Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 5. Definition of Civilians
The UK LOAC Pamphlet (1981) states: “Civilians are all persons other than those defined in paragraphs 1 to 8 above [combatants, guerrillas and commandos, spies, mercenaries, military non-combatants].” 
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. 10, § 9.
The UK LOAC Manual (2004) states: “Civilians are persons who are not members of the armed forces. In cases of doubt, persons are considered to be civilians. ‘The civilian population comprises all persons who are civilians’.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.3.1.
Upon ratification of the Convention on Certain Conventional Weapons, the United Kingdom made a declaration stating, inter alia, that the terms “civilian” and “civilian population” used in this Convention had the same meaning as in Article 50 of the 1977 Additional Protocol I. 
United Kingdom, Declaration made upon ratification of the Convention on Certain Conventional Weapons, 13 February 1995, § a(iii).