The United Kingdom had been planning to buy the air-launched Skybolt missile but the USA decided to cancel the Skybolt program in 1962 as they no longer needed the missile. The British bought the Polaris submarine-launched ballistic missile instead.
The Nassau Agreement was signed on December 18 1962 resulting in the signing of the Polaris Sales Agreement on April 6 1963. The United States would supply the United Kingdom with Polaris missiles, launch tubes, and the fire control system. The UK would make the warheads and submarines. In return, the U.S. was given certain assurances by the United Kingdom regarding the use of the missile, however the U.S. does not have any veto on the use of British nuclear weapons.
The British Polaris submarines were the Resolution-class ballistic missile submarines, the first of which entered service in 1968. The Polaris system underwent a British-designed life extension programme called Chevaline that reduced the number of warheads and added defensive measures.
British Prime Minister Margaret Thatcher had written to President Carter on July 10 1980 to request that he approve supply of Trident I missiles. However in 1982 Thatcher wrote to President Reagan to request the United Kingdom be allowed to procure the Trident II (Trident D5) system, the procurement of which had been accelerated by the US Navy. This was agreed in March 1982.
Under the Polaris Sales Agreement, the United Kingdom paid a 5% levy on the cost of equipment supplied in recognition of US research and development costs already incurred. The R&D contribution to Trident comprised payments amounting to $116M with no on-going R&D levy. U.S. suppliers are paid via the U.S. based Polaris Trust Fund.