Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice relating to Rule 66. Non-Hostile Contacts between the Parties to the Conflict
Section A. General
The UK Military Manual (1958) states:
386. It is on occasions unavoidable – and often convenient – for commanders to open direct communication with the enemy for military purposes. Furthermore, humanity and convenience may at times induce them for special reasons to relax the general prohibition of intercourse between belligerents. …
387. It is essential that in such non-hostile relations the most scrupulous good faith should be observed by both parties, and that no advantage be taken which is not intended to be given by the enemy. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, §§ 386 and 387.
The manual also provides:
There is nothing in [Articles 32–34 of the 1907 Hague Regulations] which indicates that a white flag is the only method whereby one belligerent may signify to the other its desire to open communications. In modern conditions of warfare wireless messages and loud-speakers are also used as a means of conveying the wish of one belligerent to communicate with the other. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 394, footnote 2.
The manual further emphasizes:
420. A suspension of arms is essentially a military convention of very short duration, concluded between commanders of armies, or detachments in order to arrange some local matter of urgency: most frequently to bury the dead, or to collect and succour the wounded, or, occasionally, to exchange prisoners, to permit conferences.
497. A cartel, in the wider sense of the term, is issued to signify a convention concluded between belligerents for the purpose of permitting certain kinds of non-hostile intercourse which would otherwise be prevented by the conditions of war. For instance, communication by post, trade in certain commodities, and the like, may be agreed upon by a cartel. In its strictly military sense, however, a cartel means an agreement for the exchange of prisoners of war. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, §§ 420 and 497.
The UK LOAC Pamphlet (1981) states:
It is within the legal competence of an officer to arrange for a temporary cease-fire for a specific and limited purpose, for example to permit the collection or evacuation of the wounded … Absolute good faith is required in all such dealings [the arrangement of a cease-fire] with the enemy. 
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 4, p. 17, § 18.
The UK LOAC Manual (2004) states:
Principle of Good Faith
10.2. Whenever there are non-hostile relations between parties to an armed conflict, those relations must be conducted with the utmost good faith and any agreement reached scrupulously observed. In particular, there should be no abuse of a flag of truce or emblems of identification in dealings between belligerents. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 10.2.
In its chapter on negotiations between belligerents, the manual further states:
Humanitarian arrangements to be specifically agreed may include … the provision of medical care or food supplies … An armistice agreement cannot take away the protection afforded to individuals under the law of armed conflict, though it can improve upon that protection. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 10.24.
On the basis of a meeting with an army lawyer, the Report on UK Practice comments that negotiation with the enemy “is a tricky area now” owing to the practicalities of fast-paced modern warfare. 
Report on UK Practice, 1997, Meeting with an army lawyer, 18 July 1997, Chapter 2.2.