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Share Capital and Share Premium - Summary of Share Capital and Share Premium (Parenthetical) (Detail) - £ / shares
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Ordinary shares [member]      
Disclosure Of Classes Of Share Capital [Line Items]      
Par value per share £ 0.04 £ 0.04 £ 0.04