Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 159. Amnesty
The UK Military Manual (1958) states:
Having regard to the duty of belligerents to try those who have committed grave breaches of the 1949 [Geneva] Conventions, it may now be open to doubt whether a treaty of peace would operate, as was often the case in the past, as an amnesty. It is, on the other hand, open to two or more belligerents to agree in a peace treaty, or even in a general armistice, that no further war crimes trials will be instituted by them after a certain agreed date or as from the date of the treaty of the armistice. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 641, footnote 1.
The UK LOAC Manual (2004) states with regard to internal armed conflicts in which the 1977 Additional Protocol II is applicable:
At the end of hostilities, the authorities in power, whoever they might be, “shall endeavour to grant the broadest possible amnesty to persons who have participated in the armed conflict, or those deprived of their liberty for reasons related to the armed conflict, whether they are interned or detained”. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 15.42.
The UK Military Manual (1958) states:
Having regard to the duty of belligerents to try those who have committed grave breaches of the 1949 [Geneva] Conventions, it may now be open to doubt whether a treaty of peace would operate, as was often the case in the past, as an amnesty. It is, on the other hand, open to two or more belligerents to agree in a peace treaty, or even in a general armistice, that no further war crimes trials will be instituted by them after a certain agreed date or as from the date of the treaty of the armistice. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 641, footnote 1.
The UK LOAC Manual (2004) states:
Convicted offenders, whether prisoners of war or civilians, serving sentences for war crimes have no right to release at the cessation of hostilities. This is a matter that should be dealt with in peace treaties. These should also deal with the disposal of cases involving persons accused of war crimes who were not charged or dealt with before cessation of hostilities. Any amnesty must be considered of doubtful effect as, although it would be binding as between parties to the treaty, it would not bind other states or the International Criminal Court, particularly in respect of grave breaches of the Geneva Conventions or of Additional Protocol I. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.50.
With regard to internal armed conflicts in which the 1977 Additional Protocol II is applicable, the manual states:
At the end of hostilities, the authorities in power, whoever they might be, “shall endeavour to grant the broadest possible amnesty to persons who have participated in the armed conflict, or those deprived of their liberty for reasons related to the armed conflict, whether they are interned or detained”. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 15.42.
The manual explains: “The wording [of Article 6(5) of the 1977 Additional Protocol II] is vague. It is understood to relate to offences under domestic law but not to those guilty of crimes under international law.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 15.42, footnote 97.