-dilutive as they would have decreased the loss per share and were therefore excluded from the calculation of diluted loss per share.

Loss Before Tax

Loss Before Tax	12 Months Ended
	Dec. 31, 2019
Material Income And Expense [Abstract]
Loss Before Tax	5. Loss before tax Loss before tax is stated after charging: 2019 2018 2017 (in thousands) £ £ £ Administrative expenses: Amortization 377 230 164 Depreciation 341 141 30 IPO related expenses — — 1,794 IPO related costs primarily relate to legal, accounting and other advisors’ fees in relation to the Company’s listing on Nasdaq which was completed on October 2, 2017.

Capital Commitments and Conting

Capital Commitments and Contingencies	12 Months Ended
	Dec. 31, 2019
Disclosure Of Capital Commitments And Contingencies [Abstract]
Capital commitments and contingencies	6. Capital commitments and contingencies Other commitments Collaboration and License Agreements Cardiff University License In August 2009, we entered into a research, collaboration and license agreement with Cardiff University and University College Cardiff Consultants Ltd., or Cardiff Consultants, which we refer to as the Cardiff Agreement. The Cardiff Agreement was renewed with an effective date of January 1, 2018 for an additional two years on substantially the same terms. In February 2020, we further amended the Cardiff Agreement to expire at the end of 2020, which amendment afforded us (at our sole discretion) an option to extend the expiration for one additional year until the end of 2021, and for further periods thereafter upon written agreement by the parties. Under the Cardiff Agreement, we collaborate with Cardiff University in the design, synthesis, characterization and evaluation of phosphoramidate prodrugs, which we refer to as ProTides, based on certain nucleosides. We are responsible for funding certain work performed by Cardiff University and making other payments, which totaled £292,607 in both 2018 and 2019 and which we expect will total approximately £175,000 in 2020. Cardiff University and Cardiff Consultants, which is a holder of intellectual property developed by Cardiff University, have assigned to us all rights in the results of the research under the Cardiff Agreement, and agreed not to undertake any research for any competing third party on nucleoside families of interest to us where such research would make use of ProTide-related intellectual property owned or controlled by Cardiff University as of the date of the Cardiff Agreement or which at any time thereafter becomes owned or controlled by Cardiff University, which we refer to as the Cardiff intellectual property, or to grant rights in the Cardiff intellectual property to any third party for use in connection with nucleosides of interest to us. The foregoing restrictions exclude the field of neurodegeneration for one specific nucleoside analog. 6. Capital commitments and contingencies (continued) Upon our completion of the evaluation of the ProTides, we have the right to select one or more of the evaluated ProTides as candidates for potential development of a commercial product. Cardiff University and Cardiff Consultants have granted us an exclusive worldwide license to use for all purposes the Cardiff intellectual property in respect of the nucleoside family of our selected ProTides. The exclusive dealing obligations of Cardiff University and Cardiff Consultants will continue for these nucleoside families. On our filing, or that of a sublicensee, of patent applications resulting from research under the Cardiff Agreement, we will owe Cardiff Consultants certain immaterial payments. If we or our sublicensees develop and commercialize a product resulting from such research, we will owe Cardiff Consultants clinical development milestone payments of up to £1,875,000; provided that such milestone payments are due only with respect to the first product within each nucleoside family to achieve the milestone. We will also owe Cardiff Consultants royalties equal to a low-single digit percentage on our sales of a product resulting from such research. Should we sublicense our right to commercialize a product resulting from the research, we will owe Cardiff Consultants a high-single digit percentage of payments received in consideration of the sublicense. The Cardiff Agreement currently expires on December 31, 2020, subject to the extension periods agreed to in the February 2020 amendment. Upon expiration, we have the right to extend the period in which we may evaluate products for three months, and for a further three months in exchange for an additional payment. The Cardiff Agreement may also be terminated for an uncured material breach. Licenses to use the Cardiff intellectual property in the development and commercialization of products we have selected for commercialization, and related payment obligations, will survive expiration of the Cardiff Agreement, but not on termination for an uncured material breach. Cardiff ProTides Agreement In October 2009, we entered into a license and collaboration agreement with Cardiff ProTides Ltd., or Cardiff ProTides, which agreement was subsequently amended and restated as an assignment, license and collaboration agreement in March 2012 and was further amended in May 2012, which we refer to as the ProTides Agreement. Under the ProTides Agreement, we collaborated with Cardiff ProTides in the discovery, drug design and in vitro screening of purine and pyrimidine based nucleosides as potential drug candidates. We funded certain work at Cardiff ProTides, and Cardiff ProTides has assigned to us all rights in the results of its research under the ProTides Agreement. Cardiff ProTides also assigned to us patents related to certain compounds of interest, including with respect to Acelarin, and granted us an exclusive, worldwide license, including the right to grant sublicenses, to rights in and technical information related to certain unpatented compounds for all therapeutic, diagnostic, prognostic and prophylactic applications. If we or a sublicensee develop one or more products covered by a valid claim of an assigned patent or patent resulting from Cardiff ProTides’ research, such as Acelarin, we will owe Cardiff ProTides up to approximately $4.5 million in development and approval milestone payments in the aggregate for the first such product. Additional development and approval milestones would be payable for the first additional product in a new nucleoside series covered by a valid claim of an assigned patent or a patent resulting from Cardiff ProTides’ research, although the maximum potential value of such milestone payments is approximately half the value of the milestone payments associated with the first product. We will also owe Cardiff ProTides royalties equal to a percentage in mid- to high-single digits on sales of such products, subject to reduction under certain circumstances. Royalties on sales by sublicensees are set by formula, which formula would be likely to result in a royalty in the mid-single digits. The ProTides Agreement expires, on a country by country basis, on the later of the expiration, invalidity, abandonment, lapsing or rejection of the last valid claim of an assigned patent or patent resulting from Cardiff ProTides’ research, or, if certain technical information licensed from Cardiff ProTides remains confidential or the product is covered by a period of data exclusivity, ten years from the date of first commercial sale of a product in such country. The ProTides Agreement may be sooner terminated on an uncured material breach, bankruptcy of a party or, by Cardiff ProTides, if we challenge, or assist in a challenge, of the validity or ownership of an assigned patent or patent resulting from Cardiff ProTides’ research, or fail to pay amounts payable under the ProTides Agreement. It may also be sooner terminated where sums payable by us remain unpaid for 45 days after we receive a notice from Cardiff ProTides that the relevant sums are overdue. Upon a termination of the ProTides Agreement, our license rights will terminate except where the breach results from certain breaches by Cardiff ProTides, in which case our license rights continue on a non-exclusive basis, subject to reduced payment obligations. Upon termination of the ProTides Agreement, including as a result of our breach, we will be under an obligation to assign back to Cardiff ProTides the patents which Cardiff ProTides originally assigned to us. 6. Capital commitments and contingencies (continued) CROs and Manufacturing commitments We have agreed to make payments to CROs and manufacturers under various CRO and manufacturing agreements. We have not included further details on such contingent payment obligations as the amount, timing and likelihood of such payments are not fixed or determinable. Other Contingencies Under the U.K. share-based payment plan, the Company granted unapproved share options that have fully vested. If and when these share options are exercised, the Company will be liable for the Employer Class 1 National Insurance payable to HMRC in the U.K. This contingent liability will be determined based on the market value of the shares on exercise less the exercise price paid by the option holders, at the prevailing rate of Employer National Insurance (currently 13.8%). Based on the closing share price of ADSs on the Nasdaq Global Select Market on December 31, 2019, the last trading day of the period to which these financial statements relate, and assuming full exercise of all outstanding and vested unapproved share options on that date, the Employer National Insurance contingent liability would have been £1.3 million (December 31, 2018: £3.3 million).

Income Tax Credit

Income Tax Credit	12 Months Ended
	Dec. 31, 2019
Major Components Of Tax Expense Income [Abstract]
Income tax credit	7. Income tax credit The major components of income tax for the years ended December 31, 2019, 2018 and 2017 are as follows: 2019 2018 2017 (in thousands) £ £ £ Current tax: In respect of current year U.K. 4,325 4,239 2,298 In respect of current year U.S. (1 ) — (10 ) In respect of prior years U.K. (86 ) 19 3 In respect of prior years U.S. — 3 — Total current tax 4,238 4,261 2,291 Deferred tax: In respect of current year U.S. 1 11 61 In respect of prior years U.S. — (49 ) 49 Total deferred tax 1 (38 ) 110 Income tax credit 4,239 4,223 2,401 Current income tax receivable: U.K. tax 8,477 4,239 4,207 U.S. tax 4 24 18 Current income tax receivable 8,481 4,263 4,225 Deferred tax: U.S. tax 46 47 81 Current income tax payable: U.S. tax — — — 7. Income tax credit (continued) The credit for the year can be reconciled to the loss per the statement of operations as follows: 2019 2018 2017 (in thousands) £ £ £ Loss before tax (25,651 ) (18,063 ) (25,486 ) Tax on loss at standard U.K. tax rate of 19% (2018: 19%; 2017: 19.25%) (4,874 ) (3,432 ) (4,906 ) Effects of: Expenses not deductible 2,912 1,389 3,899 Deduction for R&D (5,667 ) (5,554 ) (3,051 ) Losses surrendered for R&D tax credit 5,667 5,554 3,051 Deferred tax - PY adjustment — 49 (49 ) Overseas tax payable - current year 1 — 10 Overseas tax payable - prior years — (3 ) — R&D tax credit - U.S. (1 ) (11 ) (61 ) R&D tax credit - current year (4,325 ) (4,239 ) (2,298 ) R&D tax credit - prior years 86 (19 ) (3 ) Deferred tax asset not recognized 1,962 2,043 1,007 Income tax credit (4,239 ) (4,223 ) (2,401 ) In the United Kingdom, the Company has not recognized a deferred tax asset in respect of tax losses carried forward or temporary differences on share-based compensation arrangements Temporary differences and cumulative carry forward tax losses for which deferred tax has not been recognized amount to £46.4 million (2018: £62.1 million; 2017: £40.7 million), comprising temporary differences on share-based compensation arrangements of £14.2 million (2018: £38.9 million; 2017: £28.3 million) and cumulative carry forward tax losses of £32.2 million (2018: £23.2 million; 2017: £12.4 million). U.K. tax legislation, which was substantively enacted on October 26, 2015, includes legislation that will reduce the main rate of U.K. corporation tax from 20% to 18%. This decrease is being phased in with a rate of 19%, effective from April 1, 2017 and a planned rate of 18% from April 1, 2020. Further, as announced on March 16, 2016, that the full rate of U.K. corporation tax will reduce by a further 1% to 17% from April 1, 2020. This further reduction was included within the U.K. tax legislation, which was substantively enacted on September 6, 2016.

Intangible Assets

Intangible Assets	12 Months Ended
	Dec. 31, 2019
Disclosure Of Intangible Assets [Abstract]
Intangible Assets	8. Intangible assets Patents Computer Software Total (in thousands) £ £ £ Cost: At December 31, 2017 2,115 153 2,268 Additions 1,409 5 1,414 At December 31, 2018 3,524 158 3,682 Accumulated amortization: At December 31, 2017 302 28 330 Charge for the year 196 34 230 At December 31, 2018 498 62 560 Cost: At December 31, 2018 3,524 158 3,682 Additions 999 216 1,215 At December 31, 2019 4,523 374 4,897 Accumulated amortization: At December 31, 2018 498 62 560 Charge for the year 308 69 377 At December 31, 2019 806 131 937 Net book value: At December 31, 2019 3,717 243 3,960 At December 31, 2018 3,026 96 3,122

Property, Plant and Equipment

Property, Plant and Equipment	12 Months Ended
	Dec. 31, 2019
Disclosure Of Property Plant And Equipment [Abstract]
Property, plant and equipment	9. Property, plant and equipment Right of use assets Office and computer equipment Fixtures and Fittings Total (in thousands) £ £ £ £ Cost: At December 31, 2017 — 142 280 422 Additions — 84 126 210 Disposals — (16 ) — (16 ) Effect of foreign currency exchange differences — — — — At December 31, 2018 — 210 406 616 Depreciation: At December 31, 2017 — 51 13 64 Charge for the year — 61 80 141 Disposals — (16 ) — (16 ) Effect of foreign currency exchange differences — — — — At December 31, 2018 — 96 93 189 Cost: At December 31, 2018 — 210 406 616 Recognized on adoption of IFRS 16 Leases 455 — — 455 Additions 530 27 12 569 Re-measurement 2 — — 2 Disposals — (1 ) — (1 ) Effect of foreign currency exchange differences (5 ) — — (5 ) At December 31, 2019 982 236 418 1,636 Depreciation: At December 31, 2018 — 96 93 189 Charge for the year 193 65 83 341 Disposals — (1 ) — (1 ) Effect of foreign currency exchange differences (2 ) — — (2 ) At December 31, 2019 191 160 176 527 Net book value: At December 31, 2019 791 76 242 1,109 At December 31, 2018 — 114 313 427

Prepayments, Accrued Income and

Prepayments, Accrued Income and Other Receivables	12 Months Ended
	Dec. 31, 2019
Prepayments Accrued Income And Other Receivables [Abstract]
Prepayments, accrued income and other receivables	10. Prepayments, accrued income and other receivables 2019 2018 (in thousands) £ £ Prepayments—manufacturing and clinical 2,630 1,050 Prepayments—other 1,489 750 Accrued income 76 165 VAT 475 379 Other receivables 40 10 4,710 2,354

Share Capital and Share Premium

Share Capital and Share Premium	12 Months Ended
	Dec. 31, 2019
Disclosure Of Classes Of Share Capital [Abstract]
Share capital and share Premium	11. Share capital and share premium 2019 2018 2017 (in thousands) £ £ £ Share capital 1,299 1,289 1,272 Share premium 79,541 79,426 79,236 80,840 80,715 80,508 Number Number Number (in thousands) Issued share capital comprises: Ordinary shares of £0.04 each 32,479 32,226 31,811 32,479 32,226 31,811 Number of shares Share capital Share premium (in thousands) £ £ Fully paid shares: Balance at December 31, 2017 31,811 1,272 79,236 Exercise of share options 415 17 190 Balance at December 31, 2018 32,226 1,289 79,426 Exercise of share options 253 10 115 Balance at December 31, 2019 32,479 1,299 79,541 Ordinary shares Holders of ordinary shares are entitled to one vote for each share held of record on all matters submitted to a vote of shareholders and do not have cumulative voting rights. Capital management For the purpose of the Company’s capital management, capital includes issued capital, share premium and all other equity reserves attributable to the equity holders of the Company. The purpose of the Company’s capital management is to maximize shareholder value and ensure adequate capital is available to meet the medium-term operating plan. Review of operations and commitments is key to identifying future capital management and a full review is undertaken on a quarterly basis. No changes were made in the objectives, policies or processes for managing capital during the years ended December 31, 2019, 2018 or 2017.

