Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 154. Obedience to Superior Orders
The UK LOAC Pamphlet (1981) provides: “Military personnel are required to obey lawful commands but must not obey unlawful commands.” 
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 Pamphlet further states: “Illegal orders are not to be given nor carried out.” 
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, Annex A, p. 46, § 2.
In 2006, in the Jones and others case, the UK House of Lords were called upon to decide whether the crime of aggression was part of the domestic criminal law of England and Wales. The appellants, charged with various criminal offences committed against UK military installations in 2003, contended that their acts were legally justified, for having been intended to prevent the crime of aggression in the form of the invasion of Iraq. The House unanimously dismissed their appeals. Lord Hoffmann noted, inter alia:
83. The right of the citizen to use force on his own initiative is even more circumscribed when he is not defending his own person or property but simply wishes to see the law enforced in the interests of the community at large. The law will not tolerate vigilantes. If the citizen cannot get the courts to order the law enforcement authorities to act (compare R v Commissioner of Police of the Metropolis, Ex p Blackburn [1968] 2 QB 118) then he must use democratic methods to persuade the government or legislature to intervene.
84. Often the reason why the sovereign power will not intervene is because it takes the view that the threatened action is not a crime. In such a case too, the citizen is not entitled to take the law into his own hands. The rule of law requires that disputes over whether action is lawful should be resolved by the courts. If the citizen is dissatisfied with the law as laid down by the courts, he must campaign for Parliament to change it. …
86. My Lords, to legitimate the use of force in such cases would be to set a most dangerous precedent. As Lord Prosser said in Lord Advocate’s Reference No 1 of 2000 2001 JC 143, 160G-H:
“What one is apparently talking about are people who have come to the view that their own opinions should prevail over those of others … They might of course be persons of otherwise blameless character and of indubitable intelligence. But they might not. It is not only the good or the bright or the balanced who for one reason or another may feel unable to accept the ordinary role of a citizen in a democracy.”
87. A time of war is the extreme example of the dangers. Of course citizens are entitled, indeed required, to refuse to participate in war crimes. But if they are allowed to use force against military installations simply to give effect to their own honestly held view of the legality of what the armed forces of the Crown are doing, the Statute of Treason would become a dead letter. 
United Kingdom, House of Lords, Jones and others case, Judgment, 29 March 2006, §§ 83–84 and 86–87.