Currency conversion - Additional Information (Detail)	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
US dollar to Pound sterling [member]
Statement [Line Items]
Average exchange rate	1.39	1.26
Closing exchange rate	1.38		1.37
Euro to Pound sterling [member]
Statement [Line Items]
Average exchange rate	1.15	1.14
Closing exchange rate	1.16		1.12

The spot exchange rate at the end of the reporting period. Exchange rate is the ratio of exchange for two currencies. Spot exchange rate is the exchange rate for immediate delivery.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2020-01-01
-Paragraph 112
-Subparagraph c
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2020-en-r&anchor=para_112_c&doctype=Standard
-URIDate 2020-01-01

Name:	ifrs-full_ClosingForeignExchangeRate
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant