Revenue recognition and operating segments - Analysis of revenue by type (Details) - GBP (£) £ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Disclosure of disaggregation of revenue from contracts with customers
Revenue	£ 5,436	£ 5,375	£ 3,714
Contract service revenue
Disclosure of disaggregation of revenue from contracts with customers
Revenue	3,876	3,838	2,610
Job work
Disclosure of disaggregation of revenue from contracts with customers
Revenue	1,160	1,104	724
Sales of goods
Disclosure of disaggregation of revenue from contracts with customers
Revenue	£ 400	£ 433	£ 380

The amount of revenue from contracts with customers. A customer is a party that has contracted with an entity to obtain goods or services that are an output of the entity’s ordinary activities in exchange for consideration.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 15
-IssueDate 2024-01-01
-Paragraph 114
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=15&code=ifrs-tx-2024-en-r&anchor=para_114&doctype=Standard
-URIDate 2024-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 15
-IssueDate 2024-01-01
-Paragraph 113
-Subparagraph a
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=15&code=ifrs-tx-2024-en-r&anchor=para_113_a&doctype=Standard
-URIDate 2024-03-27

Name:	ifrs-full_RevenueFromContractsWithCustomers
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration