Règle correspondante
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 85. The Use of Incendiary Weapons against Combatants
Section B. The use of incendiary weapons against combatants in particular
The UK Military Manual (1958) states: “The use of flame throwers when directed against military targets is lawful. However, their use against personnel is contrary to the law of war in so far as it is calculated to cause unnecessary suffering.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 110, footnote 1.
The UK LOAC Manual (2004) states:
6.12.1. Although [incendiary weapons] can cause severe injury to personnel, their use is lawful provided the military necessity for their use outweighs the injury and suffering which their use may cause.
6.12.6. Use of weapons such as napalm and flame-throwers against combatant personnel is not dealt with specifically in the Conventional Weapons Convention or any other treaty. Such uses are governed by the unnecessary suffering principle so that they should not be used directly against personnel but against armoured vehicles, bunkers and built-up emplacements, even though personnel inside may be burnt. The same applies to white phosphorous, which is designed to set fire to targets such as fuel and ammunition dumps or for use to create smoke, and which should not be used directly against personnel. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 6.12.1 and 6.12.6.
In 2008, in response to a question in the House of Commons, the UK Secretary of State for Defence wrote regarding operations in Afghanistan: “In accordance with the UN third convention on conventional weapons [i.e. Protocol III to the Convention on Certain Conventional Weapons], UK training in the use of white phosphorus emphasises that it should … not [be used] as an anti-personnel weapon.” 
United Kingdom, House of Commons, Written Statement by the Secretary of State for Defence, Hansard, 23 February 2009, Vol. 488, Written Statements, col. 18W.
In 1973, in response to Resolution 2932 A (XXVII) in which the UN General Assembly asked States to comment on the report of the UN Secretary-General on napalm and other incendiary weapons and all aspects of their possible use, the United Kingdom emphasized that incendiary weapons must not be used to create unnecessary suffering and recommended further study of this issue. 
United Kingdom, Reply sent to the UN Secretary-General, reprinted in Report of the Secretary-General on napalm and other incendiary weapons and all aspects of their possible use, UN Doc. A/9207/rev.1 add.1, 11 October 1973.