Convertible preferred shares (Details) $ / shares in Units, $ in Millions						1 Months Ended	3 Months Ended	12 Months Ended
	May 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	Jun. 30, 2019 shares	Dec. 31, 2019 shares	Jan. 03, 2019 £ / shares	Dec. 20, 2018 £ / shares	Oct. 27, 2017 £ / shares	May 26, 2017 £ / shares
Convertible preferred shares
Stock split ratio						1.429
IPO
Convertible preferred shares
Number of shares issued (in shares)	4,333,333
Share issue price | $ / shares	$ 14.00
Ordinary Shares
Convertible preferred shares
Number of shares issued (in shares)								4,637,666
Conversion of convertible preferred shares to ordinary shares (in shares)							11,647,529	11,647,529
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	£ 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