Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 101. The Principle of Legality
The UK Military Manual (1958) provides: “No prisoner of war may be tried or sentenced for an act which is not forbidden by the law of the Detaining Power or by international law in force at the time the act in question was committed.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 223.
The UK LOAC Pamphlet (1981) provides that no prisoner of war “may be tried or sentenced for an act which is not forbidden by the law of the Detaining Power or by international law in force when the act in question was committed”. 
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 8, p. 33, § 19(f).
The UK LOAC Manual (2004) states in its chapter on the protection of civilians in the hands of a party to the conflict:
In the case of penal offences relating to the armed conflict, the basic principles of natural justice must be observed … These principles include the following: … no one shall be accused or convicted of a criminal offence on account of any act or omission which did not constitute a criminal offence under the national or international law to which he was subject at the time when it was committed; nor shall a heavier penalty be imposed than that which was applicable at the time when the criminal offence was committed; if, after the commission of the offence, provision is made by law for the imposition of a lighter penalty, the offender shall benefit thereby. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 9.6.
With regard to prisoners of war, the manual provides: “No prisoner of war may be tried or sentenced for an act that was not an offence under either the law of the detaining power or the international law in force at the time of its commission.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 8.130.
In its chapter on occupied territory, the manual states: “New criminal law enacted by the occupying power comes into force only after it has been duly published to the inhabitants in writing in their own language. It must not be retroactive.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 11.58 11 .
With regard to internal armed conflicts in which the 1977 Additional Protocol II is applicable, the manual provides:
no one shall be held guilty of any criminal offence on account of any act or omission which did not constitute a criminal offence, under the law, at the time when it was committed; nor shall a heavier penalty be imposed than that which was applicable at the time when the criminal offence was committed; if after the commission of the offence, provision is made by law for the imposition of a lighter penalty, the offender shall benefit thereby. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 15.42.
In its chapter on enforcement of the law of armed conflict, the manual states: “It is a fundamental principle of law that no person may be tried for a war crime unless the act in question was an offence at the time of its commission.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.30.