Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 21. Target Selection
The UK LOAC Manual (2004) states: “Where there is a choice between different military objectives whose attack will yield the same military advantage, the one whose attack is expected to cause the least incidental damage should be chosen.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.32.