Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 146. Reprisals against Protected Persons
Section A. Captured combatants and prisoners of war
The UK Military Manual (1958) states:
Measures of reprisal against prisoners of war are prohibited. This prohibition is of an absolute character, so that reprisals against prisoners of war are prohibited even if intended to be adopted as a measure of retaliation against the violation of provisions of [the 1949 Geneva Convention III] by the other party. Reprisals against such violations of the Convention are permissible, but they must not be directed against prisoners of war or any other persons protected by the 1949 Conventions. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 133(b) and footnotes 4 and 5.
In the part dealing with reprisals, the manual refers to Article 13 of the 1949 Geneva Convention III and states: “Reprisals against prisoners of war … are … prohibited.” In a footnote relating to this provision, the manual further notes: “Reprisals are unlawful against all persons except enemy combatants and those few classes of civilians who are not protected persons.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 644 and footnote 2.
In a footnote relating to another provision, the manual states: “Reprisals against … prisoners of war … constitute war crimes.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 647, footnote 1.
The UK LOAC Pamphlet (1981) provides: “The [1949] Geneva Conventions and [the 1977 Additional Protocol I] prohibit reprisals against prisoners of war.” 
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, § 16.
The UK LOAC Manual (2004) states: “Reprisals can never lawfully be taken against prisoners of war.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 8.29; see also § 16.18.
In its 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. First, it must not be directed against persons or objects against which the taking of reprisals is specifically prohibited … The Geneva Conventions of 1949 prohibit the taking of reprisals against persons or objects protected by the Conventions. 
United Kingdom, Written statement submitted to the ICJ, Nuclear Weapons case, 16 June 1995, pp. 58–59.