Règle correspondante
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 128. Release and Return of Persons Deprived of Their Liberty
Section C. Exchange of prisoners
The UK Military Manual (1958) provides:
The exchange of prisoners of war is nowadays rare. The rule generally observed is to exchange man for man and rank for rank, with due allowance if titles of ranks or grades differ or if there is no exact equivalent. A condition is often made that the men exchanged shall not participate as soldiers in the war – in fact they are paroled. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 249.
The manual further specifies:
The exchange of prisoners may be carried out by means of so-called “cartels”. Nothing more is required than a simple statement agreed by the commanders, such agreement being arrived at by parlementaires, that is, negotiations conducted during truce, or by the exchange of letters. But for exchanges on a large scale commissioners are usually appointed, and commanders ought not as a rule in such cases to act without having previously reported to their government and taken instructions. In modern war between civilised States, an exchange of prisoners will rarely be carried out except by agreement between the governments concerned. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 250.
The UK LOAC Manual (2004) states:
8.146. [The exchange of prisoners of war during hostilities] is not a matter that is dealt with in the [the 1949 Geneva Convention III] but it is allowed by the customary law of armed conflict on such terms as may be agreed between the states concerned. The practice generally observed is to exchange soldier for soldier and rank for rank, with due allowance for differences in titles of ranks or grades. The agreement between the parties, sometimes known as a cartel, may lay down other conditions, for example, that the soldiers concerned do not take any further active role in the conflict. The agreement may be negotiated during a truce by opposing commanders, or by exchange of letters between belligerent states, often through the intermediary of a neutral state.
8.146.1. Nowadays such an exchange would rarely be carried out except by agreement at government level between the parties concerned and it is likely that the assistance of a protecting power or the ICRC would be sought. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 8.146–8.146.1.