Warrant liability - Warrants information (Details) $ in Thousands						9 Months Ended	12 Months Ended
	May 28, 2019 shares	Dec. 20, 2018 shares	Oct. 27, 2017 £ / shares shares	May 26, 2017 £ / shares shares	Mar. 07, 2017	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 £ / shares shares
Warrants information
Exercise price of warrants | £ / shares										£ 0.01
Number of shares issued for warrant exercise	171,480
Remaining warrants exercisable	80,000									65,000
Number of shares into which remaining warrants is exercisable	114,320									92,885
Change in fair value of preferred stock warrant liability | $						$ 5,381	$ 5,381	$ 665	$ 119
Other expenses
Warrants information
Change in fair value of preferred stock warrant liability | $							$ 100
Expected dividend yield
Warrants information
Fair value measurement of the warrant liability										0
Series A Preferred Shares
Warrants information
Numbers of warrants issued				200,000
Exercise price of warrants | £ / shares				£ 0.01
Warrants exercisable term				10 years
Warrants expiration term following an IPO or exit				12 months
Number of warrants exercised	120,000
Series B1 Preferred Shares
Warrants information
Numbers of warrants issued		194,911	115,384	627,903
Exercise price of warrants | £ / shares			£ 0.01	£ 0.01
Number of shares issued for warrant exercise	531,077
Number of shares issued (in shares)			384,615	3,562,583
Share issue price | £ / shares				£ 11.2278
Number of warrants surrendered		194,911
Percentage of warrants exercisable in conjunction with the IPO					50.00%
Percentage of warrants to expire in conjunction with the IPO					50.00%