Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 19. Control during the Execution of Attacks
The UK LOAC Pamphlet (1981) states: “If the resulting loss or damage would be excessive in relation to the concrete and direct military advantage expected, the operation must be cancelled or suspended.” 
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. 13, § 4(b).
The UK LOAC Manual (2004) states: “There is the duty to cancel or suspend attacks if the incidental damage may be expected to be disproportionate to the military advantage anticipated.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.32.7.
In its chapter on maritime warfare, the manual provides that an attack shall be cancelled or suspended as soon as it becomes apparent that the collateral casualties or damage would be excessive.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 13.32.
With regard to internal armed conflict, the manual states:
15.22. In planning or carrying out attacks, precautions must be taken to limit attacks to military objectives and to minimize incidental loss or damage.
15.22.1. The need to take precautions can be inferred from the principle of proportionality and the principle of distinction, which require some care to be taken in the planning and execution of an attack. Attacks must be cancelled, suspended or re-planned if the rule in paragraph 15.21 [sic] cannot be complied with. The same applies in sieges. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 15.22–15.22.1.
In 1991, in a report submitted to the UN Security Council on operations in the Gulf War, the United Kingdom stated that on a number of occasions attacks had not been “pressed home” because pilots were not completely satisfied that the order to avoid damage to sites of religious or cultural significance would be met. 
United Kingdom, Letter dated 13 February 1991 to the President of the UN Security Council, UN Doc. S/22218, 13 February 1991, p. 1.