Equity - Summary of Equity Share Capital (Parenthetical) (Detail) - £ / shares	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Ordinary shares [Member] \| Equity share capital [member]
Disclosure of classes of share capital [line items]
Nominal value per share	£ 0.20852	£ 0.20852	£ 0.19810

The nominal value per share.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2020-01-01
-Paragraph 79
-Subparagraph a
-Clause iii
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_79_a_iii&doctype=Standard
-URIDate 2020-01-01

Name:	ifrs-full_ParValuePerShare
Namespace Prefix:	ifrs-full_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant