Commitments and Contingencies	12 Months Ended
Dec. 31, 2025
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Contractual obligations In July 2022, the Company renegotiated a master services agreement with Adaptive Biotechnologies Corporation (“Adaptive”), under which Adaptive's assay is used to analyze patient samples from r/r B-ALL patients. During the year ended December 31, 2023, the Company recognized all contractual milestones relating to this contract. Under the then-current agreement, the Company would be obligated to make specified payments to Adaptive upon the achievement and receipt of certain regulatory approvals and achievement of commercial milestones in connection with the Company’s use of the Adaptive assay. In previous periods, the Company has entered into agreements with certain advisory firms. The Company is obligated to make specified payments upon the achievement of certain strategic transactions involving the Company. During the year ended December 31, 2025, the Company paid a fee under these agreements. The Company has estimated the probability of the Company achieving each potential milestone in relation to the agreements with UCLB and its agreements with certain advisory firms in accordance with ASC 450. The Company considers regulatory approval, commercial milestones and execution of collaboration agreements probable when actually achieved. Furthermore, the Company recognizes expenses for clinical milestones when their achievement is deemed probable. The Company concluded that as of December 31, 2025, there were no other milestones for which the likelihood of achievement was currently probable. Capital Commitments As of December 31, 2025, the Company’s unconditional purchase obligations for capital expenditure totaled $1.8 million and included signed orders for capital equipment and capital expenditure for construction and related expenditure relating primarily to its properties in the United Kingdom. The Company expects to incur the full amount of these obligations within one year. Master Supply Commitments In March 2018, we entered into a strategic, long-term supply agreement with Miltenyi Biotec GmbH (“Miltenyi”), for the supply of Miltenyi’s CliniMACS Prodigy instruments, reagents and disposables for the manufacture of Company's programmed T cell therapies, including for pre-clinical, clinical and commercial production of AUCATZYL, as well as the provision of related support services. In September 2023, the Company amended the supply agreement. The supply agreement sets forth procedures to ensure continuity of supply to the Company of Miltenyi’s products, both during the clinical phase and any future commercial phase of the Company's product candidates. After the initial ten-year term of the agreement, the Company has two separate options to renew the agreement, each for an additional five-year term. The Company has a three-month firm commitment to purchase to reagents and disposables pursuant to the agreement. The supply agreement contains customary termination provisions, allowing for termination by a party upon the other party’s uncured material breach, upon the other party’s bankruptcy or insolvency or upon the other party being subject to an extended period of force majeure events. The Company may also terminate the supply agreement upon advance written notice, if the Company decide to suspend or discontinue the development or commercialization of the Company's product candidates. The supply agreement is governed under the laws of Germany. As of December 31, 2025, the Company’s unconditional purchase obligations for reagents and disposables totaled $1.7 million, which the Company expects to incur within one year. Distribution Commitments The Company entered into an Exclusive Distribution Agreement, effective as of April 25, 2024 (the “Effective Date”), with Cardinal Health 105, LLC (“Cardinal Health”). Pursuant to, and subject to the terms and conditions of, the Exclusive Distribution Agreement, the Company engaged Cardinal Health as its exclusive