Warrant liability - Warrants information (Details) $ in Thousands			1 Months Ended	12 Months Ended
	May 28, 2019 shares	Mar. 07, 2019	May 31, 2017 £ / shares shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 £ / shares shares
Warrants information
Number of shares issued for warrant exercise						171,480
Remaining warrants exercisable						80,000
Number of shares into which remaining warrants is exercisable						114,320
Change in fair value of warrant liability recorded as other expense | $				$ 5,381	$ 665
Other expenses
Warrants information
Change in fair value of warrant liability recorded as other expense | $				$ 100
Series A Preferred Shares
Warrants information
Numbers of warrants issued			200,000
Exercise price of warrants | £ / shares			£ 0.01
Number of warrants exercised						120,000
Series B1 Preferred Shares
Warrants information
Numbers of warrants issued						743,287
Exercise price of warrants | £ / shares						£ 0.01
Number of shares issued for warrant exercise	531,077
Shares issued (in shares)						3,947,198
Percentage of warrants exercisable in conjunction with the IPO		50.00%
Percentage of warrants to expire in conjunction with the IPO		50.00%