Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 87. Humane Treatment
Section C. Wounded and sick
The UK Military Manual (1958) provides: “The wounded and sick … must be humanely treated.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 339.
The UK LOAC Pamphlet (1981) provides: “The wounded and sick … must be humanely treated.” 
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 6, p. 22, § 2.
The UK LOAC Manual (2004) states:
7.3. The wounded and sick are to be protected and respected. They may not be attacked. They must be treated humanely. …
7.3.2. Paragraph 7.3 applies to all wounded and sick, whether United Kingdom, allied or enemy, military or civilian. They are entitled to respect and protection [and] humane treatment. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 7.3 and 7.3.2; see also 13.129 (maritime warfare).
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.