Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 155. Defence of Superior Orders
The UK Military Manual (1958) states: “Obedience to the order of a government or of a superior, whether military or civil, or to a national law or regulation, affords no defence to a charge of committing a war crime but may be considered in mitigation of punishment.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 627.
The UK LOAC Pamphlet (1981) provides: “There is no defence of ‘superior orders’. If a soldier carries out an illegal order, both he and the person giving that order are responsible.” 
United Kingdom, The Law of Armed Conflict, D/DAT/13/35/66, Army Code 71130 (Revised 1981), Ministry of Defence, prepared under the Direction of The Chief of the General Staff, 1981, Section 10, p. 38, § 1.
The UK LOAC Manual (2004) states: “The fact that a subordinate was ordered to do an act, or make an omission, which was illegal does not, of itself, absolve the subordinate from criminal responsibility.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.35.3.
The manual further states: “The defence of mistake of fact cannot succeed where the accused was ordered to do something which was manifestly illegal, such as killing prisoners of war.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.45.2.
In addition, the manual provides:
16.47. The Statute of the International Criminal Court states that superior orders do not relieve a person of criminal responsibility unless:
a. “The person was under a legal obligation to obey orders of the Government or the superior in question”;
b. “The person did not know that the order was unlawful”; and
c. “The order was not manifestly unlawful”.
16.47.1. The reference to a legal obligation to obey orders is a reference to the requirements of national law, for example, military law, which, in many countries, makes it an offence for soldiers not to obey orders. However, orders to commit genocide or crimes against humanity are considered to be manifestly unlawful.
16.47.2. Even where superior orders do not in themselves provide a defence to war crimes charges, they may be relevant to other defences such as lack of mens rea, mistake of fact or duress and may be taken into account in mitigation of punishment.
16.47.3. Orders from a superior in this context include those of a government, a superior – military or civilian – or a national law or regulation. A serviceman is under a duty not to obey a manifestly unlawful order. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 16.47–16.47.3.
[emphasis in original]
In the Kramer case (Auschwitz and Belsen concentration camps case) in 1945, the British Military Court at Lüneberg rejected the defence of superior orders. It referred to Article 8 of the 1945 IMT Charter (Nuremberg) and quoted a comment of the International Military Tribunal (IMT), which stated: “The true test, which is found in varying degrees in the criminal law of most nations, is not the existence of the order, but whether moral choice was in fact possible.” 
United Kingdom, Military Court at Lüneberg, Kramer case (Auschwitz and Belsen concentration camps case), Judgment, 17 November 1945.
At the CDDH, the United Kingdom, which opposed Article 77 of the draft Additional Protocol I submitted by the ICRC, stated that it
could not accept that there ought to be one system of law which related to grave breaches of the Conventions and Protocols, while other breaches, including breaches of customary law and of other Conventions, were subjected to an entirely different system. That state of affairs would clearly lead only to confusion in an area where it was vital to have simple rules which could be readily understood by soldiers.
The words “or should have known” in paragraph 2 appeared capable of a least two interpretations. If those words were to be taken as meaning that a soldier was to be expected to carry out his own detailed investigation of the facts of a situation before complying with an order, the result would not merely be impracticable but totally impossible in a combat situation.
Much the best course would be the omission of the article, leaving the situation to be regulated by the existing rules of international law concerning superior orders. Those rules were well understood and clearly explained in existing manuals on the law of armed conflict. 
United Kingdom, Statement at the CDDH, Official Records, Vol. VI, CDDH/SR.45, 30 May 1977, pp. 307–308, §§ 4 and 6.
In 1991, during a debate in the House of Commons, the UK Parliamentary Under-Secretary of State for Foreign and Commonwealth Affairs stated:
Anyone who breaks the provisions of the Geneva conventions may be held liable. Thus, individual Iraqis now bear personal responsibility for breaches of them. That position was reaffirmed in Security Council resolutions 670 and 674. The superior orders defence will not be accepted as an excuse. Machinery already exists under [the Geneva Conventions Act as amended (1957)] for prosecuting grave breaches of them. 
United Kingdom, House of Commons, Statement by the Parliamentary Under-Secretary of State for Foreign and Commonwealth Affairs, Hansard, 28 March 1991, Vol. 188, col. 1100.
In 1993, in a “Non-Paper” discussing the 1993 ICTY Statute and transmitted to the UN Legal Counsel, the UK Foreign and Commonwealth Office stated:
We do not believe one should depart from the principle in Article 8 of the [1945 IMT Charter (Nuremberg)] … A similar provision is found in Article 2(3) of the [1984 Convention against Torture]. These provisions are preferable to that in Article 11 of the [1991 ILC] draft Code of Crimes [against the Peace and Security of Mankind]. Under that an individual would not be relieved of criminal responsibility if “in the circumstances at the time, it was possible for him not to comply with an order of a superior”. This language, drawn from the 1954 Draft Code [of Offences against the Peace and Security of Mankind] would seem to make a large inroad into the Nuremberg rule and go against the general trend internationally towards the expansion of individual responsibility. 
United Kingdom, Foreign and Commonwealth Office, Non-Paper, Former Yugoslavia: War Crimes Implementation of Resolution 808, 22 March 1993, reprinted in BYIL, Vol. 64, 1993, p. 701.