Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 145. Reprisals
Section D. Order at the highest authority of government
The UK Military Manual (1958) states: “Although there is no clear rule of international law on the matter, reprisals should be resorted to only by order of a commander and never on the responsibility of an individual soldier.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 645.
The UK LOAC Pamphlet (1981) states that reprisals can only be taken if “they are ordered at a high level”. For cases in which the United Kingdom should have recourse to reprisals against the enemy’s civilian population or civilian objects, the manual states that “the decision to do so will be taken at Government level”. 
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. 17, §§ 14(e) and 17.
The UK LOAC Manual (2004) states: “As reprisals entail state responsibility, they must only be authorized at the highest level of government.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.17; see also § 5.18.
The manual also restates the interpretative declaration made by the UK upon ratification of the 1977 Additional Protocol I (see infra). 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.19.1.
The manual explains:
This means that reprisals taken in accordance with the statement are permissible by and against the United Kingdom. However, commanders and commanders-in-chief are not to take reprisal action on their own initiative. Requests for authority to take reprisal action must be submitted to the Ministry of Defence and require clearance at Cabinet level. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.19.2.
Upon ratification of the 1977 Additional Protocol I, the United Kingdom stated that in the event of violations of Articles 51–55 of the 1977 Additional Protocol I by the adversary, the United Kingdom would regard itself entitled to take measures otherwise prohibited by these Articles, noting, however, that such measures would be taken “only after a decision taken at the highest level of government”. 
United Kingdom, Reservations and declarations made upon ratification of the 1977 Additional Protocol I, 28 January 1998, § (m).