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RETIREMENT BENEFIT PLANS - Summary of Retirement Benefit Plans (Details) - GBP (£)
£ in Millions
Jun. 30, 2026
Dec. 31, 2025
Net assets (liabilities) [abstract]    
Total net assets £ 539 £ 502
Unfunded pension and post-retirement medical benefits    
Net assets (liabilities) [abstract]    
Surplus (deficit) in plan (21) (22)
Surplus | Funded defined benefit pension scheme - surplus    
Net assets (liabilities) [abstract]    
Surplus (deficit) in plan £ 560 £ 524