Share capital and shares held in treasury - Summary of called up share capital issued and fully paid (Details) - GBP (£) £ in Millions	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024
Share capital and shares held in treasury
Shares issued at start of year	1,880,844,719	1,906,907,605
Issue of ordinary shares	2,215,646	2,937,114
Cancellation of ordinary shares	(55,000,000)	(29,000,000)
Shares issued at end of year	1,828,060,365	1,880,844,719
At start of year	£ 272	£ 275
Cancellation of ordinary share	(8)	(3)
At end of year	£ 264	£ 272

The number of shares issued by the entity, for which full payment has been received.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Note Effective 2027-01-01
-Name IFRS
-Number 19
-IssueDate 2025-01-01
-Paragraph B10
-Subparagraph a
-Clause ii
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=19&code=ifrs-tx-2025-en-r&anchor=para_B10_a_ii&doctype=Appendix&subtype=B
-URIDate 2025-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Note Effective 2027-01-01
-Name IFRS
-Number 19
-IssueDate 2025-01-01
-Paragraph 160
-Subparagraph a
-Clause ii
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=19&code=ifrs-tx-2025-en-r&anchor=para_160_a_ii&doctype=Standard
-URIDate 2025-03-27

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Note Expiry date 2027-01-01
-Name IAS
-Number 1
-IssueDate 2025-01-01
-Paragraph 79
-Subparagraph a
-Clause ii
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2025-en-b&anchor=para_79_a_ii&doctype=Standard
-URIDate 2025-03-27

Name:	ifrs-full_NumberOfSharesIssuedAndFullyPaid
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant