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FINANCIAL INSTRUMENTS (Details) - Schedule of Valuation Adjustments - GBP (£)
£ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
FINANCIAL INSTRUMENTS (Details) - Schedule of Valuation Adjustments [Line Items]      
Valuation adjustment £ 474 £ 423 £ 562
Credit Valuation Adjustment      
FINANCIAL INSTRUMENTS (Details) - Schedule of Valuation Adjustments [Line Items]      
Valuation adjustment 358 278  
Debit Valuation Adjustment      
FINANCIAL INSTRUMENTS (Details) - Schedule of Valuation Adjustments [Line Items]      
Valuation adjustment (35) (27)  
Funding Valuation Adjustment      
FINANCIAL INSTRUMENTS (Details) - Schedule of Valuation Adjustments [Line Items]      
Valuation adjustment £ 151 £ 172