Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 3. Definition of Combatants
The UK LOAC Manual (1958) states:
A combatant is one who is permitted by the law of armed conflict to take a direct part in an armed conflict on behalf of a belligerent State. Combatant status is very closely related to entitlement to PW status. The following are entitled to combatant status:
a. Members of the organized armed forces.
b. Members of any other militias, volunteer corps or organised resistance movements. 
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 3, p. 8, § 1.
The UK LOAC Manual (2004) states: “Members of the armed forces of a party to a conflict (other than medical personnel and chaplains) are combatants”. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 4.2.
The UK LOAC Manual (2004), as amended in 2010, states:
The expression “combatant” has in the past been used in two senses. Sometimes it has been used to describe any person who actually engages in hostile acts in an armed conflict on behalf of a party to the conflict, whether or not he is permitted to do so. It has also been used to describe only those persons with a right to take a direct part in hostilities. For this reason, it has often been qualified by the use of the adjectives “lawful” or “unlawful”. The use of the words “lawful” or “unlawful” is unhelpful and possibly misleading in view of the status definition of “combatant” in [the 1977] Additional Protocol I, set out in paragraph 4.2 [quoted above]. A person who takes a direct part in hostilities when not entitled to do so does not become a “combatant” under this definition simply by such conduct. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, as amended by Amendment 3, Ministry of Defence, September 2010, § 4.2.1.