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Accounting policies - Additional Information (Detail) - GBP (£)
£ / shares in Units, £ in Millions
 6 Months Ended 12 Months Ended
Jan. 01, 2019
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Disclosure of initial application of standards or interpretations [line items]        
Right of Use Assets   £ 1,825.2    
Lease Liability   2,345.7    
Retained earnings   9,652.4 £ 10,018.8 £ 9,541.4
Deferred tax asset   £ 155.4 £ 169.9 £ 153.0
Diluted earning per share   £ 0.248 £ 0.534 £ 0.843
Weighted average discount rate 5.40%      
Impact on Initial Application of IFRS16 [member]        
Disclosure of initial application of standards or interpretations [line items]        
Right of Use Assets £ 1,895.1      
Lease Liability 2,326.2      
Reduction in other creditors 233.5      
Reduction in property provisions 68.7      
Retained earnings (128.9)      
Deferred tax asset £ 27.8      
Increase to Operating profit   £ 33.7    
Diluted earning per share   £ (0.008)