Other Reserves

Other Reserves	12 Months Ended
	Dec. 31, 2019
Disclosure Of Other Reserves [Abstract]
Other reserves	12. Other reserves 2019 2018 2017 (in thousands) £ £ £ Own share reserve (339 ) (339 ) (339 ) Foreign currency translation reserve (10 ) 1 (11 ) Capital reserve 42,466 42,466 42,466 Share option reserve Balance at beginning of year 17,564 15,955 4,406 Share-based payments 3,289 1,977 11,731 Exercise of share options (132 ) (186 ) (180 ) Forfeiture of share options (63 ) (182 ) — Lapse of share options (38 ) — (2 ) Balance at end of year 20,620 17,564 15,955 Total other reserves 62,737 59,692 58,071 Foreign currency translation reserve The foreign currency translation reserve is used to record exchange differences arising from the translation of the financial statements of foreign operations. Own share reserve The own share reserve represents the cost of 500,000 shares of NuCana plc purchased by NuCana Employee Benefit Trust and that may, at the discretion of the trustee, be used to satisfy future exercise of options under the Company’s share options plan. Capital reserve The capital reserve balance arose from the reduction of the share premium account and corresponding increase to the capital reserve account reflected as of June 30, 2017 in connection with the Company’s re-registration as a public limited company, as further described in Note 1. Share option reserve The share option reserve is used to recognize the value of equity-settled share-based payments provided to employees, directors and consultants as part of their remuneration. Refer to Note 13 for further details of these plans.

Share-based Payments

Share-based Payments	12 Months Ended
	Dec. 31, 2019
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Abstract]
Share-based payments	13. Share-based payments The Company has three share-based payment plans for employees, directors and consultants. The share options granted will be settled in equity. Options granted under each of the three plans have a maximum life of 10 years. 2017 options In 2017, share options were granted under the following share-based payment plans: U.K. share-based payment plans Options granted under these plans will vest if the option holder remains under their respective contract of employment or contract of service for the agreed vesting period. The share options granted under these plans will vest equally over a period of four years, with the exception of options granted to a director, which vested immediately. 13. Share-based payments (continued) Upon vesting, each option allows the holder to purchase one ordinary share at a specified option price determined at grant date. Stock option plan (U.S. Sub-Plan) Options granted under these plans will vest if the option holder remains under their respective employment contract for the agreed vesting period. The share options granted under these plans will vest equally over a period of four years. 2018 options In 2018, share options were granted under the following share-based payment plan: U.K. share-based payment plans Options granted under these plans will vest if the option holder remains under their respective contract of employment or contract of service for the agreed vesting period. The share options granted under these plans will vest equally over a period of four years, with the exception of options granted to a consultant, which vested immediately. Upon vesting, each option allows the holder to purchase one ordinary share at a specified option price determined at grant date. 2019 options In 2019, share options were granted under the following share-based payment plans: U.K. share-based payment plans Options granted under these plans will vest if the option holder remains under their respective contract of employment or contract of service for the agreed vesting period. The share options granted under these plans will vest equally over a period of four years. Upon vesting, each option allows the holder to purchase one ordinary share at a specified option price determined at grant date. Stock option plan (U.S. Sub-Plan) Options granted under these plans will vest if the option holder remains under their respective employment contract for the agreed vesting period. The share options granted under these plans will vest equally over a period of four years. Upon vesting, each option allows the holder to purchase one ordinary share at a specified option price determined at grant date. 13. Share-based payments (continued) Share options and weighted average exercise prices are as follows for the reporting periods presented: Number of shares Weighted average exercise price per share £ Outstanding at January 1, 2017 3,248,187 0.72 Granted 1,500,815 2.58 Forfeited (7,500 ) 0.16 Exercised 1 (30,000 ) 4.00 Outstanding at December 31, 2017 4,711,502 1.29 Granted 253,500 17.01 Forfeited (143,438 ) 12.74 Exercised 2 (415,312 ) 0.50 Outstanding at December 31, 2018 4,406,252 1.90 Granted 1,202,150 10.82 Forfeited (48,625 ) 10.42 Lapsed (8,750 ) 3.90 Exercised 3 (252,187 ) 0.50 Outstanding at December 31, 2019 4 5,298,840 3.91 Vested and exercisable at December 31, 2019 3,811,736 1.15 Vested and exercisable at December 31, 2018 3,847,305 0.68 Vested and exercisable at December 31, 2017 4,030,833 0.36 1. The weighted average share price at the date of exercise of these options was £10.43 2. The weighted average share price at the date of exercise of these options was £18.23 3. The weighted average share price at the date of exercise of these options was £7.65 4. The exercise price of outstanding share options ranges from £0.04 to £18.05 The weighted average remaining contractual life of the share options outstanding as at December 31, 2019 is 4.63 years (2018: 3.96; 2017: 4.49). 13. Share-based payments (continued) The following principal assumptions were used in the valuation for the 2017 share options. Options granted on May 16, 2017 Sep 13, 2017 Sep 14, 2017 Sep 14, 2017 Sep 14, 2017 Vesting dates Oct 28, 2017 Sep 13, 2018 Mar 06, 2018 Sep 14, 2018 Sep 14, 2018 Oct 28, 2018 Sep 13, 2019 Mar 06, 2019 Sep 14, 2019 Sep 14, 2019 Oct 28, 2019 Sep 13, 2020 Mar 06, 2020 Sep 14, 2020 Sep 14, 2020 Oct 28, 2020 Sep 13, 2021 Mar 06, 2021 Sep 14, 2021 Sep 14, 2021 Volatility 66.56 % 66.94 % 66.97 % 66.97 % 66.97 % Dividend yield 0 % 0 % 0 % 0 % 0 % Risk-free investment rate 0.12 % 0.27 % 0.36 % 0.36 % 0.36 % Fair value of option at grant date £ 7.70 £ 6.15 £ 6.08 £ 6.08 £ 6.08 Fair value of share at grant date £ 11.08 £ 10.43 £ 10.34 £ 10.34 £ 10.34 Exercise price at date of grant £ 4.00 £ 5.40 £ 5.40 £ 5.40 £ 5.40 Lapse date May 16, 2027 Sep 13, 2027 Sep 14, 2027 Sep 14, 2027 Sep 14, 2027 Expected option life (years) 2.63 2.30 2.30 2.30 2.30 Number of options granted 23,250 14,690 25,000 110,310 12,500 Sep 14, 2017 Sep 15, 2017 Sep 15, 2017 Sep 27, 2017 Sep 27, 2017 Vesting dates Sep 14, 2018 Sep 15, 2018 Sep 15, 2017 Mar 20, 2018 Sep 27, 2018 Sep 14, 2019 Sep 15, 2019 Mar 20, 2019 Sep 27, 2019 Sep 14, 2020 Sep 15, 2020 Mar 20, 2020 Sep 27, 2020 Sep 14, 2021 Sep 15, 2021 Mar 20, 2021 Sep 27, 2021 Volatility 66.97 % 67.02 % 67.02 % 67.11 % 67.11 % Dividend yield 0 % 0 % 0 % 0 % 0 % Risk-free investment rate 0.36 % 0.44 % 0.44 % 0.47 % 0.47 % Fair value of option at grant date £ 3.90 £ 6.76 £ 10.11 £ 4.36 £ 4.36 Fair value of share at grant date £ 10.34 £ 10.15 £ 10.15 £ 11.19 £ 11.19 Exercise price at date of grant £ 10.80 £ 4.00 £ 0.04 £ 11.19 £ 11.19 Lapse date Sep 14, 2027 Sep 15, 2027 Sep 15, 2027 Sep 27, 2027 Sep 27, 2027 Expected option life (years) 2.30 2.29 2.29 2.26 2.26 Number of options granted 25,000 45,750 1,028,533 37,500 178,282 The fair values of options granted were determined using the Black-Scholes model that takes into account factors specific to the share incentive plan such as the assumption that the options are exercised at a single point in time, in December 2019. This has been incorporated into the measurement by means of actuarial modelling. As NuCana plc was unlisted at the grant date of the options, it is not possible to derive historical volatility from the Company’s own share price. The underlying expected volatility was therefore determined by using the historical volatility of similar listed entities as a proxy. The volatility percentage applied to each tranche is the average of the historical volatility of companies comparable to NuCana plc. With the exception of the awards granted on September 27, 2017, the Company’s ordinary share valuations were prepared using the guideline public company, or GPC, method under the market approach. In the application of the GPC method, we considered the pricing of IPOs completed by clinical-stage oncology companies between July 2015 and June 2017. We converted prospective IPO value to present value by applying a discount rate of 25%. The discount rate was derived from studies of rates of return required by venture investors in IPO-stage companies. In addition to the IPO GPCs, we considered the enterprise values indicated by a group of eight trading GPCs. The trading prices of these clinical-stage GPCs provided contemporaneous indications of value as of each appraisal date. We applied a discount for lack of marketability to the ordinary shares to account for the lack of access to an active public market. We estimated the discount for lack of marketability using an Asian put model. In the year ended December 31, 2017, an employee remuneration expense, all of which related to equity-settled share-based payments, of £11.7 million has been included in the statement of operations and credited to equity. 13. Share-based payments (continued) The following principal assumptions were used in the valuation for 2018 share options. Options granted on Apr 11, 2018 Apr 11, 2018 May 8, 2018 Aug 14, 2018 Vesting dates Apr 11, 2019 Apr 11, 2018 May 8, 2019 Aug 14, 2019 Apr 11, 2020 May 8, 2020 Aug 14, 2020 Apr 11, 2021 May 8, 2021 Aug 14, 2021 Apr 11, 2022 May 8, 2022 Aug 14, 2022 Volatility 64.48 % 60.06 % 65.80 % 68.14 % Dividend yield 0 % 0 % 0 % 0 % Risk-free investment rate 1.04 % 0.83 % 1.02 % 0.93 % Fair value of option at grant date £ 8.97 £ 17.35 £ 8.63 £ 9.65 Fair value of share at grant date £ 17.51 £ 17.51 £ 16.57 £ 18.05 Exercise price at date of grant £ 17.51 £ 0.16 £ 16.57 £ 18.05 Lapse date Apr 11, 2028 Apr 11, 2028 May 8, 2028 Aug 14, 2028 Expected option life (years) 4.50 2.00 4.50 4.50 Number of options granted 71,500 7,500 62,000 112,500 The fair values of options granted were determined using the Black-Scholes model that takes into account factors specific to the share incentive plan such as the assumption that the options are exercised at a point in time being 2 years after vesting. This has been incorporated into the measurement by means of actuarial modelling. As NuCana plc was unlisted until October 2, 2017, it is not possible to derive historical volatility from the Company’s own share price. The underlying expected volatility was therefore determined by using the historical volatility of similar listed entities as a proxy. The volatility percentage applied to each tranche is the average of the historical volatility of companies comparable to NuCana plc. In the year ended December 31, 2018, an employee remuneration expense, all of which related to equity-settled share-based payments, of £1.8 million (2017: £11.7 million) has been included in the statement of operations and credited to equity. 13. Share-based payments (continued) The following principal assumptions were used in the valuation for 2019 share options. Options granted on Mar 13, 2019 May 15, 2019 Sept 11, 2019 Vesting dates Mar 13, 2020 May 15, 2020 Sept 11, 2020 Mar 13, 2021 May 15, 2021 Sept 11, 2021 Mar 13, 2022 May 15, 2022 Sept 11, 2022 Mar 13, 2023 May 15, 2023 Sept 11, 2023 Volatility 69.05 % 69.08 % 70.14 % Dividend yield 0 % 0 % 0 % Risk-free investment rate 0.85 % 0.77 % 0.44 % Fair value of option at grant date £ 5.46 £ 6.07 £ 4.22 Fair value of share at grant date £ 10.13 £ 11.26 £ 7.79 Exercise price at date of grant £ 10.13 £ 11.26 £ 7.79 Lapse date Mar 13, 2029 May 15, 2029 Sept 11, 2029 Expected option life (years) 4.50 4.50 4.50 Number of options granted 120,750 967,400 114,000 The fair values of options granted were determined using the Black-Scholes model that takes into account factors specific to the share incentive plan such as the assumption that the options are exercised at a point in time being 2 years after vesting. This has been incorporated into the measurement by means of actuarial modelling. As NuCana plc was unlisted until October 2, 2017, it is not possible to derive historical volatility from the Company’s own share price. The underlying expected volatility was therefore determined by using the historical volatility of similar listed entities as a proxy. The volatility percentage applied to each tranche is the average of the historical volatility of companies comparable to NuCana plc. In the year ended December 31, 2019, an employee remuneration expense, all of which related to equity-settled share-based payments, of £3.2 million (2018: £1.8 million; 2017: £11.7 million) has been included in the statement of operations and credited to equity.

Leases

Leases	12 Months Ended
	Dec. 31, 2019
Presentation Of Leases For Lessee [Abstract]
Leases	14. Leases The Company has lease contracts solely for office space with lease terms of between two and five years. Generally, the Company is restricted from assigning and subleasing the leased assets. There are a number of lease contracts that include extension and termination options and variable lease payments, which are further discussed below. Refer to Note 9 for the carrying amounts of right of use assets recognized and the movements during the period. The carrying amounts of lease liabilities and the movements during the period are as follows: 2019 (in thousands) £ Initial recognition at January 1, 2019 434 Additions 548 Re-measurement of liability 2 Accretion of interest 21 Payments (197 ) Effect of foreign currency exchange differences (2 ) At December 31, 2019 806 Classified as: Current 268 Non-current 538 806 The maturity analysis of lease liabilities are as follows: 2019 (in thousands) £ Contractual undiscounted payments Not later than 1 year 293 Later than 1 year and not later than 3 years 400 Later than 3 years and not later than 5 years 167 Total contractual undiscounted payments 860 Less: effect of discounting (54 ) Discounted lease liabilities 806 The following are the amounts recognized in the statements of operations with respect to lease contracts: 2019 2018 2017 (in thousands) £ £ £ Depreciation expense of right of use assets under IFRS 16 193 — — Interest expense on lease liabilities (included in administrative expenses) under IFRS 16 21 — — Operating lease expense under IAS 17 — 150 182 Total amount recognized in statements of operations 214 150 182 14. Leases (continued) The Company had total net cash outflows for leases of £0.2 million in 2019. The Company also had non-cash additions to right of use assets and lease liabilities of £0.5 million in 2019. The Company has one lease contract with variable payments where the lease costs after the first year of the lease are increased based upon a consumer price index. The lease liability for this lease was re-measured at December 31, 2019. All other lease contracts have fixed payments. The Company has a number of lease contracts that include extension and termination options. These options are negotiated by management to provide flexibility in managing the leased-asset portfolio and align it with the Company’s business needs. None of the termination options are expected to be exercised. All of the extension options require a market rental review and the lease cost for the extension period will typically be set at the higher of either the current lease cost or the open market lease cost. Based upon the current lease cost, the undiscounted future rental payments of potential extension options that are not included in the lease liability are as follows: 2019 (in thousands) £ Extension options not expected to be exercised Not later than 5 years 351 Later than 5 years 627 Total 978

Related Party Disclosures

Related Party Disclosures	12 Months Ended
	Dec. 31, 2019
Related Party Transactions [Abstract]
Related party disclosures	15. Related party disclosures Compensation of key management personnel of the Company 2019 2018 2017 (in thousands) £ £ £ Short-term employee benefits 1,686 1,687 1,184 Pension and other benefits 69 72 56 Share-based payments 1,976 893 11,230 3,731 2,652 12,470 The amounts disclosed in the table above are the amounts recognized as an expense during the reporting year related to key management personnel.

