United Kingdom of Great Britain and Northern Ireland
Practice Relating to Nuclear Weapons
The UK LOAC Manual (2004) states:
6.17. There is no specific rule of international law, express or implied, which prohibits the use of nuclear weapons. The legality of their use depends upon the application of the general rules of international law, including those regulating the use of force and the conduct of hostilities. Those rules cannot be applied in isolation from any factual context to imply a prohibition of a general nature. Whether the use, or threatened use, of nuclear weapons in a particular case is lawful depends on all the circumstances. Nuclear weapons fall to be dealt with by reference to the same general principles as apply to other weapons. However, the rules introduced by Additional Protocol I “apply exclusively to conventional weapons without prejudice to any other rules of international law applicable to other types of weapons. In particular, the rules so introduced do not have any effect on and do not regulate or prohibit the use of nuclear weapons.”
6.17.1. The threshold for the legitimate use of nuclear weapons is clearly a high one. The United Kingdom would only consider using nuclear weapons in self-defence including the defence of its NATO allies, and even then only in extreme circumstances.
6.17.2. The United Kingdom has given a unilateral assurance that it will not use nuclear weapons against non-nuclear weapons states parties to the Treaty on the Non-Proliferation of Nuclear Weapons 1968. The assurance does not apply in the case of an invasion or any other attack on the United Kingdom, its Overseas Territories, its armed forces, its allies or on a state towards which it has a security commitment, carried out by a non-nuclear weapon state in association or alliance with a nuclear weapon state. An assurance in virtually identical terms has been given in memoranda signed with Belarus, Kazakhstan and Ukraine. Further, the United Kingdom has given treaty based assurances in the same terms to the states in Latin America and the South Pacific which are parties to the treaties establishing nuclear weapons-free-zones in those regions. The Antarctic Treaty prohibits any nuclear explosion in Antarctica. There are various other prohibitions, for example on installing or testing nuclear weapons on the seabed and in outer space. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 6.17–6.17.2.
In 2003, in reply to a written question in the House of Commons, the UK Secretary of State for Defence wrote:
The Government do not comment on hypothetical scenarios involving the use or threat of use of nuclear weapons.
The Government have made clear on many occasions that the use of United Kingdom of Great Britain and Northern Ireland nuclear weapons would only be contemplated in extreme circumstances of self defence. We made clear in the Strategic Defence Review New Chapter that aggression against us will not secure political or military advantage, but invite a proportionately serious response. Those, at every level, responsible for any breach of international law relating to the use of weapons of mass destruction will be held personally accountable. 
United Kingdom, House of Commons, Written answer by the Secretary of State for Defence, Hansard, 10 February 2003, Vol. 399, Written Answers, col. 510W.
In 2003, in reply to a written question in the House of Commons, the UK Prime Minister wrote: “As has repeatedly been made clear, the British Government would only contemplate the use or threat of use of nuclear weapons in extreme circumstances of self-defence. We would not use our weapons, whether conventional or nuclear, contrary to international law.” 
United Kingdom, House of Commons, Written answer by the Prime Minister, Hansard, 9 December 2003, Vol. 415, Written Answers, cols. 374W–375W.
In 2006, in a written answer to a question in the House of Commons concerning “whether it is Government policy that first strike use of UK nuclear weapons is ruled out”, the UK Secretary of State for Defence stated:
The United Kingdom Government would be prepared to use nuclear weapons only in extreme circumstances of self-defence. We would not use our weapons, whether conventional or nuclear, contrary to international law.
A policy of no first use of nuclear weapons would be incompatible with our and NATO’s doctrine of deterrence. We have made clear, as have our NATO allies, that the circumstances in which any use of nuclear weapons might have to be contemplated are extremely remote. Our overall strategy is to ensure uncertainty in the mind of any aggressor about the exact nature of our response, and thus to maintain effective deterrence. 
United Kingdom, House of Commons, Written answer by the Secretary of State for Defence, Hansard, 22 May 2006, Vol. 446, Written Answers, cols. 1331W.
In 2006, in a written answer to a question in the House of Commons concerning “which international legal obligations would have to be met by the United Kingdom if it was decided to use its nuclear weapons”, the UK Secretary of State for Defence stated:
The UK would consider using nuclear weapons only in extreme circumstances of self-defence and in accordance with our international legal obligations, including those relating to the conduct of armed conflict. 
United Kingdom, House of Commons, Written answer by the Secretary of State for Defence, Hansard, 24 July 2006, Vol. 449, Written Answers, col. 779W.