Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 87. Humane Treatment
Section B. Civilians
The UK Military Manual (1958) provides that civilians “must be humanely treated”. This also applies in occupied territories. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, §§ 39 and 547.
The UK LOAC Pamphlet (1981) states: “An obligation is imposed on belligerents to deal humanely with protected persons.” With regard to enemy aliens, the manual specifies: “[The 1949 Geneva Convention III] ensures the humane treatment of those who remain.” 
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 and 11; see also Annex A, p. 49, § 20.
The UK LOAC Manual (2004) states in its chapter on the protection of civilians in the hands of a party to the conflict: “All persons are to be treated humanely in all circumstances.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 9.3.
In its chapter on internal armed conflict, the manual restates the provisions of common Article 3 of the 1949 Geneva Conventions:
Under the terms of Common Article 3, the parties to a non-international armed conflict occurring in the territory of a party to the Conventions are obliged to apply “as a minimum”, the following provisions:
(1) Persons taking no active part in the hostilities, including members of armed forces who have laid down their arms and those placed hors de combat by sickness, wounds, detention, or any other cause, shall in all circumstances be treated humanely. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 15.4.