Cash and Cash Equivalents

Cash and Cash Equivalents	12 Months Ended
	Dec. 31, 2019
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	16. Cash and cash equivalents 2019 2018 (in thousands) £ £ Cash and cash equivalents 51,962 76,972 Cash and cash equivalents comprise cash at banks with maturity of three months or less, which is subject to insignificant risk of changes in value. Cash at banks earns interest at fixed or variable rates based on the terms agreed for each account. Liquidity risk is minimal and is managed using deposits with immediate and varied fixed term dates.

Financial Instruments Risk Mana

Financial Instruments Risk Management	12 Months Ended
	Dec. 31, 2019
Disclosure Of Financial Instruments [Abstract]
Financial instruments risk management	17. Financial instruments risk management The Company is exposed to market risk arising from exposure to fluctuation in interest rates and currency exchange rates. These risks are managed by maintaining an appropriate mix of cash deposits in the two main currencies the Company operates in, placed with a variety of financial institutions for varying periods according to expected liquidity requirements. Interest Rate Risk As of December 31, 2019, the Company had cash and cash equivalents of £52.0 million. As of December 31, 2018, the company had cash and cash equivalents of £77.0 million. Exposure to interest rate sensitivity is impacted primarily by changes in the underlying bank interest rates. The Company’s surplus cash and cash equivalents are invested in interest-bearing accounts and certificates of deposit from time to time which earn interest at fixed or variable rates based on the terms agreed for each account. The Company has not entered into investments for trading or speculative purposes. Financial assets subject to fixed or variable interest rates are as follows: 2019 2018 (in thousands) Carrying amount Carrying amount £ £ Financial assets at fixed rates Cash and cash equivalents 41,827 64,267 Financial assets at variable rates Cash and cash equivalents 7,108 7,141 Non-interest bearing cash balances Cash and cash equivalents 3,027 5,564 An increase in the bank interest rates by 0.5 percentage points would increase the net annual interest income applicable to the cash and cash equivalents by £244,674 (2018: £357,041). 17. Financial instruments risk management (continued) Currency Risk The Company’s functional currency is U.K. pounds sterling, and our transactions are commonly denominated in that currency. However, a portion of expenses is incurred in other currencies, primarily U.S. dollars, and is exposed to the effects of this exchange rate. Since mid-2016, there has been significantly increased volatility in the exchange rate between the pound sterling and the U.S. dollar and an overall weakening of the pound sterling related to Britain’s exit from the European Union. Although the Company is based in the United Kingdom, it sources active pharmaceutical ingredient, raw materials, research and development, manufacturing, consulting and other services worldwide, including from the United States, the European Union and India. Any weakening of the pound sterling against the currencies of such other jurisdictions makes the purchase of such goods and services more expensive for the Company. The Company seeks to minimize this exposure by maintaining currency cash balances at levels appropriate to meet foreseeable short to mid-term expenses in these other currencies. The Company thus holds a significant portion of cash and cash equivalents in U.S. dollars and will therefore report the impact of exchange rates movements on these balances. The Company does not use derivative instruments to manage exchange rate exposure. Financial assets and liabilities in foreign currencies, primarily held in U.S. dollars, are as follows: 2019 2018 (in thousands) Carrying amount Carrying amount £ £ Financial assets Prepayments, accrued income and other receivables 2,174 1,477 Current income tax receivable 4 25 Cash and cash equivalents 28,980 44,018 Financial liabilities Trade payables 549 1,192 Payroll taxes and social security 18 26 Lease liabilities 52 — Accrued expenditure 989 1,391 A 1% increase in the value of the U.K. pound sterling relative to the U.S. dollar would reduce the carrying value of net financial assets and liabilities in foreign currencies by £295,498 (2018: £429,101). Credit risk The Company actively manages cash and cash equivalents across a number of banks and has deposits with different maturity dates. The Company monitors the credit rating of those banks. All of the Company’s cash and cash equivalents at December 31, 2019 were held at U.K. and U.S. financial institutions with short-term A-rated credit ratings, as assessed by recognized international credit rating agencies. As a result, no provision for expected credit losses has been recognized.

Events After the Reporting Peri

Events After the Reporting Period	12 Months Ended
	Dec. 31, 2019
Events After Reporting Period [Abstract]
Events After the Reporting Period	18. Events after the reporting period There have been no significant changes to the Company’s circumstances since the year end.

