Editing Vanguard-class submarine (section)

===Trident programme===
{{Main|Trident nuclear programme}}
Beginning in the late 1960s, the United Kingdom operated four {{sclass|Resolution|submarine|1}}s, each armed with sixteen US-built [[UGM-27 Polaris|UGM-27 Polaris missiles]]. The Polaris missile was supplied to Britain following the terms of the 1963 [[Polaris Sales Agreement]]. This nuclear deterrent system was known as the [[UK Polaris programme]]. In the early 1980s the British government began studies examining options for replacing the ''Resolution''-class submarines and their Polaris missiles, both of which would be approaching the end of their service lives within little over a decade.<ref name="1980DefenceCouncil">{{cite journal|last1=PYM|first1=Francis|title=The Future United Kingdom Strategic Nuclear Deterrent Force|journal=Ministry of Defence|date=July 1980|url=http://fc95d419f4478b3b6e5f-3f71d0fe2b653c4f00f32175760e96e7.r87.cf1.rackcdn.com/6AC6FE79AE5E485DB3A2B579B6AE0654.pdf|access-date=1 December 2016}}</ref> On 24 January 1980, the House of Commons backed government policy, by 308 votes to 52, to retain an independent nuclear deterrent. Options that were examined included:<ref name="1980DefenceCouncil"/>
*A British designed and built ballistic missile; Although Britain had had no capability in this field since the 1960s, it was considered "not to be impossible". However, it would be very expensive, would be full of uncertainty and would not be available within the required time period. Thus the option was considered "unattractive".
*Retain Polaris, but fitted on a new submarine class; This option would have a cheaper "initial capital cost", but would lack in terms of required capability and reliability. Also, it was concluded that any initial capital savings would have been lost beyond the 1990s, due to the high cost of sustaining a small stockpile of bespoke missiles kept only in British service.

A European solution and the US [[UGM-73 Poseidon]] were also briefly considered, but ultimately rejected, primarily on capability, cost and uncertainty grounds.<ref name="1980DefenceCouncil"/> The clear favourite was the [[UGM-96 Trident I]], which as well as being a cost-effective solution – given the US would also operate the missile in vast numbers – also delivered the overall best long-term capability insurances against Soviet advancements in [[ballistic missile defence]].<ref name="1980DefenceCouncil"/> Subsequently, on 10 July 1980, the then Prime Minister [[Margaret Thatcher]] wrote to US President [[Jimmy Carter]] requesting the purchase of Trident I missiles on a similar basis as the 1963 Polaris Sales Agreement. However, following the acceleration of the US [[UGM-133 Trident II]] missiles, Thatcher wrote to US President [[Ronald Reagan]] in 1982 requesting the United Kingdom be allowed to procure the improved system instead. An agreement was made in March 1982 between the two countries,<ref>[http://www.reagan.utexas.edu/archives/speeches/1982/31182b.htm Letter to Prime Minister Margaret Thatcher of the United Kingdom Confirming the Sale of the Trident II Missile System to the Her Country] {{Webarchive|url=https://web.archive.org/web/20100913055759/http://www.reagan.utexas.edu/archives/speeches/1982/31182b.htm |date=13 September 2010 }} 11 March 1982</ref> and under the agreement, Britain made a 5% research and development contribution.<ref name=NAO-1987>{{cite book|title=Ministry of Defence and Property Services Agency: Control and Management of the Trident Programme|publisher=[[National Audit Office (United Kingdom)|National Audit Office]]|date=29 June 1987|pages=Part 4|isbn=0-10-202788-9|no-pp=true}}</ref>