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DERIVATIVE FINANCIAL INSTRUMENTS - Narrative (Details) - GBP (£)
£ in Millions
Dec. 31, 2021
Dec. 31, 2020
DERIVATIVE FINANCIAL INSTRUMENTS (Tables) [Line Items]    
Hedged liability fair value hedge £ (1,071) £ 761
Fair value hedges | Fixed rate mortgages    
DERIVATIVE FINANCIAL INSTRUMENTS (Tables) [Line Items]    
Hedged liability fair value hedge (278)  
Fair value hedges | Fixed rate issuance    
DERIVATIVE FINANCIAL INSTRUMENTS (Tables) [Line Items]    
Hedged liability fair value hedge £ (793)