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Convertible preferred shares (Details)
$ / shares in Units, $ in Millions
1 Months Ended
May 28, 2019
shares
Mar. 28, 2019
$ / shares
shares
Jan. 03, 2019
USD ($)
shares
Dec. 20, 2018
USD ($)
shares
Oct. 27, 2017
USD ($)
shares
May 26, 2017
USD ($)
shares
May 31, 2019
Jan. 03, 2019
£ / shares
Dec. 20, 2018
£ / shares
Oct. 27, 2017
£ / shares
May 26, 2017
£ / shares
Convertible preferred shares                      
Number of bonus shares for each ordinary share             1.429        
IPO                      
Convertible preferred shares                      
Number of shares issued (in shares)   4,333,333                  
Share issue price | $ / shares   $ 14.00                  
Ordinary Shares | IPO                      
Convertible preferred shares                      
Conversion of convertible preferred shares to ordinary shares (in shares) 11,647,529                    
Conversion ratio 0.6997                    
Series B1 Convertible Preferred Shares                      
Convertible preferred shares                      
Number of shares issued (in shares)         384,615 3,562,583          
Share issue price | £ / shares                   £ 13.00 £ 11.2278
Gross proceeds from issues of shares | $         $ 6.6 $ 51.9          
Numbers of warrants issued       194,911 743,287            
Number of warrants surrendered       194,911              
Series B2 Convertible Preferred Shares                      
Convertible preferred shares                      
Number of shares issued (in shares)     80,385 1,323,248              
Share issue price | £ / shares               £ 15.55 £ 15.55    
Gross proceeds from issues of shares | $     $ 1.6 $ 26.1