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Capital and reserves (Details)
 3 Months Ended 12 Months Ended
Dec. 21, 2020
GBP (£)
shares
Mar. 02, 2020
GBP (£)
shares
Aug. 13, 2019
USD ($)
shares
Mar. 31, 2020
USD ($)
shares
Dec. 31, 2020
GBP (£)
£ / shares
shares
Dec. 31, 2020
USD ($)
shares
Dec. 31, 2019
GBP (£)
£ / shares
shares
Dec. 31, 2019
USD ($)
shares
Dec. 31, 2018
GBP (£)
Mar. 02, 2020
USD ($)
shares
Dec. 31, 2019
USD ($)
Issued share [Abstract]                      
Number of ordinary shares issued (in shares)         34,260 34,260          
Cash consideration | £         £ 3            
Ordinary shares issued (in pounds per share) | £ / shares         £ 0.01            
Nominal value of ordinary shares issued | £         £ 1,000   £ 0        
Totaling proceeds from shares issued | £         £ 83,218,000   59,874,000   £ 0    
Nominal value of repurchased and cancelled shares (in pounds per share) | £ / shares         £ 3            
Nature and purpose of reserves [Abstract]                      
Dividends paid | $           $ 0   $ 0      
Dividends declared | $           $ 0   $ 0      
Borrowings outstanding | £         £ 36,654,000   £ 0        
Gates foundation convertible loan [Member]                      
Issued share [Abstract]                      
Number of shares issued anti dilution (in shares)       43,032              
Nature and purpose of reserves [Abstract]                      
Borrowings outstanding | $       $ 25,000,000           $ 25,000,000 $ 25,500,000
Stock converted (in shares)       203,697              
Growth Shares [Member]                      
Issued share [Abstract]                      
Number of ordinary shares issued (in shares)             60,240 60,240      
Ordinary shares issued (in pounds per share) | £ / shares             £ 0.01        
Nominal value of ordinary shares issued | £             £ 6        
Repurchased and cancelled nominal shares (in shares)             29,575 29,575      
Nominal value of repurchased and cancelled shares (in pounds per share) | £ / shares             £ 3        
Nature and purpose of reserves [Abstract]                      
Treasury reserve | £         2,800,000   £ 4,420,000   £ 10,190,000    
Series B Shares [Member]                      
Issued share [Abstract]                      
Number of ordinary shares issued (in shares)   527,147                  
Totaling proceeds from shares issued | $     $ 72,250,000                
Number of shares issued (in shares)     621,556                
Series B Shares [Member] | New and Existing Investors [Member]                      
Issued share [Abstract]                      
Number of ordinary shares issued (in shares)   280,418                  
Cash consideration | £   £ 27,230,000                  
Series B Shares [Member] | Gates foundation convertible loan [Member]                      
Nature and purpose of reserves [Abstract]                      
Stock converted (in shares)       203,697           203,697  
Derivative liability, foreign exchange call option | £         £ 3,184,000            
Series C Shares [Member]                      
Issued share [Abstract]                      
Number of ordinary shares issued (in shares) 823,719                    
Cash consideration | £ £ 55,812,000                    
Ordinary Shares [Member]                      
Issued share [Abstract]                      
Number of ordinary shares issued (in shares)         163,870 163,870 45,581 45,581      
Cash consideration | £         £ 73,000   £ 27,000        
Ordinary shares issued (in pounds per share) | £ / shares         £ 0.01   £ 0.01        
Nominal value of ordinary shares issued | £         £ 16   £ 5        
Number of shares issued anti dilution (in shares)         161,094 161,094 37,007 37,007