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Accounting policies - Hyperinflation in Argentina - Additional Information (Detail) - Argentina [Member]
£ in Millions
12 Months Ended
Dec. 31, 2018
GBP (£)
Disclosure of Hyperinflationary Reporting [line items]  
Increase in goodwill due to hyperinflation £ 105.8
Increase in other intangibles due to hyperinflation £ 19.5