Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 11. Indiscriminate Attacks
The UK LOAC Pamphlet (1981) provides that it is forbidden “to carry out indiscriminate attacks.” 
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. 15, § 5(j).
The UK LOAC Manual (2004) states: “Indiscriminate attacks are prohibited.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 12.26; see also § 5.23 (conduct of hostilities), § 12.22 (air operations), § 13.28 (maritime warfare) and § 16.25 (enforcement of the law of armed conflict).
With regard to internal armed conflict, the manual states:
15.15. Attacks that are not directed against military targets are prohibited.
15.15.1. There is no specific rule on indiscriminate attacks under customary law but the principle of distinction prohibits attacks that are not aimed at a specific target. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 15.15–15.15.1.
The UK Geneva Conventions Act (1957), as amended in 1995, punishes “any person, whatever his nationality, who, whether in or outside the United Kingdom, commits, or aids, abets or procures the commission by any other person of, a grave breach of … [the 1977 Additional Protocol I]”. 
United Kingdom, Geneva Conventions Act, 1957, as amended in 1995, Section 1(1).
On 21 January 1991, in the context of the Gulf War, the UK Minister of Foreign Affairs summoned the Iraqi Ambassador to discuss Iraq’s obligations under international law. According to a statement by a spokesperson for the Foreign and Commonwealth Office after the meeting, the Minister had “expressed concern at the indiscriminate targeting of civilian sites by Iraqi SCUD missiles”. 
United Kingdom, Statement by Foreign and Commonwealth Office spokesperson, 21 January 1991, BYIL, Vol. 62, 1991, p. 680.
In 1991, in a report submitted to the UN Security Council on operations in the Gulf War, the United Kingdom accused Iraq of having had “no compunction about launching indiscriminate missile attacks directed at civilians”. 
United Kingdom, Letter dated 13 February 1991 to the President of the UN Security Council, UN Doc. S/22218, 13 February 1991, pp. 1–2.
In 1991, during a debate in the UN Security Council concerning the Gulf War, the United Kingdom reiterated its condemnation of the indiscriminate firing of missiles at civilian population centres. 
United Kingdom, Statement before the UN Security Council, UN Doc. S/PV.2977, 14 February 1991, § 72.
In 2004, in a written ministerial statement, the UK Foreign Secretary stated:
The basic obligations under international humanitarian law as regards civilian casualties in an armed conflict are set out in additional protocol I to the Geneva conventions, which also reflects customary international law. In particular, indiscriminate attacks are prohibited, and this includes any,
“attack which may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated”.
This obligation under international humanitarian law has been fully complied with by the United Kingdom in respect of all military operations in Iraq. 
United Kingdom, House of Commons, Written ministerial statement by the Foreign Secretary, Hansard, 17 November 2004, Vol. 426, Written Ministerial Statements, col. 93WS.
The UK Government Strategy on the Protection of Civilians in Armed Conflict (2010) states:
Protection of civilians in armed conflict matters from a legal perspective, because the UK has specific obligations concerning the protection of civilians in situations where it is involved in military action. International humanitarian law (IHL) … prohibits attacks that are indiscriminate. 
United Kingdom, Foreign and Commonwealth Office, Government Strategy on the Protection of Civilians in Armed Conflict, March 2010, p. 2.
[emphasis in original]