Income taxes - Components of net loss before tax provision from income taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income taxes
United Kingdom	$ (182,980)	$ (116,275)	$ (69,546)
United States	3,773	2,034	1,064
Net loss before income tax provision	$ (179,207)	$ (114,241)	$ (68,482)