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Revenue - Additional Information (Detail) - Ultragenyx collaboration agreement [Member]
£ in Millions, $ in Millions
 6 Months Ended
Jun. 30, 2022
GBP (£)
Jun. 30, 2021
GBP (£)
Jun. 30, 2021
USD ($)
Dec. 31, 2021
GBP (£)
Disclosure Of Revenue From Contracts With Customers [Line Items]        
Upfront Payment Received   £ 36.5 $ 50.0  
Milestone payment to ultragenyx collaboration agreement deductions costs £ 2.4      
Milestone payment to ultragenyx collaboraation agreement deductions costs recognised 0.4 0.9    
Milestone payment to ultragenyx collaboration agreement deductions costs yet to be recogised £ 0.9 1.5   £ 1.3
Novartis [member]        
Disclosure Of Revenue From Contracts With Customers [Line Items]        
Milestone payment for regulatory and commercial achievements   £ 7.2 $ 10.0