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Ordinary Shares - Narrative (Details)
$ in Millions
1 Months Ended 12 Months Ended 24 Months Ended
Sep. 22, 2020
£ / shares
shares
Aug. 28, 2019
USD ($)
Aug. 17, 2018
Aug. 17, 2017
Jul. 31, 2020
shares
Oct. 31, 2019
shares
Aug. 31, 2017
shares
Dec. 31, 2020
USD ($)
shares
Dec. 31, 2019
£ / shares
shares
Aug. 17, 2020
installment
Dec. 31, 2020
£ / shares
Aug. 19, 2020
£ / shares
Aug. 18, 2020
£ / shares
Class of Stock [Line Items]                          
Consideration received on transaction | $   $ 25.0                      
Initial Public Offering                          
Class of Stock [Line Items]                          
Consideration received on transaction | $               $ 132.8          
Ordinary Shares                          
Class of Stock [Line Items]                          
Common stock, issued for services (in shares)             10,551,166            
Common stock, par value (in dollars per share) | £ / shares £ 0.008               £ 0.008   £ 0.008 £ 0.001 £ 1.00
Number of monthly installments | installment                   24      
Forfeiture of ordinary shares (in shares)         63,972                
Ordinary Shares | Nonemployee                          
Class of Stock [Line Items]                          
Issuance of shares (in shares)           102,214              
Award vesting period           3 years              
Ordinary Shares | Employee                          
Class of Stock [Line Items]                          
Issuance of shares (in shares)           99,049              
Award vesting period           4 years              
Ordinary Shares | Common Stock                          
Class of Stock [Line Items]                          
Issuance of shares (in shares)               8,625,000 201,263        
Forfeiture of ordinary shares (in shares)               63,972          
Convertible preferred shares issued (in shares)               16,419,172          
Ordinary Shares | Share-based Payment Arrangement, Tranche One                          
Class of Stock [Line Items]                          
Award vesting percentage       25.00%                  
Ordinary Shares | Share-based Payment Arrangement, Tranche Two                          
Class of Stock [Line Items]                          
Award vesting percentage     25.00%                    
Ordinary Shares | Share-based Payment Arrangement, Tranche Three                          
Class of Stock [Line Items]                          
Award vesting percentage                   50.00%      
American Depositary Shares | Initial Public Offering                          
Class of Stock [Line Items]                          
Sale of stock, shares issued in transaction (in shares) 8,625,000                        
Sale of stock, price per share (in dollars per share) | £ / shares £ 17.00