third-party logistics distribution agent for sales of AUCATZYL in the US. The Exclusive Distribution Agreement runs for an initial term of three years following commercial launch and automatically renews for additional terms of one year each, unless either party elects not to renew. Under the terms of the Exclusive Distribution Agreement, the Company must pay to Cardinal Health a one-time start-up fee, and a monthly account management fee upon the Company's commercial launch of AUCATZYL, and other fees for various services, including post-launch program implementation, information systems, warehouse operations and financial services. All fees related to this Exclusive Distribution Agreement are immaterial. BioNTech Agreements BioNTech License and Option Agreement - Product Options gain contingency As the Product Options within the BioNTech License and Option Agreement were an embedded feature within a freestanding financial instrument, the Company assessed if the Product Options should be accounted for as a derivative under ASC 815. However, the Company determined the Product Options met the scope exception for derivative accounting under ASC 815 and therefore should be accounted for a gain contingency under the scope of ASC 450. As of December 31, 2025, Product Options were not exercised and therefore no amounts were recognized. Refer to Note 12, “Liabilities Related to Future Royalties and Milestone, Net” for further details about the BioNTech's Obe-cel Product Revenue Interest. Legal Proceedings From time to time, the Company may be a party to litigation or subject to claims incident to the ordinary course of business. Regardless of the outcome, litigation can have an adverse impact on the Company because of defense and settlement costs, diversion of management resources and other factors. The Company was not a party to any litigation and did not have contingency reserves established for any liabilities as of December 31, 2025 and 2024. SME R&D tax credit In the accounting period to December 2023, based on the relevant tax legislation, the Company had met the conditions of the R&D intensive scheme, and therefore submitted its corporate tax return on this basis. This is subject to agreement by the U.K. tax authority who, based on their non-statutory guidance, considers the basis for calculating whether a company meets the intensive criteria includes expenditure which is in conflict with the tax legislation. The position is uncertain and the legislation is currently untested in the U.K. courts. If the Company's claim is unsuccessful, normal SME relief will be available and there will be a material reduction in the value of the tax credit obtained (18.6% as opposed to 26.97% net benefit). Should the uncertainty be resolved in the Company’s favor, this would result in a gain and accounted for a gain contingency under the scope of ASC 450. Indemnification Agreements In the normal course of business, the Company enters into contracts and agreements that contain a variety of representations and warranties and provide for general indemnification. The Company’s exposure under these agreements is unknown because they involve claims that may be made against the Company in the future. To date, the Company has not paid any claims or been required to defend any action related to its indemnification obligations. However, the Company may record charges in the future as a result of these indemnification obligations. In accordance with the indemnification agreements entered into with relevant individuals in accordance with the Company’s Articles of Association, the Company has indemnification obligations to its directors, officers and members of senior management for certain events or occurrences, subject to certain limits, while they are serving at the Company’s request in such capacity. There have been no claims to date under these indemnification agreements, and the Company has director and officer insurance that may enable it to recover a portion of any amounts paid for future potential claims.

