Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 106. Conditions for Prisoner-of-War Status
Section A. Distinction from the civilian population
The UK LOAC Pamphlet (1981) states: “All combatants are required to distinguish themselves from the civilian population, usually by wearing uniform.” 
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. 9, § 2.
The Pamphlet also states: “It is customary for members of organised armed forces to wear uniform.” 
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:
4.4. “In order to promote the protection of the civilian population from the effects of hostilities, combatants are obliged to distinguish themselves from the civilian population while they are engaged in an attack or in a military operation preparatory to an attack.”
4.4.3. … In order that the civilian population should be adequately protected, the expression “military operation preparatory to an attack” must be given a wide meaning. Members of the armed forces who do not wear uniform, combat gear or an adequate distinctive sign and whose sole arm is a concealed weapon, or who hide their arms on the approach of the enemy, will be considered to have lost their combatant status. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 4.4 and 4.4.3.
The manual further explains that a member of armed forces failing to comply with the rule of distinction “forfeits his right to be treated as a prisoner of war unless, in spite of the circumstances, he still falls within one of the categories entitled to prisoner of war status under Geneva Convention III”. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 4.6.
In 2003, during a debate in the House of Lords, the UK Parliamentary Under-Secretary of State for Defence stated:
My Lords, I repeat what I said a few minutes ago that captured Iraqis will be given prisoner of war status until they are proved otherwise and they will be treated according to our obligations under the Geneva Conventions. Captured Iraqi forces are likely to be prisoners of war unless they conceal weapons in the conduct of operations, in which case, as the noble Lord will know, they are unlawful combatants. Although unlawful combatants do not have prisoner of war status, we would have a duty, under international humanitarian law, which we would fulfil, to treat prisoners in a reasonable and humane manner. I hope that that answers the noble Lord’s question. 
United Kingdom, House of Lords, Statement by the Parliamentary Under-Secretary of State for Defence, Hansard, 3 April 2003, Vol. 646, Debates, col. 1472.