Significant Accounting Polici_2

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
Disclosure Of Summary Of Significant Accounting Policies [Abstract]
Basis of preparation	Basis of preparation The financial statements have been prepared in accordance with International Financial Reporting Standards (“IFRS”) as issued by the International Accounting Standards Board (“IASB”). The Company’s financial statements comprise the financial statements of the Company and its subsidiaries at December 31, 2019. The financial statements are presented in pounds sterling, which is also the Company’s functional currency. All values are rounded to the nearest thousand, except where otherwise indicated.
Going concern	Going concern In common with many companies in the biopharmaceutical sector, the Company incurs significant expenditure in its early years as it researches and develops its potential products for market. The board of directors, having reviewed the operating budgets and development plans, considers that the Company has adequate resources to continue in operation for the foreseeable future. The board of directors is therefore satisfied that it is appropriate to adopt the going concern basis of accounting in preparing the financial statements. The Company believes that its cash and cash equivalents of £52.0 million at December 31, 2019, will be sufficient to fund its current operating plan for at least the next 12 months. As the Company continues to incur losses, the transition to profitability is dependent upon the successful development, approval and commercialization of its product candidates and achieving a level of revenues adequate to support its cost structure. The Company may never achieve profitability, and unless and until it does, it will continue to need to raise additional capital. There can be no assurances, however, that additional funding will be available on acceptable terms.
Judgements and estimates	Judgements and estimates The preparation of the financial statements requires management to make judgements, estimates and assumptions that affect the amounts reported for assets and liabilities at the balance sheet dates and the amounts reported for revenue and expenses during the year. The nature of estimations means that actual outcomes could differ from those estimates. The following judgements have had the most significant effect on the amounts recognized in the financial statements:
Research and development expenses	Research and development expenses The Company recognizes research and development expenses in the statement of operations in the period in which they are incurred. When development activities reach the advanced stage, as set out in the specific criteria of International Accounting Standard (“IAS”) 38, Intangible Assets
Taxation	Taxation Management judgement is required to determine the amount of deferred tax assets that should be recognized, based upon the likely timing and level of future taxable profits. Further details are contained in Note 7. The following estimates have had the most significant effect on the amounts recognized in the financial statements:
Recognition of Clinical Trial Expenses	Recognition of Clinical Trial Expenses As part of the process of preparing our consolidated financial statements, we may be required to estimate accrued or prepaid expenses related to our clinical trials. In order to obtain reasonable estimates, we review open contracts and master service agreements • fees paid to CROs for services performed on clinical trials; and • pass-through costs for activities at clinical trial investigator sites. Accruals for clinical trial expenses, including estimated amounts recognized consistent with the above policy, were £1.7 million at December 31, 2019 as compared to £1.5 million at December 31, 2018.
Recognition of Manufacturing Contract Expenses	Recognition of Contracted Manufacturing Expenses As part of the process of preparing our consolidated financial statements, we may be required to estimate accrued or prepaid expenses related to our contracted manufacturing expenses. In order to obtain reasonable estimates, we review open contracts and master service agreements. In addition, we consult with applicable personnel in order to identify services that have been performed and which have not yet been invoiced, and services not yet performed for which we have been invoiced in advance. Accruals for contracted manufacturing expenses, including estimated amounts recognized consistent with the above policy, were £0.2 million at December 31, 2019 as compared to £0.2 million at December 31, 2018.
Share-based payments	Share-based payments Estimating fair value for share-based payment transactions requires determination of the most appropriate valuation model, which depends on the terms and conditions of the grant. This estimate also requires determination of the most appropriate inputs to the valuation model, including the expected life of the share option, volatility, dividend yield and assumptions about them and, in the case of the Company, the value of an ordinary share. For the measurement of the fair value of equity-settled transactions with employees at the grant date, the Company uses the Black-Scholes model. The assumptions and models used for estimating fair value for share-based payment transactions are detailed in Note 13.
Basis of consolidation	Basis of consolidation The consolidated financial statements comprise the financial statements of the Company and its subsidiaries. Subsidiaries are consolidated from the date of acquisition, being the date on which the Company obtains control, and continue to be consolidated until the date when such control ceases. The financial statements of the subsidiaries are prepared for the same reporting period as the parent company, using consistent accounting policies. All intra-group balances, transactions, unrealized gains and losses resulting from intra-group transactions and dividends are eliminated in full. Assets, liabilities, income and expenses of a subsidiary acquired or disposed of during the year are included in the consolidated financial statements from the date the Company gains control until the date the Company ceases to control the subsidiary.
Foreign currencies	Foreign currencies The Company’s consolidated financial statements are presented in pounds sterling, which is also the parent company’s functional currency. For each group entity, the Company determines the functional currency and items included in the financial statements of each entity are measured using that functional currency. Transactions and balances Transactions in foreign currencies are initially recorded by the Company’s entities at their respective functional currency spot rates at the date the transaction first qualifies for recognition. Monetary assets and liabilities denominated in foreign currencies are translated at the functional currency spot rates of exchange at the reporting date. Differences arising on settlement or translation of monetary items are recognized in the consolidated statements of operations. Non-monetary items that are measured in terms of historical cost in a foreign currency are translated using the exchange rates at the dates of the initial transactions. Group companies On consolidation, the assets and liabilities of foreign operations are translated into pounds sterling at the rate of exchange prevailing at the reporting date and their statements of operations are translated at the average exchange rate for the financial period in which those transactions occur. The exchange differences arising on translation for consolidation are recognized in the consolidated statements of comprehensive income or loss.
Segment reporting	Segment reporting The Company operates in one operating segment. Operating segments are reported in a manner consistent with the internal reporting provided to the Company’s chief operating decision maker (“the CODM”). The Company’s CODM, its Chief Executive Officer, views the Company’s operations and manages its business as a single operating segment, which is the business of developing and commercializing for use in Oncology. The Company’s principal operations and decision-making functions are located in the United Kingdom from where global decisions are made.
Initial public offering (IPO) related expenses	Initial public offering (IPO) related expenses Incremental costs incurred and directly attributable to the offering of securities in 2017 were deducted from the related proceeds of the offering. The net amount is recorded as contributed shareholders’ equity in the period when such shares were issued. Costs that relate to the stock market listing or are otherwise not incremental and directly attributable to issuing new shares, are recorded as an expense in the statement of comprehensive income. Costs that relate to both share issuance and listing are allocated between those functions on a rational and consistent basis. In the absence of a more specific basis for apportionment, an allocation of common costs based on the proportion of new shares issued to the total number of (new and existing) shares listed has been used.
Property, plant and equipment	Property, plant and equipment Property, plant and equipment is stated at cost, net of accumulated depreciation and accumulated impairment losses, if any. There are no restrictions on title nor equipment pledged as security for liabilities. 2. Significant accounting policies (continued) Depreciation is provided on property, plant and equipment over their expected useful economic life as follows: Asset class Depreciation method and period Office and computer equipment Straight-line over 3 years Fixtures and fittings Straight-line over 5 years, or, for non-removable items, the remaining term of an associated lease, whichever is shorter Right of use assets Straight-line over the lease term, which are between two and five years, or the estimated useful lives of the assets, whichever is shorter
Intangible assets	Intangible assets Intangible assets are stated at cost, net of accumulated amortization and accumulated impairment losses, if any. Cost in relation to patents includes registration, documentation and other legal fees associated with obtaining the patent. Computer software cost represents the initial purchase price of the asset. The amortization method and amortization period for the principal categories of intangible assets are follows: Asset class Amortization method and period Patents Straight-line over 20 years Computer software Straight-line between 3 and 5 years The Company’s primary patents each have a life of 20 years. Further patents are granted in various jurisdictions to extend the territorial coverage of the primary patent. These patents are granted up to the period of the related primary patent. Costs are thus amortized over the remaining life of the relevant primary patent. The amortization expense on intangible assets with finite lives is recognized in the statements of operations as an administrative expense. The amortization method and the amortization period for an intangible asset with a finite useful life are reviewed at least at each financial year end. Changes in the expected useful economic life or the expected pattern of consumption of future economic benefits embodied in the asset are accounted for by changing the amortization period or method, as appropriate. Intangible assets are tested for impairment when there is an indicator of impairment.
Cash, cash equivalents and short-term deposits	Cash and cash equivalents Cash and cash equivalents in the statement of financial position include cash at banks with a maturity of less than three months, which is subject to an insignificant risk of changes in value.
Research and development	Research and development Research and development expenses are currently recognized in the statement of operations in the year in which they are incurred. Development expenditures on an individual project will be recognized as an intangible asset when the Company can demonstrate: • the technical feasibility of completing the intangible asset so that the asset will be available for use or sale; • its intention to complete and its ability and intention to use or sell the asset; • how the asset will generate future economic benefits; • the availability of resources to complete the asset; and • the ability to measure reliably the expenditure during development.
Income Taxes	2. Significant accounting policies (continued) Income Taxes Current income tax Current income tax assets and liabilities are measured at the amount expected to be recovered from or paid to the taxation authorities. The tax rates and tax laws used to compute the amount are those that are enacted or substantively enacted at the reporting date in the countries where the Company operates and generates taxable income. Deferred income tax Deferred income tax is provided in full, using the liability method, on temporary differences arising between the tax bases of assets and liabilities and their carrying amounts in the Company’s financial statements. However, the deferred income tax is not accounted for if it arises from initial recognition of an asset or liability in a transaction other than a business combination that at the time of the transaction affects neither accounting nor taxable profit or loss. Deferred income tax is determined using tax rates and laws that have been enacted or substantially enacted by the year end date and are expected to apply when the related deferred income tax asset is realized or the deferred tax liability is settled. Deferred tax assets are recognized to the extent that it is probable that future taxable profit will be available against which the temporary differences can be utilized. Income tax credit The Company benefits from the U.K. and U.S. research and development tax credit regimes. In the U.K. a portion of the Company’s losses can be surrendered for a cash rebate of up to 33.35% of eligible expenditures. In the U.S. the Company is able to offset the research and development credits against corporation tax payable. Such credits are accounted for within the tax provision, in the year in which the expenditures are incurred.
Operating leases	Operating leases (prior to January 1, 2019) Leases where the lessor retains substantially all the risks and benefits of ownership of the asset are classified as operating leases and rentals payable are charged in the consolidated statements of operations on a straight-line basis over the lease term. Leases (from January 1, 2019) The Company assesses, at contract inception, whether a contract is, or contains, a lease. That is, if the contract conveys the right to control the use of an identified asset for a period of time in exchange for consideration. The Company applies a single recognition and measurement approach for all leases, except for short-term leases and leases of low-value assets. The Company recognizes lease liabilities to make lease payments and right of use assets representing the right to use the underlying assets. Right of use assets The Company recognizes right of use assets at the commencement date of the lease (i.e. the date the underlying asset is available for use). Right of use assets are measured at cost, less any accumulated depreciation and impairment losses, and adjusted for any remeasurement of lease liabilities. The cost of right of use assets includes the amount of lease liabilities recognized, initial direct costs incurred, and lease payments made at or before the commencement date less any lease incentives received. Right of use assets, which relate solely to office space, are depreciated on a straight-line basis over the shorter of the lease terms, which are between two and five years, or the estimated useful lives of the assets. Lease At the commencement date of the lease, the Company recognizes a lease liability measured at the present value of lease payments to be made over the lease term. The lease payments include fixed payments less any lease incentives receivable, and any variable lease payments that depend on an index. 2. Significant accounting policies (continued) In calculating the present value of lease payments, the Company uses its incremental borrowing rate at the lease commencement date because the interest rate implicit in the lease is not readily determinable. After the commencement date, the amount of the lease liability is increased to reflect the accretion of interest and reduced for the lease payments made. In addition, the carrying amount of the lease liability is remeasured if there is a modification, a change in the lease term or a change in the lease payments. The Company determines the lease term as the non-cancellable term of the lease, together with any periods covered by an option to extend the lease if it is reasonably certain to be exercised, or any periods covered by an option to terminate the lease, if it is reasonably certain not to be exercised. The Company has a number of lease contracts that include extension and termination options. The Company applies judgement in evaluating whether it is reasonably certain whether or not to exercise the option to renew or terminate the lease. That is, it considers all relevant factors that create an economic incentive for it to exercise either the renewal or termination. After the commencement date, the Company reassesses the lease term if there is a significant event or change in circumstances that is within its control and affects its ability to exercise or not to exercise the option to renew or to terminate, such as the construction of significant leasehold improvements. Refer to Note 14 for information on potential future rental payments relating to periods following the exercise date of extension options that are not included in the lease liability.
Impairment of non-financial assets	Impairment of non-financial assets The Company assesses, at each reporting date, whether there is an indication that an asset may be impaired. If any indication exists, the Company estimates the recoverable amount of the asset. An impairment loss is recognized whenever the carrying amount of an asset or its cash-generating unit exceeds its recoverable amount. Impairment losses are recognized in the consolidated statements of operations. A cash-generating unit is the smallest identifiable group of assets that generates cash inflows that are largely independent of the cash inflows from other assets or groups of assets.
Calculation of recoverable amount	Calculation of recoverable amount The recoverable amount of assets and cash-generating units is the higher of their fair value less costs to sell and value in use. In assessing value in use, the estimated future cash flows are discounted to their present value using a pre-tax discount rate that reflects current market assessments of the time value of money and the risks specific to the asset. For an asset that does not generate largely independent cash inflows, the recoverable amount is determined for the cash-generating unit to which the asset belongs.
Reversal of impairment	Reversal of impairment An assessment is made at each reporting date as to whether there is an indication that a previously recognized impairment loss may no longer exist or may have decreased. If such an indication exists, the recoverable amount is estimated. A previously recognized impairment loss is reversed only if there has been a change in the estimates used to determine the recoverable amount since the last impairment loss was recognized. If that is the case, the carrying value is increased to its recoverable amount. An impairment loss is reversed only to the extent that the asset’s carrying amount does not exceed the carrying amount that would have been determined, net of depreciation or amortization, if no impairment loss had been recognized.
Share-based payments	Share-based payments Employees, directors and consultants of the Company receive remuneration in the form of share options, whereby individuals render services as consideration for equity instruments and the cost is recognized as share-based payments under IFRS 2. 2. Significant accounting policies (continued) Under IFRS 2 Share-based Payment The fair value determined at the grant date of equity settled share-based payments is expensed on a straight line basis over the vesting period, with a corresponding increase in equity to the share option reserve.
Fair value measurement	Fair value measurement The fair value of the financial assets and liabilities is included at the amount at which an instrument could be exchanged in a current transaction between willing parties, other than in a forced liquidation or sale. Fair value is based on the price that would be received from the sale of an asset or that would be paid to transfer a liability in an orderly transaction between market participants at the measurement date. In order to increase consistency and comparability in fair value measurements, IFRS 13 establishes a fair value hierarchy that prioritizes observable and unobservable inputs used to measure fair value into three broad levels, which are described as follows: Level 1: Quoted (unadjusted) prices in active markets for identical assets or liabilities. Level 2: Other techniques for which all inputs that have a significant effect on the recorded fair value are observable, either directly or indirectly. Level 3: Techniques that use inputs that have a significant effect on the recorded fair value that are not based on observable market data. The fair values of cash, cash equivalents, short-term deposits, other receivables, trade payables and other payables approximate their carrying amounts largely due to the short-term maturities of these instruments.
Accounting Standards	Accounting Standards In preparing these financial statements, the Company has applied all relevant IAS, IFRS and International Financial Reporting Interpretations Committee (“IFRIC”) Interpretations as of the date of approval of these financial statements and which are mandatory for the financial year ended December 31, 2019. The following accounting standards, interpretations and amendments have been adopted as of January 1, 2019 in these financial statements and, with the exception of IFRS 16 Leases • IFRS 16 Leases • IFRIC 23 Uncertainty over Income Tax Treatments • Amendments to IFRS 3 – Annual Improvements to IFRSs 2015–2017 Cycle • Amendments to IFRS 9 – Prepayment Features with Negative Compensation • Amendments to IFRS 11 – Annual Improvements to IFRSs 2015–2017 Cycle • Amendments to IAS 12 – Annual Improvements to IFRSs 2015–2017 Cycle • Amendments to IAS 19 – Plan Amendment, Curtailment or Settlement • Amendments to IAS 23 – Annual Improvements to IFRSs 2015–2017 Cycle • Amendments to IAS 28 – Long-term Interests in Associates and Joint Ventures 2. Significant accounting policies (continued) The IASB and IFRIC have issued the following standards and amendments with an effective date after the date of these financial statements: • Amendments to References to the Conceptual Framework in IFRS Standards (effective from January 1, 2020) • Amendments to IFRS 3 – Definition of a Business (effective from January 1, 2020) • Amendments to IFRS 9, IAS 39, and IFRS 7 – Interest Rate Benchmark Reform (effective from January 1, 2020) • Amendments to IAS 1 and IAS 8 – Definition of Material (effective from January 1, 2020) • IFRS 17 Insurance Contracts (effective from January 1, 2021) • Amendments to IFRS 4 – Applying IFRS 9 Financial Instruments with IFRS 4 Insurance Contracts (effective from January 1, 2021) • Amendments to IAS 1 Presentation of Financial Statements – Classification of Liabilities as Current or Non-Current (effective from January 1, 2022) The Company has reviewed and considered that the above standards and amendments either do not apply to the Company or will not have a material impact in future periods. IFRS 16 Leases IFRS 16 supersedes IAS 17 Leases Determining whether an Arrangement contains a Lease Operating Leases-Incentives Evaluating the Substance of Transactions Involving the Legal Form of a Lease The Company adopted IFRS 16 using the modified retrospective method of adoption with the date of initial application of January 1, 2019. Under this method, the standard is applied retrospectively with the cumulative effect of initially applying the standard recognized at the date of initial application. The Company elected to use the transition practical expedient to not reassess whether a contract is, or contains a lease at January 1, 2019. Instead, the Company applied the standard only to contracts that were previously identified as leases applying IAS 17 and IFRIC 4 at the date of initial application. The effect of adoption of IFRS 16 as at January 1, 2019 (increase/(decrease)) is, as follows: 2019 (in thousands) £ Assets Right of use assets (note 9) 455 Prepayments (21 ) Total assets 434 Liabilities Lease liabilities - non-current 287 Lease liabilities - current 147 Total liabilities 434 The Company has lease contracts solely for office space. Before the adoption of IFRS 16, the Company classified each of its leases (as lessee) as an operating lease. Refer to Note 2 Operating leases 2. Significant accounting policies (continued) Upon adoption of IFRS 16, the Company applied a single recognition and measurement approach for all leases except for short-term leases and leases of low-value assets. Refer to Note 2 Leases The Company recognized right of use assets and lease liabilities for the leases previously classified as operating leases. The right of use assets were recognized based on the amount equal to the lease liabilities, adjusted for any related prepaid lease payments previously recognized, and are included in property, plant and equipment on the balance sheet. Lease liabilities were recognized based on the present value of the remaining lease payments, discounted using the incremental borrowing rate at the date of initial application. The Company also applied the following available practical expedients wherein it: • Used a single discount rate to a portfolio of leases with reasonably similar characteristics. • Excluded the initial direct costs from the measurement of the right of use asset at the date of initial application. The lease liabilities as at January 1, 2019 can be reconciled to the operating lease commitments as of December 31, 2018, as follows: 2019 (in thousands, except borrowing rate) £ Operating lease obligations as at December 31, 2018 433 Weighted average incremental borrowing rate as at January 1, 2019 4.17 % Discounted operating lease obligations as at January 1, 2019 413 Less: Lease not commenced as at December 31, 2018 (138 ) Lease obligations prepaid as at December 31, 2018 (21 ) Service component included in operating lease obligations as at December 31, 2018 (3 ) Add: Lease payments relating to renewal periods not included in operating lease obligations as at December 31, 2018 183 Lease liabilities as at January 1, 2019 434

Significant Accounting Polici_3

Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2019
Disclosure Of Summary Of Significant Accounting Policies [Abstract]
Summary of Depreciation on Property, Plant and Equipment Over Their Expected Useful Economic Life	2. Significant accounting policies (continued) Depreciation is provided on property, plant and equipment over their expected useful economic life as follows: Asset class Depreciation method and period Office and computer equipment Straight-line over 3 years Fixtures and fittings Straight-line over 5 years, or, for non-removable items, the remaining term of an associated lease, whichever is shorter Right of use assets Straight-line over the lease term, which are between two and five years, or the estimated useful lives of the assets, whichever is shorter
Summary of Amortization Method and Amortization Period for the Principal Categories of Intangible Assets	The amortization method and amortization period for the principal categories of intangible assets are follows: Asset class Amortization method and period Patents Straight-line over 20 years Computer software Straight-line between 3 and 5 years
Summary of Effect of Adoption IFRS 16	The effect of adoption of IFRS 16 as at January 1, 2019 (increase/(decrease)) is, as follows: 2019 (in thousands) £ Assets Right of use assets (note 9) 455 Prepayments (21 ) Total assets 434 Liabilities Lease liabilities - non-current 287 Lease liabilities - current 147 Total liabilities 434
Summary of Operating Lease Commitments	The lease liabilities as at January 1, 2019 can be reconciled to the operating lease commitments as of December 31, 2018, as follows: 2019 (in thousands, except borrowing rate) £ Operating lease obligations as at December 31, 2018 433 Weighted average incremental borrowing rate as at January 1, 2019 4.17 % Discounted operating lease obligations as at January 1, 2019 413 Less: Lease not commenced as at December 31, 2018 (138 ) Lease obligations prepaid as at December 31, 2018 (21 ) Service component included in operating lease obligations as at December 31, 2018 (3 ) Add: Lease payments relating to renewal periods not included in operating lease obligations as at December 31, 2018 183 Lease liabilities as at January 1, 2019 434

Staff Costs (Tables)

Staff Costs (Tables)	12 Months Ended
	Dec. 31, 2019
Staff Costs [Line Items]
Schedule of Average Number of Staff Employed Under Contracts of Service Explanatory	The average number of staff employed under contracts of service were: 2019 2018 2017 (number) Research and development activities 23 18 18 Administrative activities 5 4 2 28 22 20
Summary of Directors’ Remuneration and Other Benefits	Directors’ remuneration & other benefits 2019 2018 2017 (in thousands) £ £ £ Directors’ remuneration 1,222 1,143 793 Pension and other benefits 52 51 37 1,274 1,194 830
Highest Paid Director
Staff Costs [Line Items]
Summary of Directors’ Remuneration and Other Benefits	The above amounts for remuneration include the following in respect of the highest paid director: 2019 2018 2017 (in thousands) £ £ £ Director’s remuneration 780 739 601 Pension and other benefits 48 47 33 828 786 634
Research and Development Expenses
Staff Costs [Line Items]
Disclosure of Employee Benefit Expense	Included in research and development expenses: 2019 2018 2017 (in thousands) £ £ £ Wages and salaries 3,304 2,761 2,391 Social security costs 395 314 272 Pension costs 140 139 119 Share-based payments 1,878 835 8,868 5,717 4,049 11,650
Administrative Expenses
Staff Costs [Line Items]
Disclosure of Employee Benefit Expense	3. Staff costs (continued) Included in administrative expenses: 2019 2018 2017 (in thousands) £ £ £ Wages and salaries 1,190 790 408 Social security costs 115 75 37 Pension costs 37 37 21 Share-based payments 1,348 960 2,863 2,690 1,862 3,329 Total employee benefit expense 8,407 5,911 14,979

Basic and Diluted Loss Per Sh_2

Basic and Diluted Loss Per Share (Tables)	12 Months Ended
	Dec. 31, 2019
Earnings Per Share [Abstract]
Schedule of Basic and Diluted Loss Per Share	2019 2018 2017 (in thousands, except per share data) £ £ £ Loss for the year (21,412 ) (13,840 ) (23,085 ) Basic and diluted weighted average number of shares 32,327 31,972 26,069 £ £ £ Basic and diluted loss per share (0.66 ) (0.43 ) (0.89 )

