Convertible preferred shares (Details) $ / shares in Units, $ in Millions				1 Months Ended	12 Months Ended
	May 28, 2019 $ / shares shares	Jan. 03, 2019 USD ($) shares	Dec. 20, 2018 USD ($) shares	May 31, 2019	Dec. 31, 2020 shares	Dec. 31, 2019 shares	Jan. 03, 2019 £ / shares	Dec. 20, 2018 £ / shares
Convertible preferred shares
Number of bonus shares for each ordinary share				1.429
IPO
Convertible preferred shares
Issuance of ADSs in initial public offering, net of underwriting discounts, commissions and offering expenses (in shares)	4,333,333
Share issue price | $ / shares	$ 14.00
Ordinary Shares
Convertible preferred shares
Issuance of ADSs in initial public offering, net of underwriting discounts, commissions and offering expenses (in shares)					2,928,813	4,637,666
Conversion of convertible preferred shares to ordinary shares (in shares)						11,647,529
Ordinary Shares | IPO
Convertible preferred shares
Conversion of convertible preferred shares to ordinary shares (in shares)	11,647,529
Conversion ratio	1.429
Series B2 Convertible Preferred Shares
Convertible preferred shares
Issuance of ADSs in initial public offering, net of underwriting discounts, commissions and offering expenses (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