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Other Reserves - Schedule of Other Reserves (Detail) - GBP (£)
£ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share option reserve      
Total other reserves £ 66,887 £ 62,737  
Other Reserves      
Disclosure Of Other Reserves [Line Items]      
Own share reserve (339) (339) £ (339)
Foreign currency translation reserve (22) (10) 1
Capital reserve 42,466 42,466 42,466
Share option reserve      
Balance at beginning of year 20,620 17,564 15,955
Share-based payments 4,823 3,289 1,977
Exercise of share options (68) (132) (186)
Forfeiture of share options (518) (63) (182)
Lapse of share options (75) (38)  
Balance at end of year 24,782 20,620 17,564
Total other reserves £ 66,887 £ 62,737 £ 59,692