Loss Before Tax (Tables)

Loss Before Tax (Tables)	12 Months Ended
	Dec. 31, 2019
Material Income And Expense [Abstract]
Summary of Loss Before Tax Stated After Charging	Loss before tax is stated after charging: 2019 2018 2017 (in thousands) £ £ £ Administrative expenses: Amortization 377 230 164 Depreciation 341 141 30 IPO related expenses — — 1,794

Income Tax Credit (Tables)

Income Tax Credit (Tables)	12 Months Ended
	Dec. 31, 2019
Major Components Of Tax Expense Income [Abstract]
Schedule of Major Components of Income Tax	The major components of income tax for the years ended December 31, 2019, 2018 and 2017 are as follows: 2019 2018 2017 (in thousands) £ £ £ Current tax: In respect of current year U.K. 4,325 4,239 2,298 In respect of current year U.S. (1 ) — (10 ) In respect of prior years U.K. (86 ) 19 3 In respect of prior years U.S. — 3 — Total current tax 4,238 4,261 2,291 Deferred tax: In respect of current year U.S. 1 11 61 In respect of prior years U.S. — (49 ) 49 Total deferred tax 1 (38 ) 110 Income tax credit 4,239 4,223 2,401 Current income tax receivable: U.K. tax 8,477 4,239 4,207 U.S. tax 4 24 18 Current income tax receivable 8,481 4,263 4,225 Deferred tax: U.S. tax 46 47 81 Current income tax payable: U.S. tax — — —
Schedule of Reconciliation of Accounting Loss per Statement of Operations	7. Income tax credit (continued) The credit for the year can be reconciled to the loss per the statement of operations as follows: 2019 2018 2017 (in thousands) £ £ £ Loss before tax (25,651 ) (18,063 ) (25,486 ) Tax on loss at standard U.K. tax rate of 19% (2018: 19%; 2017: 19.25%) (4,874 ) (3,432 ) (4,906 ) Effects of: Expenses not deductible 2,912 1,389 3,899 Deduction for R&D (5,667 ) (5,554 ) (3,051 ) Losses surrendered for R&D tax credit 5,667 5,554 3,051 Deferred tax - PY adjustment — 49 (49 ) Overseas tax payable - current year 1 — 10 Overseas tax payable - prior years — (3 ) — R&D tax credit - U.S. (1 ) (11 ) (61 ) R&D tax credit - current year (4,325 ) (4,239 ) (2,298 ) R&D tax credit - prior years 86 (19 ) (3 ) Deferred tax asset not recognized 1,962 2,043 1,007 Income tax credit (4,239 ) (4,223 ) (2,401 )

Intangible Assets (Tables)

Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2019
Disclosure Of Intangible Assets [Abstract]
Summary of Intangible Assets	Patents Computer Software Total (in thousands) £ £ £ Cost: At December 31, 2017 2,115 153 2,268 Additions 1,409 5 1,414 At December 31, 2018 3,524 158 3,682 Accumulated amortization: At December 31, 2017 302 28 330 Charge for the year 196 34 230 At December 31, 2018 498 62 560 Cost: At December 31, 2018 3,524 158 3,682 Additions 999 216 1,215 At December 31, 2019 4,523 374 4,897 Accumulated amortization: At December 31, 2018 498 62 560 Charge for the year 308 69 377 At December 31, 2019 806 131 937 Net book value: At December 31, 2019 3,717 243 3,960 At December 31, 2018 3,026 96 3,122

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	12 Months Ended
	Dec. 31, 2019
Disclosure Of Property Plant And Equipment [Abstract]
Summary of Property, Plant and Equipment	Right of use assets Office and computer equipment Fixtures and Fittings Total (in thousands) £ £ £ £ Cost: At December 31, 2017 — 142 280 422 Additions — 84 126 210 Disposals — (16 ) — (16 ) Effect of foreign currency exchange differences — — — — At December 31, 2018 — 210 406 616 Depreciation: At December 31, 2017 — 51 13 64 Charge for the year — 61 80 141 Disposals — (16 ) — (16 ) Effect of foreign currency exchange differences — — — — At December 31, 2018 — 96 93 189 Cost: At December 31, 2018 — 210 406 616 Recognized on adoption of IFRS 16 Leases 455 — — 455 Additions 530 27 12 569 Re-measurement 2 — — 2 Disposals — (1 ) — (1 ) Effect of foreign currency exchange differences (5 ) — — (5 ) At December 31, 2019 982 236 418 1,636 Depreciation: At December 31, 2018 — 96 93 189 Charge for the year 193 65 83 341 Disposals — (1 ) — (1 ) Effect of foreign currency exchange differences (2 ) — — (2 ) At December 31, 2019 191 160 176 527 Net book value: At December 31, 2019 791 76 242 1,109 At December 31, 2018 — 114 313 427

Prepayments, Accrued Income a_2

Prepayments, Accrued Income and Other Receivables (Tables)	12 Months Ended
	Dec. 31, 2019
Prepayments Accrued Income And Other Receivables [Abstract]
Summary of Prepayments, Accrued Income and Other Receivables	2019 2018 (in thousands) £ £ Prepayments—manufacturing and clinical 2,630 1,050 Prepayments—other 1,489 750 Accrued income 76 165 VAT 475 379 Other receivables 40 10 4,710 2,354

Share Capital and Share Premi_2

Share Capital and Share Premium (Tables)	12 Months Ended
	Dec. 31, 2019
Disclosure Of Classes Of Share Capital [Abstract]
Summary of Share Capital and Share Premium	2019 2018 2017 (in thousands) £ £ £ Share capital 1,299 1,289 1,272 Share premium 79,541 79,426 79,236 80,840 80,715 80,508 Number Number Number (in thousands) Issued share capital comprises: Ordinary shares of £0.04 each 32,479 32,226 31,811 32,479 32,226 31,811
Summary of Movement in the Share Capital	Number of shares Share capital Share premium (in thousands) £ £ Fully paid shares: Balance at December 31, 2017 31,811 1,272 79,236 Exercise of share options 415 17 190 Balance at December 31, 2018 32,226 1,289 79,426 Exercise of share options 253 10 115 Balance at December 31, 2019 32,479 1,299 79,541

Other Reserves (Tables)

Other Reserves (Tables)	12 Months Ended
	Dec. 31, 2019
Other Reserves
Disclosure Of Other Reserves [Line Items]
Schedule of Other Reserves	2019 2018 2017 (in thousands) £ £ £ Own share reserve (339 ) (339 ) (339 ) Foreign currency translation reserve (10 ) 1 (11 ) Capital reserve 42,466 42,466 42,466 Share option reserve Balance at beginning of year 17,564 15,955 4,406 Share-based payments 3,289 1,977 11,731 Exercise of share options (132 ) (186 ) (180 ) Forfeiture of share options (63 ) (182 ) — Lapse of share options (38 ) — (2 ) Balance at end of year 20,620 17,564 15,955 Total other reserves 62,737 59,692 58,071

Share-based Payments (Tables)

Share-based Payments (Tables)	12 Months Ended
	Dec. 31, 2019
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Abstract]
Summary of Share Options and Weighted Average Exercise Prices	13. Share-based payments (continued) Share options and weighted average exercise prices are as follows for the reporting periods presented: Number of shares Weighted average exercise price per share £ Outstanding at January 1, 2017 3,248,187 0.72 Granted 1,500,815 2.58 Forfeited (7,500 ) 0.16 Exercised 1 (30,000 ) 4.00 Outstanding at December 31, 2017 4,711,502 1.29 Granted 253,500 17.01 Forfeited (143,438 ) 12.74 Exercised 2 (415,312 ) 0.50 Outstanding at December 31, 2018 4,406,252 1.90 Granted 1,202,150 10.82 Forfeited (48,625 ) 10.42 Lapsed (8,750 ) 3.90 Exercised 3 (252,187 ) 0.50 Outstanding at December 31, 2019 4 5,298,840 3.91 Vested and exercisable at December 31, 2019 3,811,736 1.15 Vested and exercisable at December 31, 2018 3,847,305 0.68 Vested and exercisable at December 31, 2017 4,030,833 0.36 1. The weighted average share price at the date of exercise of these options was £10.43 2. The weighted average share price at the date of exercise of these options was £18.23 3. The weighted average share price at the date of exercise of these options was £7.65 4. The exercise price of outstanding share options ranges from £0.04 to £18.05
Summary of Principal Assumptions Used for Valuation of Share Options	The following principal assumptions were used in the valuation for the 2017 share options. Options granted on May 16, 2017 Sep 13, 2017 Sep 14, 2017 Sep 14, 2017 Sep 14, 2017 Vesting dates Oct 28, 2017 Sep 13, 2018 Mar 06, 2018 Sep 14, 2018 Sep 14, 2018 Oct 28, 2018 Sep 13, 2019 Mar 06, 2019 Sep 14, 2019 Sep 14, 2019 Oct 28, 2019 Sep 13, 2020 Mar 06, 2020 Sep 14, 2020 Sep 14, 2020 Oct 28, 2020 Sep 13, 2021 Mar 06, 2021 Sep 14, 2021 Sep 14, 2021 Volatility 66.56 % 66.94 % 66.97 % 66.97 % 66.97 % Dividend yield 0 % 0 % 0 % 0 % 0 % Risk-free investment rate 0.12 % 0.27 % 0.36 % 0.36 % 0.36 % Fair value of option at grant date £ 7.70 £ 6.15 £ 6.08 £ 6.08 £ 6.08 Fair value of share at grant date £ 11.08 £ 10.43 £ 10.34 £ 10.34 £ 10.34 Exercise price at date of grant £ 4.00 £ 5.40 £ 5.40 £ 5.40 £ 5.40 Lapse date May 16, 2027 Sep 13, 2027 Sep 14, 2027 Sep 14, 2027 Sep 14, 2027 Expected option life (years) 2.63 2.30 2.30 2.30 2.30 Number of options granted 23,250 14,690 25,000 110,310 12,500 Sep 14, 2017 Sep 15, 2017 Sep 15, 2017 Sep 27, 2017 Sep 27, 2017 Vesting dates Sep 14, 2018 Sep 15, 2018 Sep 15, 2017 Mar 20, 2018 Sep 27, 2018 Sep 14, 2019 Sep 15, 2019 Mar 20, 2019 Sep 27, 2019 Sep 14, 2020 Sep 15, 2020 Mar 20, 2020 Sep 27, 2020 Sep 14, 2021 Sep 15, 2021 Mar 20, 2021 Sep 27, 2021 Volatility 66.97 % 67.02 % 67.02 % 67.11 % 67.11 % Dividend yield 0 % 0 % 0 % 0 % 0 % Risk-free investment rate 0.36 % 0.44 % 0.44 % 0.47 % 0.47 % Fair value of option at grant date £ 3.90 £ 6.76 £ 10.11 £ 4.36 £ 4.36 Fair value of share at grant date £ 10.34 £ 10.15 £ 10.15 £ 11.19 £ 11.19 Exercise price at date of grant £ 10.80 £ 4.00 £ 0.04 £ 11.19 £ 11.19 Lapse date Sep 14, 2027 Sep 15, 2027 Sep 15, 2027 Sep 27, 2027 Sep 27, 2027 Expected option life (years) 2.30 2.29 2.29 2.26 2.26 Number of options granted 25,000 45,750 1,028,533 37,500 178,282 The following principal assumptions were used in the valuation for 2018 share options. Options granted on Apr 11, 2018 Apr 11, 2018 May 8, 2018 Aug 14, 2018 Vesting dates Apr 11, 2019 Apr 11, 2018 May 8, 2019 Aug 14, 2019 Apr 11, 2020 May 8, 2020 Aug 14, 2020 Apr 11, 2021 May 8, 2021 Aug 14, 2021 Apr 11, 2022 May 8, 2022 Aug 14, 2022 Volatility 64.48 % 60.06 % 65.80 % 68.14 % Dividend yield 0 % 0 % 0 % 0 % Risk-free investment rate 1.04 % 0.83 % 1.02 % 0.93 % Fair value of option at grant date £ 8.97 £ 17.35 £ 8.63 £ 9.65 Fair value of share at grant date £ 17.51 £ 17.51 £ 16.57 £ 18.05 Exercise price at date of grant £ 17.51 £ 0.16 £ 16.57 £ 18.05 Lapse date Apr 11, 2028 Apr 11, 2028 May 8, 2028 Aug 14, 2028 Expected option life (years) 4.50 2.00 4.50 4.50 Number of options granted 71,500 7,500 62,000 112,500 The following principal assumptions were used in the valuation for 2019 share options. Options granted on Mar 13, 2019 May 15, 2019 Sept 11, 2019 Vesting dates Mar 13, 2020 May 15, 2020 Sept 11, 2020 Mar 13, 2021 May 15, 2021 Sept 11, 2021 Mar 13, 2022 May 15, 2022 Sept 11, 2022 Mar 13, 2023 May 15, 2023 Sept 11, 2023 Volatility 69.05 % 69.08 % 70.14 % Dividend yield 0 % 0 % 0 % Risk-free investment rate 0.85 % 0.77 % 0.44 % Fair value of option at grant date £ 5.46 £ 6.07 £ 4.22 Fair value of share at grant date £ 10.13 £ 11.26 £ 7.79 Exercise price at date of grant £ 10.13 £ 11.26 £ 7.79 Lapse date Mar 13, 2029 May 15, 2029 Sept 11, 2029 Expected option life (years) 4.50 4.50 4.50 Number of options granted 120,750 967,400 114,000

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2019
Presentation Of Leases For Lessee [Abstract]
Summary of Carrying Amounts of Lease Liabilities	The carrying amounts of lease liabilities and the movements during the period are as follows: 2019 (in thousands) £ Initial recognition at January 1, 2019 434 Additions 548 Re-measurement of liability 2 Accretion of interest 21 Payments (197 ) Effect of foreign currency exchange differences (2 ) At December 31, 2019 806 Classified as: Current 268 Non-current 538 806
Summary of Maturity Analysis of Lease Liabilities	The maturity analysis of lease liabilities are as follows: 2019 (in thousands) £ Contractual undiscounted payments Not later than 1 year 293 Later than 1 year and not later than 3 years 400 Later than 3 years and not later than 5 years 167 Total contractual undiscounted payments 860 Less: effect of discounting (54 ) Discounted lease liabilities 806
Summary of Amounts Recognized in Statements of Operations With Respect to Lease Contracts	The following are the amounts recognized in the statements of operations with respect to lease contracts: 2019 2018 2017 (in thousands) £ £ £ Depreciation expense of right of use assets under IFRS 16 193 — — Interest expense on lease liabilities (included in administrative expenses) under IFRS 16 21 — — Operating lease expense under IAS 17 — 150 182 Total amount recognized in statements of operations 214 150 182
Summary of Maturity Analysis of Lease Liabilities Extension Options Not Expected to be Exercised	Based upon the current lease cost, the undiscounted future rental payments of potential extension options that are not included in the lease liability are as follows: 2019 (in thousands) £ Extension options not expected to be exercised Not later than 5 years 351 Later than 5 years 627 Total 978

