Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 159. Amnesty
Section A. Amnesty for participation in non-international armed conflicts
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.