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Cash and Cash Equivalents (Excluding Overdrafts) - Summary of Cash and Cash Equivalents for Purpose of Cash Flow Statement (Detail) - GBP (£)
£ in Millions
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Cash and cash equivalents [line items]        
Cash and cash equivalents £ 568 £ 518    
Bank overdrafts - continuing operations (43) (15)    
Total cash and cash equivalents £ 525 630 £ 1,424 £ 1,671
Asset held for sale [member]        
Cash and cash equivalents [line items]        
Cash and cash equivalents   £ 127