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Share-based Payments - Additional Information (Details)
£ in Millions
12 Months Ended
Dec. 31, 2020
GBP (£)
yr
Plan
Dec. 31, 2019
GBP (£)
Dec. 31, 2018
GBP (£)
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]      
Number of equity settled share-based payment plans | Plan 6    
Weighted average remaining contractual life of outstanding share options 5 years 9 months 7 days 3 years 11 months 15 days 4 years 5 months 26 days
Employee remuneration expense, related to equity-settled share-based payments | £ £ 4.3 £ 3.2 £ 1.8
U.K. Share-Based Payment Plans [member]      
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]      
Share options granted, vesting period 4 years 4 years 4 years
U.S Share Option Sub-Plan [Member]      
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]      
Share options granted, vesting period   4 years  
Long-Term Incentive Plan [member]      
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]      
Share options granted, vesting period 4 years    
2018 Share Options [member]      
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]      
Term of share options exercised     2 years
2019 Share Options [member]      
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]      
Term of share options exercised   2 years  
2020 Share Options [member]      
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]      
Term of share options exercised 2 years    
Maximum [member] | Employees [member]      
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]      
Expected option life (years) 10    
Maximum [member] | Directors [member]      
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]      
Expected option life (years) 10    
Maximum [member] | Consultants [member]      
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]      
Expected option life (years) 10