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Share Capital and Share Premium - Summary of Movement in the Share Capital (Detail)
£ in Thousands
12 Months Ended
Dec. 31, 2020
GBP (£)
shares
Dec. 31, 2019
GBP (£)
shares
Dec. 31, 2018
GBP (£)
shares
Disclosure Of Classes Of Share Capital [Line Items]      
Beginning balance £ 63,522 £ 81,594 £ 93,420
Beginning balance, Shares | shares 32,479,000 32,226,000  
Exercise of share options £ 15 £ 125 £ 207
Exercise of share options | shares 32,500 252,187 415,312
Issue of share capital, Shares | shares 18,663,000    
Ending balance £ 99,230 £ 63,522 £ 81,594
Ending balance, Shares | shares 51,175,000 32,479,000 32,226,000
Share Capital [member]      
Disclosure Of Classes Of Share Capital [Line Items]      
Beginning balance £ 1,299 £ 1,289 £ 1,272
Exercise of share options 1 10 17
Issue of share capital 747    
Ending balance 2,047 1,299 1,289
Share Premium [member]      
Disclosure Of Classes Of Share Capital [Line Items]      
Beginning balance 79,541 79,426 79,236
Exercise of share options 14 115 190
Issue of share capital 61,335    
Ending balance £ 140,890 £ 79,541 £ 79,426