Règle correspondante
United Kingdom of Great Britain and Northern Ireland
Practice relating to Rule 66. Non-Hostile Contacts between the Parties to the Conflict
Section B. Use of the white flag of truce
The UK Military Manual (1958) states:
From time immemorial a white flag has been used as a signal by an armed force which wishes to open communications with the enemy. This is the only meaning which the flag possesses in international law. The hoisting of a white flag, therefore, means in itself nothing else than one party is asked whether it will receive a communication from the other. It may indicate merely that the party which hoists it wishes to make an arrangement for the suspension of arms for some purpose; but it may also mean that the party wishes to negotiate for surrender. Everything depends on the circumstances and conditions of the particular case. For instance, in practice, the white flag has come to indicate surrender if hoisted by individual soldiers or a small party in the course of an action. Great vigilance is always necessary, for the question in every case is whether the hoisting of the white flag was authorised by the commander. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 394.
The UK LOAC Pamphlet (1981) states: “The white flag, or flag of truce, indicates no more than an intention to enter into negotiations with the enemy. It does not necessarily mean a wish to surrender.” 
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. 16, § 10.
The UK LOAC Manual (2004) states:
10.5. From time immemorial, a white flag has been used as a signal of a desire to open communications with the enemy. This is the only meaning that the white flag possesses in the law of armed conflict. Wilful abuse of a white flag that results in death or serious injury is a grave breach of Additional Protocol I.
10.5.1. The display of a white flag means only that one party is asked whether it will receive a communication from the other. In some cases it may also mean that the party that displays it wishes to make an arrangement for a temporary suspension of hostilities for a purpose, such as the evacuation of the wounded, but in other cases it may mean that the party wishes to negotiate for surrender. Everything depends on the circumstances and conditions of the particular case. For instance, in practice, the white flag has come to indicate surrender if displayed by individual soldiers or a small party in the course of an action.
10.5.2. Those who display a white flag should cease firing until the invitation has been answered. Any abuse of a white flag is likely to be a war crime. Great vigilance must, however, always be displayed in dealing with enemy forces that have displayed a white flag, because other enemy soldiers in the vicinity may be unaware of the display of the white flag and continue firing. This is especially likely where the decision to display a white flag was taken not by the enemy commander on behalf of the entire force under his command but by individual soldiers. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 10.5–10.5.2.
A training video produced by the UK Ministry of Defence emphasizes that the white flag is protective and that it only indicates a wish to negotiate, not to surrender. 
United Kingdom, Ministry of Defence, Training Video: The Geneva Conventions, 1986, Report on UK Practice, 1997, Chapter 2.2.