Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 12. Definition of Indiscriminate Attacks
Section B. Attacks which cannot be directed at a specific military objective
The UK LOAC Manual (2004) states:
Indiscriminate attacks are:
(b) “those which employ a method or means of combat which cannot be directed at a specific military objective”;
“and consequently, in each such case, are of a nature to strike military objectives and civilians or civilian objects without distinction.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.23.1; see also §§ 12.22 (air operations) and 13.28 (maritime warfare).
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.