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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Financial Instruments - Schedule of Financial Assets Subject to Fixed or Variable Interest Rates (Details) - GBP (£) £ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure Of Financial Instruments By Type Of Interest Rate [Line Items]
Cash and cash equivalents	£ 86,703	£ 19,990	£ 14,112	£ 18,761
Financial Assets at Fixed Rates
Disclosure Of Financial Instruments By Type Of Interest Rate [Line Items]
Cash and cash equivalents	51,745	5,000
Financial Assets at Variable Rates
Disclosure Of Financial Instruments By Type Of Interest Rate [Line Items]
Cash and cash equivalents	£ 13,708	£ 14,433