In July 2022, the Company renegotiated a master services agreement with Adaptive Biotechnologies Corporation (“Adaptive”), under which Adaptive's assay is used to analyze patient samples from r/r B-ALL patients. During the year ended December 31, 2023, the Company recognized all contractual milestones relating to this contract. Under the then-current agreement, the Company would be obligated to make specified payments to Adaptive upon the achievement and receipt of certain regulatory approvals and achievement of commercial milestones in connection with the Company’s use of the Adaptive assay.

The Company has estimated the probability of the Company achieving each potential milestone in relation to the agreements with UCLB and its agreements with certain advisory firms in accordance with ASC 450. The Company considers regulatory approval, commercial milestones and execution of collaboration agreements probable when actually achieved. Furthermore, the Company recognizes expenses for clinical milestones when their achievement is deemed probable. The Company concluded that as of December 31, 2025, there were no other milestones for which the likelihood of achievement was currently probable.

As of December 31, 2025, the Company’s unconditional purchase obligations for capital expenditure totaled $1.8 million and included signed orders for capital equipment and capital expenditure for construction and related expenditure relating primarily to its properties in the United Kingdom. The Company expects to incur the full amount of these obligations within one year.

In March 2018, we entered into a strategic, long-term supply agreement with Miltenyi Biotec GmbH (“Miltenyi”), for the supply of Miltenyi’s CliniMACS Prodigy instruments, reagents and disposables for the manufacture of Company's programmed T cell therapies, including for pre-clinical, clinical and commercial production of AUCATZYL, as well as the provision of related support services. In September 2023, the Company amended the supply agreement. The supply agreement sets forth procedures to ensure continuity of supply to the Company of Miltenyi’s products, both during the clinical phase and any future commercial phase of the Company's product candidates. After the initial ten-year term of the agreement, the Company has two separate options to renew the agreement, each for an additional five-year term. The Company has a three-month firm commitment to purchase to reagents and disposables pursuant to the agreement. The supply agreement contains customary termination provisions, allowing for termination by a party upon the other party’s uncured material breach, upon the other party’s bankruptcy or insolvency or upon the other party being subject to an extended period of force majeure events. The Company may also terminate the supply agreement upon advance written notice, if the Company decide to suspend or discontinue the development or commercialization of the Company's product candidates. The supply agreement is governed under the laws of Germany. As of December 31, 2025, the Company’s unconditional purchase obligations for reagents and disposables totaled $1.7 million, which the Company expects to incur within one year.

The Company entered into an Exclusive Distribution Agreement, effective as of April 25, 2024 (the “Effective Date”), with Cardinal Health 105, LLC (“Cardinal Health”). Pursuant to, and subject to the terms and conditions of, the Exclusive Distribution Agreement, the Company engaged Cardinal Health as its exclusive third-party logistics distribution agent for sales of AUCATZYL in the US. The Exclusive Distribution Agreement runs for an initial term of three years following commercial launch and automatically renews for additional terms of one year each, unless either party elects not to renew. Under the terms of the Exclusive Distribution Agreement, the Company must pay to Cardinal Health a one-time start-up fee, and a monthly account management fee upon the Company's commercial launch of AUCATZYL, and other fees for various services, including post-launch program implementation, information systems, warehouse operations and financial services. All fees related to this Exclusive Distribution Agreement are immaterial.

As the Product Options within the BioNTech License and Option Agreement were an embedded feature within a freestanding financial instrument, the Company assessed if the Product Options should be accounted for as a derivative under ASC 815. However, the Company determined the Product Options met the scope exception for derivative accounting under ASC 815 and therefore should be accounted for a gain contingency under the scope of ASC 450. As of December 31, 2025, Product Options were not exercised and therefore no amounts were recognized.

From time to time, the Company may be a party to litigation or subject to claims incident to the ordinary course of business. Regardless of the outcome, litigation can have an adverse impact on the Company because of defense and settlement costs, diversion of management resources and other factors. The Company was not a party to any litigation and did not have contingency reserves established for any liabilities as of December 31, 2025 and 2024.

In the accounting period to December 2023, based on the relevant tax legislation, the Company had met the conditions of the R&D intensive scheme, and therefore submitted its corporate tax return on this basis. This is subject to agreement by the U.K. tax authority who, based on their non-statutory guidance, considers the basis for calculating whether a company meets the intensive criteria includes expenditure which is in conflict with the tax legislation. The position is uncertain and the legislation is currently untested in the U.K. courts. If the Company's claim is unsuccessful, normal SME relief will be available and there will be a material reduction in the value of the tax credit obtained (18.6% as opposed to 26.97% net benefit). Should the uncertainty be resolved in the Company’s favor, this would result in a gain and accounted for a gain contingency under the scope of ASC 450.

In the normal course of business, the Company enters into contracts and agreements that contain a variety of representations and warranties and provide for general indemnification. The Company’s exposure under these agreements is unknown because they involve claims that may be made against the Company in the future. To date, the Company has not paid any claims or been required to defend any action related to its indemnification obligations. However, the Company may record charges in the future as a result of these indemnification obligations.

In accordance with the indemnification agreements entered into with relevant individuals in accordance with the Company’s Articles of Association, the Company has indemnification obligations to its directors, officers and members of senior management for certain events or occurrences, subject to certain limits, while they are serving at the Company’s request in such capacity. There have been no claims to date under these indemnification agreements, and the Company has director and officer insurance that may enable it to recover a portion of any amounts paid for future potential claims.