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Nature of the business and basis of presentation - Share activity (Details) $ / shares in Units, $ in Thousands			12 Months Ended
	May 28, 2024 USD ($)	Jul. 17, 2023 USD ($) shares	Dec. 31, 2025 £ / shares shares	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($) shares	Dec. 31, 2024 £ / shares shares	May 23, 2024 £ / shares shares	May 23, 2024 $ / shares shares	Jul. 17, 2023 £ / shares	Jul. 17, 2023 $ / shares
Significant agreements.
Ordinary shares, shares issued \| shares			69,367,896			69,061,418
Ordinary shares, nominal value \| £ / shares			£ 0.01			£ 0.01
Aggregate purchase price				$ 544,127	$ 249,345
Commissions and offering expenses				11,400	16,000
Proceeds from the issuance of ADSs and non-voting ordinary shares, net of issuance costs				$ 544,127	249,345
Investors Purchase Agreement
Significant agreements.
Purchase price per share \| $ / shares								$ 21.42
Aggregate purchase price	$ 555,500
Commissions and offering expenses	11,400
Proceeds from the issuance of ADSs and non-voting ordinary shares, net of issuance costs	$ 544,100
Investors Purchase Agreement \| American Depositary Shares
Significant agreements.
Ordinary shares, shares issued \| shares							6,764,705	6,764,705
Ordinary shares, nominal value \| £ / shares							£ 0.01
Investors Purchase Agreement \| Non-voting ordinary shares
Significant agreements.
Ordinary shares, shares issued \| shares							19,169,001	19,169,001
Ordinary shares, nominal value \| £ / shares							£ 0.01
Underwritten Public Offering
Significant agreements.
Aggregate purchase price		$ 230,000
Commissions and offering expenses		14,900
Proceeds from the issuance of ADSs and non-voting ordinary shares, net of issuance costs		$ 215,100
Underwritten Public Offering \| American Depositary Shares
Significant agreements.
Ordinary shares, nominal value \| £ / shares									£ 0.01
Purchase price per share \| $ / shares										$ 21.25
Shares issued (in shares) \| shares		6,117,648
Underwritten Public Offering \| Non-voting ordinary shares
Significant agreements.
Ordinary shares, nominal value \| £ / shares									£ 0.01
Shares issued (in shares) \| shares		4,705,882
Underwriters \| American Depositary Shares
Significant agreements.
Shares issued (in shares) \| shares		1,411,764
At the market offering
Significant agreements.
Aggregate purchase price					35,300
Commissions and offering expenses					1,100
Proceeds from the issuance of ADSs and non-voting ordinary shares, net of issuance costs					$ 34,200
Shares issued (in shares) \| shares			0
At the market offering \| American Depositary Shares
Significant agreements.
Shares issued (in shares) \| shares					1,561,176