OTHER RESERVES - Schedule of Merger Reserve (Details) - Hedges of net investment in foreign operations - GBP (£) £ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Merger reserve
At 1 January	£ 7,763	£ 7,763	£ 7,766
Redemption of preference shares (note 39)	(614)	0	(3)
At 31 December	£ 7,149	£ 7,763	£ 7,763

The cash outflow to acquire equity or debt instruments of other entities (other than payments for those instruments considered to be cash equivalents or those held for dealing or trading purposes), classified as investing activities.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Name IAS
-Number 7
-IssueDate 2021-01-01
-Paragraph 16
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=7&code=ifrs-tx-2021-en-r&anchor=para_16_c&doctype=Standard
-URIDate 2021-03-24

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