Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 18. Assessment of the Effects of Attacks
The UK LOAC Manual (2004) provides that “those who plan or decide upon an attack” shall
refrain from deciding to launch 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. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.32; see also § 13.32 (maritime warfare).
In 1991, in reply to a question in the House of Lords concerning the use of conventional weapons against nuclear facilities, chemical weapons plants and dumps, and petrochemical enterprises situated in towns or cities, the UK Minister of State, Foreign and Commonwealth Office, stated:
International law requires that, in planning an attack on any military objective, account is taken of certain principles. These include the [principle] … that an attack should not be launched if it can be expected to cause civilian losses which would be disproportionate to the military advantage expected from the attack as a whole. 
United Kingdom, House of Lords, Statement by the Minister of State, Foreign and Commonwealth Office, 4 February 1991, Hansard, Vol. 525, Written Answers, col. 37.
In 1991, in response to a question in the Defence Committee of the UK House of Commons on whether or not there were occasions during the Gulf War when he decided that it would not be appropriate for the Royal Air Force to attack a particular target, Air Vice Marshal Wratten stated:
Yes, there were such occasions. In particular, when we were experiencing collateral damage, such as it was, and some of the targets were in locations where with any weapon system malfunction severe collateral damage would have resulted inevitably, then there were one or two occasions that I chose not to go against those targets, but they were very few and far between and they were not – and this is the most important issue – in my judgment and in the judgment of the Americans of a critical nature, that is to say, they were not fundamental to the timely achievement of the victory. Had that been the case, then regrettably, irrespective of what collateral damage might have resulted, one would have been responsible and had a responsibility for accepting those targets and for going against them. 
United Kingdom, Statement of Air Vice Marshall Wratten, Minutes of Evidence taken before the Defence Committee, 22 May 1991, p. 38, § 274.