Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 71. Weapons That Are by Nature Indiscriminate
The UK LOAC Manual (2004) states:
6.4. It is prohibited to employ weapons which cannot be directed at a specific military objective or the effects of which cannot be limited as required by Additional Protocol I and consequently are of a nature to strike military objectives and civilians or civilian objects without distinction.
6.4.1. This provision operates as an effective prohibition on the use of weapons that are so inaccurate that they cannot be directed at a military target. The V1 flying bomb used in the Second World War and the Scud rocket used during the Gulf conflict of 1990–91 are examples of weapons likely to be caught by this provision. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 6.4–6.4.1.
Under the UK ICC Act (2001), it is a punishable offence to commit a war crime as defined in Article 8(2)(b)(xx) of the 1998 ICC Statute. 
United Kingdom, ICC Act, 2001, Sections 50(1) and 51(1) (England and Wales) and Section 58(1) (Northern Ireland).
At the CDDH, the United Kingdom stated:
The definition of indiscriminate attacks given in [Article 51(4) of the 1977 Additional Protocol I] was not intended to mean that there were means of combat the use of which would constitute an indiscriminate attack in all circumstances. The paragraph did not in itself prohibit the use of any specific weapon, but it took account of the fact that the lawful use of means of combat depended on the circumstances. 
United Kingdom, Statement at the CDDH, Official Records, Vol. VI, CDDH/SR.41, 26 May 1977, p. 164, § 119.
In 1991, in a briefing note on the Gulf crisis, the UK Foreign and Commonwealth Office criticized Iraq’s policy of launching Scud missiles against Israel and Saudi Arabia, “since these missiles are not precision weapons and are clearly intended to hit civilian targets”. 
United Kingdom, Foreign and Commonwealth Office, Briefing Note on the Gulf Crisis, January 1991, BYIL, Vol. 62, 1991, p. 678.
In 1995, in a letter to the UK House of Lords, the government spokesman deplored the use of weapons by the Israeli artillery in southern Lebanon that “may be deemed … to have indiscriminate effects”. 
United Kingdom, House of Lords, Letter from the government spokesman, 6 February 1995, BYIL, Vol. 66, 1995, p. 713.
In its written statement submitted to the ICJ in the Nuclear Weapons case in 1995, the United Kingdom stated:
3.67 A further argument which has been raised is that the use of any nuclear weapon would necessarily have such terrible effects upon civilians that it would violate those rules of the law of armed conflict which exist for their protection. There are two principles of particular relevance in this respect. First, it is a well established principle of customary international law that the civilian population and individual civilians are not a legitimate target in their own right. The parties to an armed conflict are required to discriminate between civilians and civilian objects on the one hand and combatants and military objectives on the other hand and to direct their attacks only against the latter …
3.68 … Modern nuclear weapons are capable of far more precise targeting and can therefore be directed against specific military objectives without the indiscriminate effect on the civilian population which the older literature assumed to be inevitable. 
United Kingdom, Written statement submitted to the ICJ, Nuclear Weapons case, 16 June 1995, p. 52, §§ 3.67–3.68.