Earnings Per Share - Summary of Earnings Per Share (Detail) - GBP (£) £ / shares in Units, £ in Millions, shares in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Earnings per share [abstract]
Profit for the year	£ 160	£ 310	£ 266
Non-controlling interest	(1)	0	(2)
Earnings attributable to equity shareholders	£ 159	£ 310	£ 264
Weighted average number of shares (millions)	754.1	755.4	777.0
Effect of dilutive share options (millions)	5.0	0.0	0.5
Weighted average number of shares (millions) for diluted earnings	759.1	755.4	777.5
Earnings per share (in pence per share)
Basic	£ 0.211	£ 0.410	£ 0.340
Diluted	£ 0.209	£ 0.410	£ 0.340

The weighted average number of ordinary shares outstanding plus the weighted average number of ordinary shares that would be issued on the conversion of all the dilutive potential ordinary shares into ordinary shares. [Refer: Ordinary shares [member]; Weighted average [member]]

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The amount of profit (loss) attributable to ordinary equity holders of the parent entity (the numerator) divided by the weighted average number of ordinary shares outstanding during the period (the denominator).

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The amount of profit (loss) attributable to ordinary equity holders of the parent entity (the numerator), divided by the weighted average number of ordinary shares outstanding during the period (the denominator), both adjusted for the effects of all dilutive potential ordinary shares. [Refer: Ordinary shares [member]; Weighted average [member]]

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The profit (loss) from continuing and discontinued operations attributable to non-controlling interests. [Refer: Profit (loss); Non-controlling interests]

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