Derivatives and Financial Instruments - Schedule of Derivative Instruments (Details) - Designated as Hedging Instrument £ in Millions, $ in Millions	Dec. 31, 2020 USD ($)	Dec. 31, 2019 GBP (£) $ / £ ¥ / £ € / £ $ / £
U.S. dollars | Sales hedges
Derivative [Line Items]
Contract totals/£m	$ 3.0	£ 0.1
Exchange rates | $ / £		1.2914
U.S. dollars | Sales hedges | Minimum
Derivative [Line Items]
Exchange rates	1.3045
U.S. dollars | Sales hedges | Maximum
Derivative [Line Items]
Exchange rates	1.3667
U.S. dollars | Purchase hedges
Derivative [Line Items]
Contract totals/£m	$ 4.8	£ 1.3
Exchange rates | $ / £		1.3228
U.S. dollars | Purchase hedges | Minimum
Derivative [Line Items]
Exchange rates	1.3046
U.S. dollars | Purchase hedges | Maximum
Derivative [Line Items]
Exchange rates	1.3667
Euros | Sales hedges
Derivative [Line Items]
Contract totals/£m	$ 11.1	£ 7.6
Euros | Sales hedges | Minimum
Derivative [Line Items]
Exchange rates	1.0917	1.1551
Euros | Sales hedges | Maximum
Derivative [Line Items]
Exchange rates	1.1181	1.1750
Euros | Purchase hedges
Derivative [Line Items]
Contract totals/£m	$ 1.7	£ 0.8
Exchange rates | € / £		1.1663
Euros | Purchase hedges | Minimum
Derivative [Line Items]
Exchange rates	1.1065
Euros | Purchase hedges | Maximum
Derivative [Line Items]
Exchange rates	1.0944
Japanese Yen | Sales hedges
Derivative [Line Items]
Contract totals/£m	$ 0.1	£ 0.1
Exchange rates	136.89	142.86
Canadian dollars | Sales hedges
Derivative [Line Items]
Contract totals/£m	$ 0.1
Exchange rates	1.7409
Canadian dollars | Purchase hedges
Derivative [Line Items]
Contract totals/£m	$ 9.4	£ 7.0
Canadian dollars | Purchase hedges | Minimum
Derivative [Line Items]
Exchange rates	1.7409	1.7137
Canadian dollars | Purchase hedges | Maximum
Derivative [Line Items]
Exchange rates	1.7201	1.7664
Australian dollars | Purchase hedges
Derivative [Line Items]
Contract totals/£m	$ 0.9
Exchange rates	1.7729
Chinese yuan | Purchase hedges
Derivative [Line Items]
Contract totals/£m	$ 0.9
Exchange rates	8.9184