Related Party Disclosures (Tabl

Related Party Disclosures (Tables)	12 Months Ended
	Dec. 31, 2019
Related Party Transactions [Abstract]
Summary of Compensation of Key Management Personnel	Compensation of key management personnel of the Company 2019 2018 2017 (in thousands) £ £ £ Short-term employee benefits 1,686 1,687 1,184 Pension and other benefits 69 72 56 Share-based payments 1,976 893 11,230 3,731 2,652 12,470

Cash and Cash Equivalents (Tabl

Cash and Cash Equivalents (Tables)	12 Months Ended
	Dec. 31, 2019
Cash And Cash Equivalents [Abstract]
Schedule of Cash and Cash Equivalents	2019 2018 (in thousands) £ £ Cash and cash equivalents 51,962 76,972

Financial Instruments Risk Ma_2

Financial Instruments Risk Management (Tables)	12 Months Ended
	Dec. 31, 2019
Disclosure Of Financial Instruments [Abstract]
Schedule of Financial Assets Subject to Fixed or Variable Interest Rates	Financial assets subject to fixed or variable interest rates are as follows: 2019 2018 (in thousands) Carrying amount Carrying amount £ £ Financial assets at fixed rates Cash and cash equivalents 41,827 64,267 Financial assets at variable rates Cash and cash equivalents 7,108 7,141 Non-interest bearing cash balances Cash and cash equivalents 3,027 5,564
Summary of Financial Assets and Liabilities in Foreign Currencies, Primarily Held in U.S Dollars	Financial assets and liabilities in foreign currencies, primarily held in U.S. dollars, are as follows: 2019 2018 (in thousands) Carrying amount Carrying amount £ £ Financial assets Prepayments, accrued income and other receivables 2,174 1,477 Current income tax receivable 4 25 Cash and cash equivalents 28,980 44,018 Financial liabilities Trade payables 549 1,192 Payroll taxes and social security 18 26 Lease liabilities 52 — Accrued expenditure 989 1,391

General Information- Additional

General Information- Additional Information (Details)	12 Months Ended
	Dec. 31, 2019Subsidiary
Disclosure Of General Information About Financial Statements [Abstract]
Name of reporting entity	NuCana plc
Description of nature of entity's operations	clinical-stage biopharmaceutical company developing a portfolio of new medicines to treat cancer
Domicile of entity	United Kingdom
Country of incorporation	England and Wales
Name of subsidiary	NuCana, Inc. and NuCana BioMed Trustee Company Limited
Number of subsidiaries	2

Significant Accounting Polici_4

Significant Accounting Policies - Additional Information (Details) £ in Thousands	12 Months Ended
	Dec. 31, 2019GBP (£)Segment	Dec. 31, 2018GBP (£)	Dec. 31, 2017GBP (£)	Dec. 31, 2016GBP (£)
Disclosure Of Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents	£ 51,962	£ 76,972	£ 86,703	£ 19,990
Accrued clinical trial expenses	1,700	1,500
Accrued contracted manufacturing expense	£ 200	£ 200
Number of operating segment | Segment	1
Cash at banks maturity period	Less than three months
Right of Use Assets
Disclosure Of Summary Of Significant Accounting Policies [Line Items]
Depreciation method	Straight-line
Maximum
Disclosure Of Summary Of Significant Accounting Policies [Line Items]
Income tax credit, cash rebate percentage	33.35%
Maximum | Right of Use Assets
Disclosure Of Summary Of Significant Accounting Policies [Line Items]
Depreciation rate	5 years
Bottom Of Range [Member] | Right of Use Assets
Disclosure Of Summary Of Significant Accounting Policies [Line Items]
Depreciation rate	2 years
Patents
Disclosure Of Summary Of Significant Accounting Policies [Line Items]
Amortization rate	20 years

Significant Accounting Polici_5

Significant Accounting Policies - Summary of Depreciation on Property, Plant and Equipment Over Their Expected Useful Economic Life (Details)	12 Months Ended
	Dec. 31, 2019
Office and Computer Equipment
Disclosure Of Property Plant And Equipment [Line Items]
Depreciation method	Straight-line
Depreciation rate	3 years
Right of Use Assets
Disclosure Of Property Plant And Equipment [Line Items]
Depreciation method	Straight-line
Right of Use Assets | Minimum
Disclosure Of Property Plant And Equipment [Line Items]
Depreciation rate	2 years
Right of Use Assets | Maximum
Disclosure Of Property Plant And Equipment [Line Items]
Depreciation rate	5 years
Fixtures and Fittings
Disclosure Of Property Plant And Equipment [Line Items]
Depreciation method	Straight-line
Depreciation rate	5 years

Significant Accounting Polici_6

Significant Accounting Policies - Summery of Amortization Method and Amortization Period for the Principal Categories of Intangible Assets (Details)	12 Months Ended
	Dec. 31, 2019
Patents
Disclosure Of Intangible Assets [Line Items]
Amortization method	Straight-line
Amortization rate	20 years
Computer Software
Disclosure Of Intangible Assets [Line Items]
Amortization method	Straight-line
Computer Software | Bottom Of Range [Member]
Disclosure Of Intangible Assets [Line Items]
Amortization rate	3 years
Computer Software | Maximum
Disclosure Of Intangible Assets [Line Items]
Amortization rate	5 years

Significant Accounting Polici_7

Significant Accounting Policies - Summary of Effect of Adoption IFRS 16 (Details) - GBP (£) £ in Thousands	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Disclosure Of Quantitative Information About Leases For Lessee [Abstract]
Right of use assets (note 9)		£ 455
Prepayments	£ (860)	(21)
Total assets		434
Liabilities
Lease liabilities - non-current	538	287
Lease liabilities - current	268	147
Total liabilities	£ 806	£ 434	£ 434

Significant Accounting Polici_8

Significant Accounting Policies - Summary of Operating Lease Commitments (Details) - GBP (£) £ in Thousands	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Lease Liabilities [Abstract]
Operating lease obligations as at December 31, 2018			£ 433
Weighted average incremental borrowing rate as at January 1, 2019		4.17%
Discounted operating lease obligations as at January 1, 2019		£ 413
Lease not commenced as at December 31, 2018			(138)
Lease obligations prepaid as at December 31, 2018			(21)
Service component included in operating lease obligations as at December 31, 2018			(3)
Lease payments relating to renewal periods not included in operating lease obligations as at December 31, 2018			183
Total liabilities	£ 806	£ 434	£ 434

Staff Costs - Disclosure of Emp

Staff Costs - Disclosure of Employee Benefit Expense (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Staff Costs [Line Items]
Share-based payments	£ 3,200	£ 1,800	£ 11,700
Total employee benefit expense	8,407	5,911	14,979
Research and Development Expenses
Staff Costs [Line Items]
Wages and salaries	3,304	2,761	2,391
Social security costs	395	314	272
Pension costs	140	139	119
Share-based payments	1,878	835	8,868
Total employee benefit expense	5,717	4,049	11,650
Administrative Expenses
Staff Costs [Line Items]
Wages and salaries	1,190	790	408
Social security costs	115	75	37
Pension costs	37	37	21
Share-based payments	1,348	960	2,863
Total employee benefit expense	£ 2,690	£ 1,862	£ 3,329

Staff Costs - Schedule of Avera

Staff Costs - Schedule of Average Number of Staff Employed Under Contracts of Service Explanatory (Details) - Employee	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Staff Costs [Line Items]
Average number of staff employed under contracts of service	28	22	20
Research and Development Expenses
Staff Costs [Line Items]
Average number of staff employed under contracts of service	23	18	18
Administrative Expenses
Staff Costs [Line Items]
Average number of staff employed under contracts of service	5	4	2

Staff Costs - Summary of Direct

Staff Costs - Summary of Directors' Remuneration and Other Benefits (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Staff Costs [Line Items]
Directors’ remuneration	£ 1,222	£ 1,143	£ 793
Pension and other benefits	52	51	37
Directors’ remuneration & other benefits	1,274	1,194	830
Highest Paid Director
Staff Costs [Line Items]
Directors’ remuneration	780	739	601
Pension and other benefits	48	47	33
Directors’ remuneration & other benefits	£ 828	£ 786	£ 634

Staff Costs - Additional Inform

Staff Costs - Additional Information (Details) £ in Millions	12 Months Ended
	Dec. 31, 2019GBP (£)Director	Dec. 31, 2018GBP (£)Director	Dec. 31, 2017GBP (£)Director
Classes Of Employee Benefits Expense [Abstract]
Number of directors, exercised share options | Director	1	1	0
Gain on exercise of option | £	£ 1.4	£ 4.2	£ 0

Basic and Diluted Loss Per Sh_3

Basic and Diluted Loss Per Share - Schedule of Basic and Diluted Loss Per Share (Details) - GBP (£) £ / shares in Units, £ in Thousands, shares in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Loss for the year	£ (21,412)	£ (13,840)	£ (23,085)
Basic and diluted weighted average number of shares	32,327	31,972	26,069
Basic and diluted loss per share	£ (0.66)	£ (0.43)	£ (0.89)

Loss Before Tax - Summary of Lo

Loss Before Tax - Summary of Loss Before Tax Stated After Charging (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Material Income And Expense [Abstract]
Amortization	£ 377	£ 230	£ 164
Depreciation	£ 341	£ 141	30
IPO related expenses			£ 1,794

Capital Commitments and Conti_2

Capital Commitments and Contingencies - Additional Information (Details) - GBP (£)	1 Months Ended	12 Months Ended		24 Months Ended
	Feb. 29, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Capital Commitments [Line Items]
Employer national insurance contingent liability			£ 1,300,000	£ 1,300,000	£ 3,300,000
Prevailing rate of employer national insurance			13.80%
Cardiff University
Capital Commitments [Line Items]
Research, collaboration and license agreement additional term			2 years
Payments for work performed by sub-licensee				£ 292,607
Cardiff University | Subsequent Event
Capital Commitments [Line Items]
Research, collaboration and license agreement additional term	1 year
Cardiff University | Scenario Forecast
Capital Commitments [Line Items]
Payments for work performed by sub-licensee		£ 175,000
Cardiff Consultants
Capital Commitments [Line Items]
Milestone payments for sub-licensee			£ 1,875,000
Cardiff ProTides
Capital Commitments [Line Items]
Milestone payments for sub-licensee			£ 4,500,000

Income Tax Credit - Schedule of

Income Tax Credit - Schedule of Major Components of Income Tax (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current tax:
Total current tax	£ 4,238	£ 4,261	£ 2,291
Deferred tax:
In respect of prior years, deferred tax	(1,962)	(2,043)	(1,007)
Total deferred tax	1	(38)	110
Income tax credit	4,239	4,223	2,401
Current income tax receivable	8,481	4,263	4,225
Deferred tax	46	47
U.K.
Current tax:
In respect of current year	4,325	4,239	2,298
In respect of prior years	(86)	19	3
Deferred tax:
Current income tax receivable	8,477	4,239	4,207
U.S.
Current tax:
In respect of current year	(1)		(10)
In respect of prior years		3
Deferred tax:
In respect of current year, deferred tax	1	11	61
In respect of prior years, deferred tax		(49)	49
Current income tax receivable	4	24	18
Deferred tax	£ 46	£ 47	£ 81

Income Tax Credit - Schedule _2

Income Tax Credit - Schedule of Reconciliation of Accounting Loss per Statement of Operations (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation Of Accounting Profit Multiplied By Applicable Tax Rates [Abstract]
Loss before tax	£ (25,651)	£ (18,063)	£ (25,486)
Tax on loss at standard U.K. tax rate	(4,874)	(3,432)	(4,906)
Expenses not deductible	2,912	1,389	3,899
Deduction for R&D	(5,667)	(5,554)	(3,051)
Losses surrendered for R&D tax credit	5,667	5,554	3,051
Deferred tax - PY adjustment		49	(49)
Overseas tax payable - current year	1		10
Overseas tax payable - prior years		(3)
R&D tax credit - U.S.	(1)	(11)	(61)
R&D tax credit - current year	(4,325)	(4,239)	(2,298)
R&D tax credit - prior years	86	(19)	(3)
Deferred tax asset not recognized	1,962	2,043	1,007
Income tax credit	£ (4,239)	£ (4,223)	£ (2,401)

Income Tax Credit - Schedule _3

Income Tax Credit - Schedule of Reconciliation of Accounting Loss per Statement of Operations (Parenthetical) (Details)	Apr. 01, 2017	Oct. 26, 2015	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
U.K.
Reconciliation Of Accounting Profit Multiplied By Applicable Tax Rates [Line Items]
Applicable tax rate	19.00%	20.00%	19.00%	19.00%	19.25%

Income Tax Credit - Additional

Income Tax Credit - Additional Information (Details) - GBP (£) £ in Millions	Apr. 01, 2020	Apr. 01, 2017	Oct. 26, 2015	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disclosure Of Temporary Difference Unused Tax Losses And Unused Tax Credits [Line Items]
Temporary differences and cumulative carry forward tax losses for which deferred tax has not been recognized				£ 46.4	£ 62.1	£ 40.7
Temporary differences on share based compensation arrangements				14.2	38.9	28.3
Cumulative carry forward tax losses				£ 32.2	£ 23.2	£ 12.4
U.K.
Disclosure Of Temporary Difference Unused Tax Losses And Unused Tax Credits [Line Items]
Applicable tax rate		19.00%	20.00%	19.00%	19.00%	19.25%
Applicable tax rate initially declared			18.00%
Scenario Forecast | U.K.
Disclosure Of Temporary Difference Unused Tax Losses And Unused Tax Credits [Line Items]
Applicable tax rate	17.00%
Future tax rate	18.00%
Percentage of change in tax rate	1.00%

Intangible Assets - Summary of

Intangible Assets - Summary of Intangible Assets (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	£ 3,122
Ending Balance	3,960	£ 3,122
Cost
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	3,682	2,268
Additions	1,215		£ 1,414
Ending Balance	4,897	3,682	2,268
Accumulated Amortization
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	560	330
Charge for the year	377	230
Ending Balance	937	560	330
Patents
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	3,026
Ending Balance	3,717	3,026
Patents | Cost
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	3,524	2,115
Additions	999		1,409
Ending Balance	4,523	3,524	2,115
Patents | Accumulated Amortization
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	498	302
Charge for the year	308	196
Ending Balance	806	498	302
Computer Software
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	96
Ending Balance	243	96
Computer Software | Cost
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	158	153
Additions	216		5
Ending Balance	374	158	153
Computer Software | Accumulated Amortization
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	62	28
Charge for the year	69	34
Ending Balance	£ 131	£ 62	£ 28

