Соответствующая норма
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 145. Reprisals
Section B. Reasonable notice and measure of last resort
The UK Military Manual (1958) states:
An infraction of the laws of war having been definitely established, every effort should first be made to detect and punish the actual offenders. Only if this is impossible may recourse be had to reprisals, if the injured belligerent is of the opinion that the facts warrant them. As a rule, the injured party must not at once resort to reprisals, but must first lodge a complaint with the enemy (or with a neutral Power, for transmission to the enemy) with a view to preventing any repetition of the offence and to securing the punishment of the guilty. This course should always be pursued unless the safety of the troops requires immediate drastic action and the persons who actually committed the offences cannot be secured. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 646.
The UK LOAC Pamphlet (1981) states that reprisals can only be taken if “prior warning is given”. 
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(b).
The UK LOAC Manual (2004) states:
16.17. In order to qualify as a legitimate reprisal, an act must comply with the following conditions when employed:
b. It must be for the purpose of compelling the adversary to observe the law of armed conflict. Reprisals serve as an ultimate legal sanction or law enforcement mechanism. Thus, if one party to an armed conflict breaches the law but then expresses regret, declares that it will not be repeated and takes measures to punish those immediately responsible, then any action taken by another party in response to the original unlawful act cannot be justified as a reprisal;
c. Reasonable notice must be given that reprisals will be taken. What degree of notice is required will depend upon the particular circumstances of the case;
d. The victim of a violation must first exhaust other reasonable means of securing compliance before reprisals can be justified. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.17.
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.
In a written statement submitted to the ICJ in the Nuclear Weapons case in 1995, the United Kingdom stated: “To be lawful, a belligerent reprisal must meet two conditions … It must meet the criteria for the regulation of reprisals, namely that it is … a means of last resort.” 
United Kingdom, Written statement submitted to the ICJ, Nuclear Weapons case, 16 June 1995, p. 58.
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 this would be true “only after [a] formal warning to the adverse party requiring cessation of the violations has been disregarded”. 
United Kingdom, Reservations and declarations made upon ratification of the 1977 Additional Protocol I, 28 January 1998, § (m).