Retirement benefit schemes - additional information (Details) - GBP (£) £ in Millions		6 Months Ended
Apr. 23, 2026	Jun. 30, 2026	Dec. 31, 2025	Jun. 30, 2025
Disclosure of defined benefit plans [abstract]
Increase (decrease) in net defined benefit liability (asset) resulting from other changes	£ 1,766
Frequency of valuations		3 years
Net asset and (liability) for pension and healthcare schemes		£ 132	£ 79	£ 72

The increase (decrease) in the net defined benefit liability (asset) resulting from changes that the entity does not separately disclose in the same statement or note. [Refer: Net defined benefit liability (asset)]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2025-01-01
-Paragraph 141
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=19&code=ifrs-tx-2025-en-r&anchor=para_141&doctype=Standard
-URIDate 2025-03-27

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