Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 103. Collective Punishments
The UK Military Manual (1958) states:
The Hague Rules forbid collective punishment, in the form of a general pecuniary or other penalty, of the population for acts of individuals for which the population as a whole cannot be regarded as jointly and severally responsible. It was formerly thought that the prohibition did not exclude reprisals against a locality or community for some act committed by its inhabitants or members who cannot be identified. However, the Civilian Convention, Art. 33 has prohibited collective penalties and has expressly adopted the principle that “no protected person may be punished for an offence he or she has not personally committed”. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 647; see also §§ 42 (civilians), 205 (prisoners of war) and 553 (occupied territory).
The manual also states: “Violations of the Geneva Conventions not amounting to ‘grave breaches’, are also war crimes, for example, … imposing collective disciplinary measures affecting food of prisoners of war.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 626.
The UK LOAC Pamphlet (1981) forbids collective punishments. 
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 9, p. 35, § 9; see also Annex A, p. 48, § 20.
The UK LOAC Manual (2004) states in its chapter on the protection of civilians in the hands of a party to the conflict:
The following acts are prohibited “at any time and in any place whatsoever”:
d. collective punishments;
e. threats to commit any of the foregoing acts. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 9.4.
In its chapter on prisoners of war, the manual provides that collective punishments for individual acts are prohibited. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 8.121.
In its chapter on internal armed conflict, the manual states: “Indispensable judicial guarantees include as a minimum … individual criminal responsibility (so that collective punishments would be unlawful).” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 15.30.5.