Income Taxes - Components of net loss before tax provision from income taxes (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Taxes
United Kingdom	$ (69,546)	$ (52,521)	$ (31,906)
United States	1,064	787	1,044
Net loss before income tax provision	$ (68,482)	$ (51,734)	$ (30,862)

Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

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+ Details