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Income Tax Credit - Additional Information (Details) - GBP (£) £ in Millions |  |  | 12 Months Ended
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Apr. 01, 2020 | Sep. 06, 2016 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
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 |  |  | £ 57.5 | £ 46.4 | £ 62.1
Temporary differences on share based compensation arrangements |  |  | 9.9 | 14.2 | 38.9
Cumulative carry forward tax losses |  |  | £ 47.6 | £ 32.2 | £ 23.2
U.K. [member] |  |  |  |  |
Disclosure Of Temporary Difference Unused Tax Losses And Unused Tax Credits [Line Items] |  |  |  |  |
Applicable tax rate | 19.00% | 19.00% | 19.00% | 19.00% | 19.00%
Applicable tax rate initially declared | 17.00% | 17.00% |  |  |

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