Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 108. Mercenaries
The UK LOAC Pamphlet (1981) states: “A mercenary is a person who takes part in the conflict for private gain, who is not a member of any organised armed forces and has no connection with the countries involved in the 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. 10, § 7.
The UK LOAC Manual (2004) states:
A mercenary is any person who:
a. is specially recruited locally or abroad in order to fight in an armed conflict;
b. does, in fact, take a direct part in the hostilities;
c. is motivated to take part in the hostilities essentially by the desire for private gain and, in fact, is promised, by or on behalf of a party to the conflict, material compensation substantially in excess of that promised or paid to combatants of similar ranks and functions in the armed forces of that party;
d. is neither a national of a party to the conflict nor a resident of territory controlled by a party to the conflict;
e. is not a member of the armed forces of a party to the conflict; and
f. has not been sent by a state which is not a party to the conflict on official duty as a member of its armed forces. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 4.10.1.
The United Kingdom's LOAC Pamphlet (1981) states:
Mercenaries are neither combatants nor entitled to PW [prisoner-of-war] status. A mercenary is a person who takes part in the conflict for private gain, who is not a member of any organized armed forces and has no connection with the countries involved in the 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. 10, § 7.
The UK LOAC Manual (2004) states: “Mercenaries are not entitled to be prisoners of war unless their captors so decide. Even if not treated as prisoners of war, captured mercenaries remain entitled to the basic humanitarian guarantees provided by Additional Protocol I.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 8.12; see also 4.10.