Cash, Cash Equivalents and Short-Term Deposits - Schedule of Cash, Cash Equivalents and Short-Term Deposits (Detail) - GBP (£) £ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	£ 86,703	£ 19,990	£ 14,112	£ 18,761

The amount of cash on hand and demand deposits, along with short-term, highly liquid investments that are readily convertible to known amounts of cash and that are subject to an insignificant risk of changes in value. [Refer: Cash; Cash equivalents]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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