Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 1. The Principle of Distinction between Civilians and Combatants
Section B. Attacks against combatants
The UK Military Manual (1958) states:
The most important powers of resistance possessed by a belligerent … are his armed forces with their military stores and equipment, and his defence installations of all kinds. The means of reducing these powers of resistance [include] killing and disabling enemy combatants. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 108.
The UK LOAC Manual (2004), as amended in 2010, states:
The principle of distinction separates those who may be legitimately the subject of direct attack, namely combatants and those who take a direct part in hostilities, from those who may not be so subject. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, as amended by Amendment 3, Ministry of Defence, September 2010, §§ 2.5–2.5.1.
Upon ratification of the 1977 Additional Protocol I, the United Kingdom stated:
It is the understanding of the United Kingdom that … the first sentence of paragraph 2 [of Article 52] prohibits only such attacks as may be directed against non-military objectives; it does not deal with the question of collateral damage resulting from attacks directed against military objectives. 
United Kingdom, Reservations and declarations made upon ratification of the 1977 Additional Protocol I, 28 January 1998, § j.
At the CDDH, the United Kingdom stated that it did not interpret the obligation in the first sentence of Article 47(2) of the draft Additional Protocol I (now Article 52(2))
as dealing with the question of incidental damage caused by attacks directed against military objectives. In its view, the purpose of the first sentence of the paragraph was to prohibit only such attacks as might be directed against non-military objectives. 
United Kingdom, Statement at the CDDH, Official Records, Vol. VI, CDDH/SR.41, 26 May 1977, p. 169, § 153.
In 2003, in reply to an oral question in the House of Lords asking “what in the circumstances of the Iraq of today constitutes the enemy”, the UK Parliamentary Under-Secretary of State for Defence stated: “My Lords, the potential enemy are all those, wherever and whoever they are, who seek to engage British forces in a hostile manner.” 
United Kingdom, House of Lords, Statement by the Parliamentary Under-Secretary of State for Defence, Hansard, 13 October 2003, Vol. 653, Debates, col. 600.
The UK Government Strategy on the Protection of Civilians in Armed Conflict (2010) states: “IHL requires parties to a conflict to respect and protect civilians. In the conduct of military operations they must distinguish at all times between combatants and civilians, and only direct attacks against suspected combatants”. 
United Kingdom, Foreign and Commonwealth Office, Government Strategy on the Protection of Civilians in Armed Conflict, March 2010, p. 4.