Warrant liability - Warrants information (Details) $ in Thousands						3 Months Ended		6 Months Ended
	May 28, 2019 shares	Mar. 07, 2019	Dec. 20, 2018 shares	Oct. 27, 2017 £ / shares shares	May 26, 2017 £ / shares shares	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($) shares	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($) shares
Warrants information
Number of shares issued for warrant exercise	171,480
Remaining warrants exercisable	80,000						80,000		80,000
Number of shares into which remaining warrants is exercisable	114,320
Change in fair value of preferred stock warrant liability | $						$ 0	$ 2,200	$ 0	$ 5,381
Other expenses
Warrants information
Change in fair value of preferred stock warrant liability | $									$ 100
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%