Futter, Andrew. "Trident Replacement and UK Nuclear Deterrence: Requirements in an Uncertain Future." The RUSI Journal, Vol.160, No.5 (2015): 60-67.

Futter, Andrew. "Trident Replacement and UK Nuclear Deterrence: Requirements in an Uncertain Future." The RUSI Journal, Vol.160, No.5 (2015): 60-67.

• The British government announced an intention to replace the current nuclear submarine fleet -- currently composed of four Vanguard-class nuclear-powered submarines loaded with Trident nuclear missile -- in 2006, sparking a debate on the future of the Trident program (60).
• At the instance of the Liberal Democrats, in 2013 the British government opened an investigation into the possibility of using a nuclear deterrence system that was not based on Trident. Although the report raised the question of reducing the number of submarines to 3, it is likely that British deterrence will remain based on the Trident system (62).
• The Trident system was developed to meet the needs of the Cold War and it is unlikely that the future security threats to the UK will resemble those posed in the 2010s or those posed during the Cold War. The primary risks are the development of superior anti-ballistic-missile technology, anti-submarine technology, and cyber warfare capacities that could all disrupt the operating capacity of the Trident system and thus compromise its value as a deterrent (60, 65).
• The current Trident fleet was designed with the intent of deterring Soviet aggression by being able to deliver an unacceptable amount of damage to Russia, a goal called the 'Moscow criterion' (62). 
• The continuation of the Trident program is justified on the principle that a conventional nuclear deterrent will continue to be relevant to the security threat that may face the UK in the future, particularly from small nuclear states posing a threat to Britain's vital interests (62).
• The development of anti-ballistic-missile technology poses a threat to the continued ability of the Trident system to act as a deterrent, as it may become increasingly possible for Trident missiles to be intercepted. This may also make it increasingly necessary to develop new ballistic missile technologies capable of defeating this deterrent (63).
• Enemies could potentially detect, and thus damage or destroy, the Vanguard submarines or their missiles if new technology is developed. They are particularly vulnerable to cyberattack or similar computer tracking and digital espionage (63). 
• Changes in politics and public perceptions of nuclear weapons also call into account the reliability of British deterrence since it is increasingly doubtful that Britain would respond to an existential threat will a nuclear attack on a civilian area, especially since such a move would violate human rights law (64).
• Britain acquired nuclear weapons in October 1952 and has since maintained a minimum arsenal needed to act as a deterrent. The WE 177 aerial nuclear bomb was retired from service in March 1998, leaving the Trident submarine-based nuclear missile system as UK's only nuclear arsenal (61).
• The current Trident fleet was commissioned in 1980, entered service in the 1990s, and is expected to be replaced throughout the 2020s (61). Each submarine carries as many as eight ballistic missiles with a maximum total of 40 nuclear warheads (62).
• The British nuclear stockpile is set at 180 nuclear warheads, with between 120 and 160 kept at operation level at any given time (62).
• The basis of British nuclear strategy is minimal deterrence, as in keeping the smallest possible nuclear arsenal required to inflict unacceptable harm to a potential aggressor (61).
• In order to maintain this deterrence, there is constantly one Vanguard submarine on active patrol at a state of partial readiness at all times. At current technological levels, these submarines are essentially undetectable and extremely unlikely to be disabled by a preemptive strike (62).
• The UK possesses three main options to maintain deterrence: the continuation of the Trident program of nuclear deterrence, the development of an advanced conventional deterrence strategy in addition to Trident, and the abandonment of Trident in favor of conventional deterrence (64).
• The continuation of the current Trident program would see the UK replace three or four of the submarines in the 2020s and likely have these missiles and submarines undergo several upgrades over the course of decades. This approach carries the risk that nuclear threat will not continue to act as an effective deterrence, that the deterrence capability of these systems is challenged by new technologies, and that shifting nuclear norms makes British retaliation less credible (64).
• The development of an alternative conventional deterrence would see the UK continue Trident, albeit with reduced readiness and fewer patrols, alongside missile defense systems and precision-strike capabilities for conventional deterrence. This option would offer the largest range of deterrence to both conventional threats and newer nuclear threats from rogue states, but would be the most expensive to develop (65).
• The development of an entirely conventional system of deterrence would see the UK nuclearly disarm and develop an array of missile defense system, air defenses, precision-strike capabilities, and an enlarged conventional military. This would necessitate the development of new conventional capabilities. The impact to British safety from nuclear attacks would be limited since the UK would still be covered by American nuclear guarantees (65).