[7][8] During the 1950s, Britain's nuclear deterrent was based around the V-bombers of the Royal Air Force (RAF), but developments in radar and surface-to-air missiles made it clear that bombers were becoming increasingly vulnerable, and would be unlikely to penetrate Soviet airspace by the mid-1970s.

[14] John F. Kennedy, then President of the United States, and Harold Macmillan, then UK Prime Minister, negotiated the Nassau Agreement, under which the US would sell Polaris systems for UK-built submarines.

[29] By the 1980s, possession of nuclear weapons was considered a visible sign of Britain's enduring status as a great power in spite of the loss of the British Empire, and had become a component of the national self-image.

[40] Callaghan was succeeded by Margaret Thatcher after the general election on 3 May 1979,[41] and she discussed the issue with Carter in October, who agreed to supply C-4, but he asked that the UK delay a formal request until December in order that he could get SALT II ratified beforehand.

[42] In the meantime, the MDA, without which the UK would not be able to access US nuclear weapons technology, was renewed for five more years on 5 December, and the MISC 7 cabinet committee formally approved the decision to purchase C-4 the following day.

[40] The British government hoped that Trident could be secured on the same terms as Polaris, but when its chief negotiator, Robert Wade-Gery, sat down with his American counterpart, David L. Aaron, in March 1980, he found this was not the case.

On 2 June 1980, Thatcher and the US Secretary of Defense, Harold Brown, agreed to $2.5 billion for the C-4 missile system, plus a 5 per cent R&D levy, British personnel for the Rapier batteries, and an expansion of the US base on Diego Garcia, which had assumed great importance since the Soviet invasion of Afghanistan.

The Americans were disturbed at the proposed British defence cuts, and pressed for an undertaking that the aircraft carrier HMS Invincible be retained in service, which they felt was necessary to avert trouble over the Belizean–Guatemalan territorial dispute.

[65] To provide an effective deterrent, the Trident system was intended to "pose a potential threat to key aspects of Soviet state power" whilst being invulnerable to a surprise or pre-emptive nuclear strike.

As with Polaris, Trident was owned and operated by the UK but committed to NATO and targeted in accordance with plans set out by the organisation's Supreme Allied Commander Europe, who is traditionally a senior figure in the US military.

[66] Under the terms of the Polaris Sales Arrangement, the US does not have a veto on the use of British nuclear weapons, which the UK may launch independently,[67] but this would only occur if "supreme national interests" required it.

[66] The final decision on firing the missiles is the responsibility of the prime minister, who, upon taking office, writes four identical letters of last resort, one of which is locked in a safe on board each of the Vanguard-class submarines.

If contact with the UK is lost, the commanding officer of a submarine has to follow the instructions in the letter if they believe that the United Kingdom has suffered an overwhelming attack.

A later statement read: "We also intend to exploit the flexibility of Trident to provide the vehicle for both sub-strategic and strategic elements of our deterrent … as an insurance against potential adverse trends in the international situation".

[86][87] Labour Party Leader Keir Starmer, although supporting nuclear deterrence, criticised the new policy and questioned why Prime Minister Boris Johnson believed that increasing the stockpile was necessary.

[119] Due to the distance of 720 km (450 mi) between AWE Aldermaston and the UK's nuclear weapon storage depot at RNAD Coulport, Holbrook (Trident) warheads are transported by road in heavily armed convoys by Ministry of Defence Police.

[120] According to a pressure group, between 2000 and 2016 the vehicles were involved in 180 incidents, ranging from delays and diversions because of accidents, protests, or bad weather, to a sudden loss of power in one of the lorries, which halted a convoy and caused a double lane closure and a tailback on the M6 motorway.

[125] The missile is 13 metres (43 ft) long, weighs 58.5 tonnes (129,000 lb), has a range of 11,300 kilometres (7,000 mi), a top speed of over 21,600 km/h (13,400 mph) (Mach 17.4) and a circular error probable (CEP) accuracy to within "a few feet".

[149] Two personnel are required to authenticate each stage of the process before launching, with the submarine commander only able to activate the firing trigger after two safes have been opened with keys held by the ship's executive and weapon engineer officers.

[154] According to senior RAF officers the retirement of the Nimrod maritime patrol aircraft in 2011 gave Russia the potential to gain "valuable intelligence" on the country's nuclear deterrent.

It reached its peak around 1960, by which time it had evolved into a broader movement calling for Britain to unilaterally give up nuclear weapons, withdraw from NATO, and end the basing of nuclear-armed aircraft in the UK.

Membership leapt from 3,000 in 1980 to 50,000 a year later, and rallies for unilateral nuclear disarmament in London in October 1981 and June 1982 attracted 250,000 marchers, the largest ever mass demonstrations in the UK up to that time.

[166] During the 2020 coronavirus pandemic a letter was circulated to MPs by the Bertrand Russell Peace Foundation which stated, “It is completely unacceptable that the UK continues to spend billions of pounds on deploying and modernising the Trident Nuclear Weapon System when faced with the threats to health, climate change and world economies that Coronavirus poses.” Signatories included Commander Robert Forsyth RN (Ret'd), a former nuclear submariner; Commander Robert Green RN (Ret'd), a former nuclear-armed aircraft bombardier-navigator and Staff Officer (Intelligence) to CINFLEET in the Falklands War; and Commander Colin Tabeart RN (Ret'd), a former Senior Engineer Officer on a Polaris submarine.

[168] The Royal United Services Institute (RUSI), a British defence and security think tank, released a paper in July 2010 assessing four possible options for maintaining both an effective nuclear deterrent and also reducing costs in light of anticipated budget restrictions.

[169] The paper concluded that "given the opportunity costs for conventional capabilities that current plans for Trident renewal are due to incur over the next decade...there is now a growing case for a re-examination of whether there are less expensive means of pursuing this objective.

The review asserted that whether or not cruise missile-based systems offer a credible alternative was contingent upon a political judgement on whether the UK could accept a "significant increase in vulnerability" and a reduction in who it could deter.

[173][174] In 2011, BASIC launched an independent cross-party Commission to initiate a deeper national debate on the UK's nuclear weapons policy and examine questions around the contentious issue of Trident renewal.

The conclusion acknowledged that "it remains crucial that the UK show keen regard for its position within the international community and for the shared responsibility to achieve progress in global nuclear disarmament.

"[176] BASIC's interpretation of the report also focused on this point, emphasising that the commissioners "agreed that the health of the global strategic environment, particularly nuclear non-proliferation, is critical to national security and is a central consideration.

[200] For the 2018/19 financial year the National Audit Office found that spending on the nuclear deterrent will cost £5.2 billion, or 14% of the defence budget, with £600 million of contingency funding used.