Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 106. Conditions for Prisoner-of-War Status
Section C. Situations where combatants cannot distinguish themselves
The United Kingdom’s LOAC Pamphlet (1981) states:
2. All combatants are required to distinguish themselves from the civilian population, usually by wearing uniform. However, where because of unusual combat conditions they are unable to do so, they do not lose their combatant status provided that they carry their arms openly:
(a) during each military engagement, and
(b) while visible to the enemy when deploying before attack.
3. These unusual combat conditions can only occur in occupied territories and during wars of national liberation.
5. “Deployment” in this context means any movement towards the place from which an attack is to be launched. 
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, 3 and 5.
[emphasis in original]
The UK LOAC Manual (2004) states:
4.5. “There are situations in armed conflicts where, owing to the nature of the hostilities, an armed combatant cannot so distinguish himself”. In such situations, a special rule applies and the individual will retain his status as a combatant provided that he “carries his arms openly: (a) during each military engagement, and (b) during such time as he is visible to the adversary while he is engaged in a military deployment preceding the launching of an attack in which he is to participate”.
In these exceptional situations, there is no obligation for a combatant to distinguish himself from the civilian population at any other time. The combatant is permitted, in effect, to disguise himself as a member of the civilian population and thereby seek to obtain the protection from attack given to the latter, except during the period of the attack and deployment.
4.5.1. Wide application of this special rule would reduce the protection of civilians to vanishing point. Members of the opposing armed forces would come to regard every civilian as likely to be a combatant in disguise and, for their own protection, would see them as proper targets for attack. The special rule is thus limited to those exceptional situations where a combatant is truly unable to operate effectively whilst distinguishing himself in accordance with the normal requirements. The United Kingdom, together with other states, made a formal statement on ratifying Additional Protocol I that this exception could only apply in occupied territory or in conflicts to which Additional Protocol I, Article 1(4) apply. Even in those cases, there are many occasions on which combatants can still comply with the general rule of distinction, which remains in force, when the special rule would not apply.
4.5.2. Members of irregular armed forces, whether they comply with the rule of distinction or not, are legitimate objects of attack when taking a direct part in hostilities.
4.5.3. Even when the special rule applies, it requires, as a condition of retaining combatant status, that arms be carried openly in two cases. The first is during a military engagement, that is, when the combatant is in contact with the enemy. The second is during such time as he is visible to the adversary while engaged in a military deployment preceding the launching of an attack in which he is to participate. The term “deployment” includes individual as well as group deployments. On ratifying Additional Protocol I, the United Kingdom, together with other states, made a formal declaration that the expression “military deployment” means any movement towards a place from which an attack is to be launched. The requirement to carry arms openly during any such movement is limited to such time as the combatant is visible to the adversary. In the light of modern technical developments, “visible” cannot be construed as meaning only “visible to the naked eye”. A combatant is accordingly required to carry his arms openly if he is visible through binoculars or, during the night, visible by the use of infra-red or image intensification devices. The test is whether the adversary is able, using such devices, to distinguish a civilian from a combatant carrying a weapon. If such distinction can be made, the combatant is “visible to the adversary”. The wide availability of these devices means that combatants who are seeking to take advantage of the special rule should carry their arms openly well before they are actually in contact with the enemy.
4.5.4. Combatants who comply with the requirements set out above will not be regarded as committing perfidious acts, such as the feigning of civilian, non-combatant status. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 4.5–4.5.4.
[emphasis in original]
At the CDDH, the United Kingdom abstained in the vote on Article 42 of the draft Additional Protocol I (now Article 44) and stated:
Any failure to distinguish between combatants and civilians could only put the latter at risk. That risk might well become unacceptable unless a satisfactory interpretation could be given to certain provisions of Article 42. 
United Kingdom, Statement at the CDDH, Official Records, Vol. VI, CDDH/SR.40, 26 May 1977, p. 132, § 73.
The United Kingdom further stated that it considered that “the situations in which a guerrilla fighter was unable to distinguish himself from the civilian population could exist only in occupied territory” and that the word “deployment” must be interpreted as meaning “any movement towards a place from which an attack was to be launched”. 
United Kingdom, Statement at the CDDH, Official Records, Vol. VI, CDDH/SR.40, 26 May 1977, p. 132, § 74.
Upon signature of Additional Protocol I in 1977, the United Kingdom stated in relation to Article 44 of the Protocol:
that the situation described in the second sentence of paragraph 3 of the Article can exist only in occupied territories or in armed conflicts covered by paragraph 4 of Article 1, and that the Government of the United Kingdom will interpret the word “deployment” in paragraph 3 (b) of the Article as meaning “any movement towards a place from which an attack is to be launched”. 
United Kingdom, Declarations made upon signature of the 1977 Additional Protocol I, 12 December 1977, § c.
The United Kingdom made a similar statement upon ratification of the Additional Protocol I in 1998. 
United Kingdom, Reservations and declarations made upon ratification of the 1977 Additional Protocol I, 28 January 1998, § g.