Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 4. Definition of Armed Forces
The UK Military Manual (1958) defines armed forces with reference to Article 4(A) of the 1949 Geneva Convention III. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 89.
The UK LOAC Pamphlet (1981) defines armed forces as:
a. Members of the organised armed forces, even if they belong to a government or authority not recognised by the adversary, if those forces:
1. are under a commander who is responsible for the conduct of his subordinates to one of the Parties in conflict; and
2. are subject to an internal disciplinary system which enforces compliance with the law of armed conflict.
It is customary for members of organised armed forces to wear uniform. The definition is wide enough to cover auxiliary and reserve forces.
b. Members of any other militias, volunteer corps or organised resistance movements if:
(1) they are subject to a system of internal discipline; and
(2) they have a fixed distinctive sign; and
(3) they carry their arms openly; and
(4) they comply with the law of armed conflict. 
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:
The armed forces of a party to a conflict are “all organized armed forces, groups and units which are under a command responsible to that Party for the conduct of its subordinates, even if that Party is represented by a government or an authority not recognized by an adverse Party”. Armed forces are to be “subject to an internal disciplinary system which, inter alia, shall enforce compliance with the rules of international law applicable in armed conflict.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 4.33.
The UK LOAC Manual (2004) states:
To avoid confusion, the law requires that “whenever a Party to a conflict incorporates a paramilitary or armed law enforcement agency into its armed forces it shall so notify the other Parties to the conflict”. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 4.3.6.