Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 12. Definition of Indiscriminate Attacks
Section A. Attacks which are not directed at a specific military objective
The UK LOAC Pamphlet (1981) defines indiscriminate attacks as “attacks which are not directed at a military objective and which are likely to strike at military objectives and civilian objects without distinction”. 
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:
(a) “those which are not 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(a); see also §§ 12.22 (air operations) and 13.28 (maritime warfare).
In 1991, in a report submitted to the UN Security Council on operations in the Gulf War, the United Kingdom criticized Iraq for launching indiscriminate missile attacks against 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.