Property, Plant and Equipment -

Property, Plant and Equipment - Summary of Property, Plant and Equipment (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Disclosure Of Property Plant And Equipment [Line Items]
Beginning Balance	£ 427
Ending Balance	1,109	£ 427
Cost
Disclosure Of Property Plant And Equipment [Line Items]
Beginning Balance	616	422
Recognized on adoption of IFRS 16 Leases	455
Additions	569	210
Re-measurement	2
Disposals	(1)	(16)
Effect of foreign currency exchange differences	(5)
Ending Balance	1,636	616
Depreciation
Disclosure Of Property Plant And Equipment [Line Items]
Beginning Balance	189	64
Charge for the year	341	141
Disposals	(1)	(16)
Effect of foreign currency exchange differences	(2)
Ending Balance	527	189
Right of Use Assets
Disclosure Of Property Plant And Equipment [Line Items]
Ending Balance	791
Right of Use Assets | Cost
Disclosure Of Property Plant And Equipment [Line Items]
Recognized on adoption of IFRS 16 Leases	455
Additions	530
Re-measurement	2
Effect of foreign currency exchange differences	(5)
Ending Balance	982
Right of Use Assets | Depreciation
Disclosure Of Property Plant And Equipment [Line Items]
Charge for the year	193
Effect of foreign currency exchange differences	(2)
Ending Balance	191
Office and Computer Equipment
Disclosure Of Property Plant And Equipment [Line Items]
Beginning Balance	114
Ending Balance	76	114
Office and Computer Equipment | Cost
Disclosure Of Property Plant And Equipment [Line Items]
Beginning Balance	210	142
Additions	27	84
Disposals	(1)	(16)
Ending Balance	236	210
Office and Computer Equipment | Depreciation
Disclosure Of Property Plant And Equipment [Line Items]
Beginning Balance	96	51
Charge for the year	65	61
Disposals	(1)	(16)
Ending Balance	160	96
Fixtures and Fittings
Disclosure Of Property Plant And Equipment [Line Items]
Beginning Balance	313
Ending Balance	242	313
Fixtures and Fittings | Cost
Disclosure Of Property Plant And Equipment [Line Items]
Beginning Balance	406	280
Additions	12	126
Ending Balance	418	406
Fixtures and Fittings | Depreciation
Disclosure Of Property Plant And Equipment [Line Items]
Beginning Balance	93	13
Charge for the year	83	80
Ending Balance	£ 176	£ 93

Prepayments, Accrued Income a_3

Prepayments, Accrued Income and Other Receivables - Summary of Prepayments, Accrued Income and Other Receivables Explanatory (Details) - GBP (£) £ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Prepayments Accrued Income And Other Receivables [Abstract]
Prepayments—manufacturing and clinical	£ 2,630	£ 1,050
Prepayments—other	1,489	750
Accrued income	76	165
VAT	475	379
Other receivables	40	10
Prepayments, accrued income and other receivables	£ 4,710	£ 2,354

Share Capital and Share Premi_3

Share Capital and Share Premium - Summary of Share Capital and Share Premium (Details) - GBP (£) £ in Thousands, shares in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disclosure Of Classes Of Share Capital [Line Items]
Share capital	£ 1,299	£ 1,289	£ 1,272
Share premium	79,541	79,426	79,236
Share capital and share premium	£ 80,840	£ 80,715	£ 80,508
Number of shares issued	32,479	32,226	31,811
Ordinary shares [member]
Disclosure Of Classes Of Share Capital [Line Items]
Number of shares issued	32,479	32,226	31,811

Share Capital and Share Premi_4

Share Capital and Share Premium - Summary of Share Capital and Share Premium (Parenthetical) (Details) - £ / shares	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Ordinary shares [member]
Disclosure Of Classes Of Share Capital [Line Items]
Par value per share	£ 0.04	£ 0.04	£ 0.04

Share Capital and Share Premi_5

Share Capital and Share Premium - Summary of Movement in the Share Capital (Details) £ in Thousands	12 Months Ended
	Dec. 31, 2019GBP (£)shares	Dec. 31, 2018GBP (£)shares	Dec. 31, 2017GBP (£)shares
Disclosure Of Classes Of Share Capital [Line Items]
Beginning balance	£ 81,594	£ 93,420	£ 25,241
Beginning balance, Shares | shares	32,226,000	31,811,000
Exercise of share options	£ 125	£ 207	£ 120
Exercise of share options, Shares | shares	252,187	415,312	30,000
Ending balance	£ 63,522	£ 81,594	£ 93,420
Ending balance, Shares | shares	32,479,000	32,226,000	31,811,000
Share Capital
Disclosure Of Classes Of Share Capital [Line Items]
Beginning balance	£ 1,289	£ 1,272	£ 663
Exercise of share options	10	17	1
Ending balance	1,299	1,289	1,272
Share Premium
Disclosure Of Classes Of Share Capital [Line Items]
Beginning balance	79,426	79,236	42,770
Exercise of share options	115	190	119
Ending balance	£ 79,541	£ 79,426	£ 79,236

Other Reserves - Schedule of Ot

Other Reserves - Schedule of Other Reserves (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share option reserve
Total other reserves	£ 62,737	£ 59,692
Other Reserves
Disclosure Of Other Reserves [Line Items]
Own share reserve	(339)	(339)	£ (339)
Foreign currency translation reserve	(10)	1	(11)
Capital reserve	42,466	42,466	42,466
Share option reserve
Balance at beginning of year	17,564	15,955	4,406
Share-based payments	3,289	1,977	11,731
Exercise of share options	(132)	(186)	(180)
Forfeiture of share options	(63)	(182)
Lapse of share options	(38)		(2)
Balance at end of year	20,620	17,564	15,955
Total other reserves	£ 62,737	£ 59,692	£ 58,071

Other Reserves - Additional Inf

Other Reserves - Additional Information (Details)	Dec. 31, 2019shares
Disclosure Of Other Reserves [Abstract]
Number of shares purchased by employee benefit trust under share options plan	500,000

Share-based Payments - Addition

Share-based Payments - Additional Information (Details) £ in Millions	12 Months Ended
	Dec. 31, 2019GBP (£)yrPlan	Dec. 31, 2018GBP (£)	Dec. 31, 2017GBP (£)
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Number of equity settled share-based payment plans | Plan	3
Weighted average remaining contractual life of outstanding share options	4 years 7 months 17 days	3 years 11 months 15 days	4 years 5 months 26 days
Employee remuneration expense, related to equity-settled share-based payments | £	£ 3.2	£ 1.8	£ 11.7
U.K. Share-Based Payment Plans
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Share options granted, vesting period	4 years	4 years	4 years
Stock Option Plan (U.S. Sub-Plan)
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Share options granted, vesting period	4 years		4 years
2017 Share Options
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Discount rate used to compute present value			25.00%
2018 Share Options
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Term of share options exercised		2 years
2019 Share Options
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Term of share options exercised	2 years
Maximum | Employees
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Expected option life (years)	10
Maximum | Directors
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Expected option life (years)	10
Maximum | Consultants
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Expected option life (years)	10

Share-based Payments - Summary

Share-based Payments - Summary of Share Options and Weighted Average Exercise Prices (Details)	12 Months Ended
	Dec. 31, 2019shares£ / shares	Dec. 31, 2018shares£ / shares	Dec. 31, 2017shares£ / shares
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Abstract]
Number of shares, Outstanding, Beginning balance | shares	4,406,252	4,711,502	3,248,187
Number of shares, Granted | shares	1,202,150	253,500	1,500,815
Number of shares, Forfeited | shares	(48,625)	(143,438)	(7,500)
Number of shares, Lapsed | shares	(8,750)
Number of shares, Exercised | shares	(252,187)	(415,312)	(30,000)
Number of shares, Outstanding, Ending balance | shares	5,298,840	4,406,252	4,711,502
Number of shares, Vested and exercisable, Ending balance | shares	3,811,736	3,847,305	4,030,833
Weighted average exercise price per share, Outstanding, Beginning balance | £ / shares	£ 1.90	£ 1.29	£ 0.72
Weighted average exercise price per share, Granted | £ / shares	10.82	17.01	2.58
Weighted average exercise price per share, Forfeited | £ / shares	10.42	12.74	0.16
Weighted average exercise price per share, Lapsed | £ / shares	3.90
Weighted average exercise price per share, Exercised | £ / shares	0.50	0.50	4
Weighted average exercise price per share, Outstanding, Ending balance | £ / shares	3.91	1.90	1.29
Weighted average exercise price per share, Vested and exercisable, Ending balance | £ / shares	£ 1.15	£ 0.68	£ 0.36

Share-based Payments - Summar_2

Share-based Payments - Summary of Share Options and Weighted Average Exercise Prices (Parenthetical) (Details) - £ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Weighted average share price for share options at date of exercise	£ 7.65	£ 18.23	£ 10.43
Bottom Of Range [Member]
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Exercise price of outstanding share options	0.04
Maximum
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Exercise price of outstanding share options	£ 18.05

