Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 135. Children
Section D. Death penalty on children
The UK Military Manual (1958) provides: “In any case, the death penalty may not be pronounced against a protected person who was under 18 years of age at the time of the offence.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 566(c).
The UK LOAC Manual (2004) states:
Death penalties for offences related to the armed conflict are not to be carried out on those aged under eighteen years at the time of the offence. In any event, United Kingdom courts may not impose death sentences. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 9.9.2; see also § 16.48 (enforcement of the law of armed conflict).