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Significant Accounting Policies - Additional Information (Detail)
£ in Thousands
 12 Months Ended
Dec. 31, 2020
GBP (£)
Segment
Dec. 31, 2019
GBP (£)
Dec. 31, 2018
GBP (£)
Dec. 31, 2017
GBP (£)
Disclosure Of Summary Of Significant Accounting Policies [Line Items]        
Cash and cash equivalents £ 87,356 £ 51,962 £ 76,972 £ 86,703
Accrued clinical trial expenses 3,500 1,700    
Prepayments for clinical trial expenses 2,200 2,600    
Accrued contracted manufacturing expense £ 100 £ 200    
Number of operating segment | Segment 1      
Cash at banks maturity period Less than three months      
Right of Use Assets [member]        
Disclosure Of Summary Of Significant Accounting Policies [Line Items]        
Depreciation method Straight-line      
Maximum [member]        
Disclosure Of Summary Of Significant Accounting Policies [Line Items]        
Income tax credit, cash rebate percentage 33.35%      
Maximum [member] | Right of Use Assets [member]        
Disclosure Of Summary Of Significant Accounting Policies [Line Items]        
Depreciation rate 5 years      
Bottom Of Range [Member] | Right of Use Assets [member]        
Disclosure Of Summary Of Significant Accounting Policies [Line Items]        
Depreciation rate 2 years      
Patents [member]        
Disclosure Of Summary Of Significant Accounting Policies [Line Items]        
Amortization rate 20 years