Share-based Payments - Summar_3

Share-based Payments - Summary of Principal Assumptions used for Valuation of Share Options (Details)	12 Months Ended
	Dec. 31, 2019GBP (£)sharesyr£ / shares	Dec. 31, 2018GBP (£)sharesyr£ / shares	Dec. 31, 2017GBP (£)sharesyr£ / shares
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Number of options granted | shares	1,202,150	253,500	1,500,815
2017 Share Options | Grant Date May 16, 2017
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Volatility			66.56%
Dividend yield			0.00%
Risk-free investment rate			0.12%
Fair value of option at grant date | £			£ 7.70
Fair value of share at grant date			£ 11.08
Exercise price at date of grant			£ 4
Lapse date			May 16, 2027
Expected option life (years) | yr			2.63
Number of options granted | shares			23,250
2017 Share Options | Grant Date Sep 14, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Volatility			66.94%
Dividend yield			0.00%
Risk-free investment rate			0.27%
Fair value of option at grant date | £			£ 6.15
Fair value of share at grant date			£ 10.43
Exercise price at date of grant			£ 5.40
Lapse date			Sep. 13, 2027
Expected option life (years) | yr			2.30
Number of options granted | shares			14,690
2017 Share Options | Grant Date Sep 14, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Volatility			66.97%
Dividend yield			0.00%
Risk-free investment rate			0.36%
Fair value of option at grant date | £			£ 6.08
Fair value of share at grant date			£ 10.34
Exercise price at date of grant			£ 5.40
Lapse date			Sep. 14, 2027
Expected option life (years) | yr			2.30
Number of options granted | shares			25,000
2017 Share Options | Grant Date Sep 14, 2017 Two
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Volatility			66.97%
Dividend yield			0.00%
Risk-free investment rate			0.36%
Fair value of option at grant date | £			£ 6.08
Fair value of share at grant date			£ 10.34
Exercise price at date of grant			£ 5.40
Lapse date			Sep. 14, 2027
Expected option life (years) | yr			2.30
Number of options granted | shares			110,310
2017 Share Options | Grant Date Sep 14, 2017 Three
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Volatility			66.97%
Dividend yield			0.00%
Risk-free investment rate			0.36%
Fair value of option at grant date | £			£ 6.08
Fair value of share at grant date			£ 10.34
Exercise price at date of grant			£ 5.40
Lapse date			Sep. 14, 2027
Expected option life (years) | yr			2.30
Number of options granted | shares			12,500
2017 Share Options | Grant Date Sep 14, 2017 Four
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Volatility			66.97%
Dividend yield			0.00%
Risk-free investment rate			0.36%
Fair value of option at grant date | £			£ 3.90
Fair value of share at grant date			£ 10.34
Exercise price at date of grant			£ 10.80
Lapse date			Sep. 14, 2027
Expected option life (years) | yr			2.30
Number of options granted | shares			25,000
2017 Share Options | Grant Date Sep 15, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Volatility			67.02%
Dividend yield			0.00%
Risk-free investment rate			0.44%
Fair value of option at grant date | £			£ 6.76
Fair value of share at grant date			£ 10.15
Exercise price at date of grant			£ 4
Lapse date			Sep. 15, 2027
Expected option life (years) | yr			2.29
Number of options granted | shares			45,750
2017 Share Options | Grant Date Sep 15, 2017 Two
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Volatility			67.02%
Dividend yield			0.00%
Risk-free investment rate			0.44%
Fair value of option at grant date | £			£ 10.11
Fair value of share at grant date			£ 10.15
Exercise price at date of grant			£ 0.04
Lapse date			Sep. 15, 2027
Expected option life (years) | yr			2.29
Number of options granted | shares			1,028,533
2017 Share Options | Grant Date Sep 27, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Volatility			67.11%
Dividend yield			0.00%
Risk-free investment rate			0.47%
Fair value of option at grant date | £			£ 4.36
Fair value of share at grant date			£ 11.19
Exercise price at date of grant			£ 11.19
Lapse date			Sep. 27, 2027
Expected option life (years) | yr			2.26
Number of options granted | shares			37,500
2017 Share Options | Grant Date Sep 27, 2017 Two
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Volatility			67.11%
Dividend yield			0.00%
Risk-free investment rate			0.47%
Fair value of option at grant date | £			£ 4.36
Fair value of share at grant date			£ 11.19
Exercise price at date of grant			£ 11.19
Lapse date			Sep. 27, 2027
Expected option life (years) | yr			2.26
Number of options granted | shares			178,282
2017 Share Options | Vesting Dates One | Grant Date May 16, 2017
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Oct. 28, 2017
2017 Share Options | Vesting Dates One | Grant Date Sep 14, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 13, 2018
2017 Share Options | Vesting Dates One | Grant Date Sep 14, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Mar. 6, 2018
2017 Share Options | Vesting Dates One | Grant Date Sep 14, 2017 Two
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 14, 2018
2017 Share Options | Vesting Dates One | Grant Date Sep 14, 2017 Three
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 14, 2018
2017 Share Options | Vesting Dates One | Grant Date Sep 14, 2017 Four
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 14, 2018
2017 Share Options | Vesting Dates One | Grant Date Sep 15, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 15, 2018
2017 Share Options | Vesting Dates One | Grant Date Sep 15, 2017 Two
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 15, 2017
2017 Share Options | Vesting Dates One | Grant Date Sep 27, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Mar. 20, 2018
2017 Share Options | Vesting Dates One | Grant Date Sep 27, 2017 Two
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 27, 2018
2017 Share Options | Vesting Dates Two | Grant Date May 16, 2017
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Oct. 28, 2018
2017 Share Options | Vesting Dates Two | Grant Date Sep 14, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 13, 2019
2017 Share Options | Vesting Dates Two | Grant Date Sep 14, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Mar. 6, 2019
2017 Share Options | Vesting Dates Two | Grant Date Sep 14, 2017 Two
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 14, 2019
2017 Share Options | Vesting Dates Two | Grant Date Sep 14, 2017 Three
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 14, 2019
2017 Share Options | Vesting Dates Two | Grant Date Sep 14, 2017 Four
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 14, 2019
2017 Share Options | Vesting Dates Two | Grant Date Sep 15, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 15, 2019
2017 Share Options | Vesting Dates Two | Grant Date Sep 27, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Mar. 20, 2019
2017 Share Options | Vesting Dates Two | Grant Date Sep 27, 2017 Two
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 27, 2019
2017 Share Options | Vesting Dates Three | Grant Date May 16, 2017
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Oct. 28, 2019
2017 Share Options | Vesting Dates Three | Grant Date Sep 14, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 13, 2020
2017 Share Options | Vesting Dates Three | Grant Date Sep 14, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Mar. 6, 2020
2017 Share Options | Vesting Dates Three | Grant Date Sep 14, 2017 Two
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 14, 2020
2017 Share Options | Vesting Dates Three | Grant Date Sep 14, 2017 Three
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 14, 2020
2017 Share Options | Vesting Dates Three | Grant Date Sep 14, 2017 Four
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 14, 2020
2017 Share Options | Vesting Dates Three | Grant Date Sep 15, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 15, 2020
2017 Share Options | Vesting Dates Three | Grant Date Sep 27, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Mar. 20, 2020
2017 Share Options | Vesting Dates Three | Grant Date Sep 27, 2017 Two
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 27, 2020
2017 Share Options | Vesting Dates Four | Grant Date May 16, 2017
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Oct. 28, 2020
2017 Share Options | Vesting Dates Four | Grant Date Sep 14, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 13, 2021
2017 Share Options | Vesting Dates Four | Grant Date Sep 14, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Mar. 6, 2021
2017 Share Options | Vesting Dates Four | Grant Date Sep 14, 2017 Two
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 14, 2021
2017 Share Options | Vesting Dates Four | Grant Date Sep 14, 2017 Three
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 14, 2021
2017 Share Options | Vesting Dates Four | Grant Date Sep 14, 2017 Four
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 14, 2021
2017 Share Options | Vesting Dates Four | Grant Date Sep 15, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 15, 2021
2017 Share Options | Vesting Dates Four | Grant Date Sep 27, 2017 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Mar. 20, 2021
2017 Share Options | Vesting Dates Four | Grant Date Sep 27, 2017 Two
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates			Sep. 27, 2021
2018 Share Options | Grant Date April 11, 2018
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Volatility		64.48%
Dividend yield		0.00%
Risk-free investment rate		1.04%
Fair value of option at grant date | £		£ 8.97
Fair value of share at grant date		£ 17.51
Exercise price at date of grant		£ 17.51
Lapse date		Apr. 11, 2028
Expected option life (years) | yr		4.50
Number of options granted | shares		71,500
2018 Share Options | Grant Date April 11, 2018 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Volatility		60.06%
Dividend yield		0.00%
Risk-free investment rate		0.83%
Fair value of option at grant date | £		£ 17.35
Fair value of share at grant date		£ 17.51
Exercise price at date of grant		£ 0.16
Lapse date		Apr. 11, 2028
Expected option life (years) | yr		2
Number of options granted | shares		7,500
2018 Share Options | Grant Date May 8, 2018
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Volatility		65.80%
Dividend yield		0.00%
Risk-free investment rate		1.02%
Fair value of option at grant date | £		£ 8.63
Fair value of share at grant date		£ 16.57
Exercise price at date of grant		£ 16.57
Lapse date		May 8, 2028
Expected option life (years) | yr		4.50
Number of options granted | shares		62,000
2018 Share Options | Grant Date Aug 14, 2018
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Volatility		68.14%
Dividend yield		0.00%
Risk-free investment rate		0.93%
Fair value of option at grant date | £		£ 9.65
Fair value of share at grant date		£ 18.05
Exercise price at date of grant		£ 18.05
Lapse date		Aug. 14, 2028
Expected option life (years) | yr		4.50
Number of options granted | shares		112,500
2018 Share Options | Vesting Dates One | Grant Date April 11, 2018
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates		Apr. 11, 2019
2018 Share Options | Vesting Dates One | Grant Date April 11, 2018 One
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates		Apr. 11, 2018
2018 Share Options | Vesting Dates One | Grant Date May 8, 2018
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates		May 8, 2019
2018 Share Options | Vesting Dates One | Grant Date Aug 14, 2018
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates		Aug. 14, 2019
2018 Share Options | Vesting Dates Two | Grant Date April 11, 2018
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates		Apr. 11, 2020
2018 Share Options | Vesting Dates Two | Grant Date May 8, 2018
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates		May 8, 2020
2018 Share Options | Vesting Dates Two | Grant Date Aug 14, 2018
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates		Aug. 14, 2020
2018 Share Options | Vesting Dates Three | Grant Date April 11, 2018
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates		Apr. 11, 2021
2018 Share Options | Vesting Dates Three | Grant Date May 8, 2018
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates		May 8, 2021
2018 Share Options | Vesting Dates Three | Grant Date Aug 14, 2018
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates		Aug. 14, 2021
2018 Share Options | Vesting Dates Four | Grant Date April 11, 2018
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates		Apr. 11, 2022
2018 Share Options | Vesting Dates Four | Grant Date May 8, 2018
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates		May 8, 2022
2018 Share Options | Vesting Dates Four | Grant Date Aug 14, 2018
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates		Aug. 14, 2022
2019 Share Options | Grant Date Mar 13, 2019
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Volatility	69.05%
Dividend yield	0.00%
Risk-free investment rate	0.85%
Fair value of option at grant date | £	£ 5.46
Fair value of share at grant date	£ 10.13
Exercise price at date of grant	£ 10.13
Lapse date	Mar. 13, 2029
Expected option life (years) | yr	4.50
Number of options granted | shares	120,750
2019 Share Options | Grant Date May 15, 2019
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Volatility	69.08%
Dividend yield	0.00%
Risk-free investment rate	0.77%
Fair value of option at grant date | £	£ 6.07
Fair value of share at grant date	£ 11.26
Exercise price at date of grant	£ 11.26
Lapse date	May 15, 2029
Expected option life (years) | yr	4.50
Number of options granted | shares	967,400
2019 Share Options | Grant Date Sept, 2019
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Volatility	70.14%
Dividend yield	0.00%
Risk-free investment rate	0.44%
Fair value of option at grant date | £	£ 4.22
Fair value of share at grant date	£ 7.79
Exercise price at date of grant	£ 7.79
Lapse date	Sep. 11, 2029
Expected option life (years) | yr	4.50
Number of options granted | shares	114,000
2019 Share Options | Vesting Dates One | Grant Date Mar 13, 2019
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates	Mar. 13, 2020
2019 Share Options | Vesting Dates One | Grant Date May 15, 2019
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates	May 15, 2020
2019 Share Options | Vesting Dates One | Grant Date Sept, 2019
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates	Sep. 11, 2020
2019 Share Options | Vesting Dates Two | Grant Date Mar 13, 2019
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates	Mar. 13, 2021
2019 Share Options | Vesting Dates Two | Grant Date May 15, 2019
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates	May 15, 2021
2019 Share Options | Vesting Dates Two | Grant Date Sept, 2019
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates	Sep. 11, 2021
2019 Share Options | Vesting Dates Three | Grant Date Mar 13, 2019
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates	Mar. 13, 2022
2019 Share Options | Vesting Dates Three | Grant Date May 15, 2019
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates	May 15, 2022
2019 Share Options | Vesting Dates Three | Grant Date Sept, 2019
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates	Sep. 11, 2022
2019 Share Options | Vesting Dates Four | Grant Date Mar 13, 2019
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates	Mar. 13, 2023
2019 Share Options | Vesting Dates Four | Grant Date May 15, 2019
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates	May 15, 2023
2019 Share Options | Vesting Dates Four | Grant Date Sept, 2019
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Vesting dates	Sep. 11, 2023

Leases - Additional information

Leases - Additional information (Details) £ in Millions	12 Months Ended
	Dec. 31, 2019GBP (£)LeaseContract
Presentation Of Leases For Lessee [Abstract]
Information about nature of lessee's leasing activities	lease terms of between two and five years
Total net cash outflows for leases	£ 0.2
Non-cash additions to right-of-use assets and lease liabilities	£ 0.5
Information about lessee's exposure arising from variable lease payments	The Company has one lease contract with variable payments where the lease costs after the first year of the lease are increased based upon a consumer price index.
Number of lease contracts with variable lease payments | LeaseContract	1
Information about lessee's exposure arising from extension options and termination options	The Company has a number of lease contracts that include extension and termination options. These options are negotiated by management to provide flexibility in managing the leased-asset portfolio and align it with the Company’s business needs. None of the termination options are expected to be exercised.

Leases - Summary of Carrying Am

Leases - Summary of Carrying Amounts of Lease Liabilities (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2019	Jan. 01, 2019
Lease Liabilities [Abstract]
Initial recognition at January 1, 2019	£ 434
Additions	548
Re-measurement of liability	2
Accretion of interest	21
Payments	(197)
Effect of foreign currency exchange differences	(2)
At December 31, 2019	806
Current	268	£ 147
Non-current	£ 538	£ 287

Leases - Summary of Maturity An

Leases - Summary of Maturity Analysis of Lease Liabilities (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Disclosure Of Maturity Analysis Of Operating Lease Payments [Line Items]
Total contractual undiscounted payments	£ 860	£ 21
Less: effect of discounting	(54)
Discounted lease liabilities	806	£ 434	£ 434
Not later than 1 year
Disclosure Of Maturity Analysis Of Operating Lease Payments [Line Items]
Total contractual undiscounted payments	293
Later than 1 year and not later than 3 years
Disclosure Of Maturity Analysis Of Operating Lease Payments [Line Items]
Total contractual undiscounted payments	400
Later than 3 years and not later than 5 years
Disclosure Of Maturity Analysis Of Operating Lease Payments [Line Items]
Total contractual undiscounted payments	£ 167

Leases - Summary of Amounts Rec

Leases - Summary of Amounts Recognized in Statements of Operations With Respect to Lease Contracts (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Presentation Of Leases For Lessee [Abstract]
Depreciation expense of right of use assets under IFRS 16	£ 193
Interest expense on lease liabilities (included in administrative expenses) under IFRS 16	21
Operating lease expense under IAS 17		£ 150	£ 182
Total amount recognized in statements of operations	£ 214	£ 150	£ 182

Leases - Summary of Maturity _2

Leases - Summary of Maturity Analysis of Lease Liabilities Extension Options Not Expected to be Exercised (Details) £ in Thousands	Dec. 31, 2019GBP (£)
Disclosure Of Maturity Analysis Of Operating Lease Payments [Line Items]
Extension options not expected to be exercised	£ 978
Not later than 5 years
Disclosure Of Maturity Analysis Of Operating Lease Payments [Line Items]
Extension options not expected to be exercised	351
Later than 5 years
Disclosure Of Maturity Analysis Of Operating Lease Payments [Line Items]
Extension options not expected to be exercised	£ 627

Related Party Disclosures - Sum

Related Party Disclosures - Summary of Compensation of Key Management Personnel (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disclosure Of Key Management Personnel Compensation [Abstract]
Short-term employee benefits	£ 1,686	£ 1,687	£ 1,184
Pension and other benefits	69	72	56
Share-based payments	1,976	893	11,230
Key management personnel compensation	£ 3,731	£ 2,652	£ 12,470

Cash and Cash Equivalents - Sch

Cash and Cash Equivalents - Schedule of Cash and Cash Equivalents (Detail) - GBP (£) £ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	£ 51,962	£ 76,972	£ 86,703	£ 19,990

Financial Instruments Risk Ma_3

Financial Instruments Risk Management - Additional Information (Details) - GBP (£)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Financial Instruments [Line Items]
Cash and cash equivalents	£ 51,962,000	£ 76,972,000	£ 86,703,000	£ 19,990,000
Interest Rate Risk
Disclosure Of Financial Instruments [Line Items]
Cash and cash equivalents	£ 52,000,000	77,000,000
Increase in interest rates	0.50%
Increase in interest income on cash and cash equivalents	£ 244,674	357,041
Currency Risk
Disclosure Of Financial Instruments [Line Items]
Cash and cash equivalents	£ 28,980,000	44,018,000
Percentage of increase in foreign exchange rate	1.00%
Reduction in financial assets and liabilities in foreign currencies	£ (295,498)	£ (429,101)

Financial Instruments Risk Ma_4

Financial Instruments Risk Management - Schedule of Financial Assets Subject to Fixed or Variable Interest Rates (Details) - GBP (£) £ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Financial Instruments By Type Of Interest Rate [Line Items]
Cash and cash equivalents	£ 51,962	£ 76,972	£ 86,703	£ 19,990
Financial Assets at Fixed Rates
Disclosure Of Financial Instruments By Type Of Interest Rate [Line Items]
Cash and cash equivalents	41,827	64,267
Financial Assets at Variable Rates
Disclosure Of Financial Instruments By Type Of Interest Rate [Line Items]
Cash and cash equivalents	7,108	7,141
Non-Interest Bearing Cash Balances
Disclosure Of Financial Instruments By Type Of Interest Rate [Line Items]
Cash and cash equivalents	£ 3,027	£ 5,564

Financial Instruments Risk Ma_5

Financial Instruments Risk Management - Summary of Financial Assets and Liabilities in Foreign Currencies, Primarily Held in U.S Dollars (Details) - GBP (£) £ in Thousands	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financial assets
Current income tax receivable	£ 8,481		£ 4,263	£ 4,225
Cash and cash equivalents	51,962		76,972	£ 86,703	£ 19,990
Financial liabilities
Payroll taxes and social security	160		127
Lease liabilities	806	£ 434	434
Accrued expenditure	3,342		2,983
Currency Risk
Financial assets
Prepayments, accrued income and other receivables	2,174		1,477
Current income tax receivable	4		25
Cash and cash equivalents	28,980		44,018
Financial liabilities
Trade payables	549		1,192
Payroll taxes and social security	18		26
Lease liabilities	52
Accrued expenditure	£ 989		£ 1,391