Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 125. Correspondence of Persons Deprived of Their Liberty
The UK Military Manual (1958) states:
Internees must be permitted to send and receive letters and postcards. If the Detaining Power deems it necessary to impose limitations, the number permitted must not be less than two letters and four cards monthly. Letters and cards must be conveyed with reasonable dispatch and must not be held up as a disciplinary measure. Internees who have been a long time without news or cannot obtain news from their relatives and those who are a long distance from their homes must be allowed to send telegrams at their own expense. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 67.
The manual further specifies:
Prisoners of war must be allowed to send and receive letters and cards. In addition to the capture card, they must be allowed to send at least two letters and four cards every month. Limitations on correspondence addressed to prisoners of war may be imposed only by the state on which they depend. The Detaining Power may request such a limitation. All correspondence must be conveyed as rapidly as possible, and must not be delayed or retained for disciplinary reasons. Prisoners of war who have not been in touch with their families for a long time or who are at a great distance from their homes must be allowed to send telegrams at their own expense. The same applies in case of emergency. As a general rule they must be permitted to correspond in their native language. Bags containing prisoner of war mail must be labelled as such, sealed and addressed to offices of destination. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 189.
In addition, the manual provides: “Prisoners undergoing disciplinary punishment must be allowed to read and write and to send and receive letters.” 
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, § 219.
The UK LOAC Pamphlet (1981) provides: “PW [prisoners of war] must be allowed to send a capture card to the Protecting Power and to their next of kin no later than the time of their arrival in a PW camp.” 
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 8, p. 31, § 16(f).
The UK LOAC Manual (2004) states in its chapter on the protection of civilians in the hands of a party to the conflict, when discussing conditions of internment:
9.45. Immediately on internment, or within one week of arrival at an internment camp, each internee must be allowed to write one card to his family and another to the Central Information Agency informing them of his internment, address and state of health. This also applies in the event of sickness, admission to hospital or transfer to another camp. Internment cards must be forwarded as rapidly as possible.
9.61. Internees must be permitted to send letters and cards. If the detaining power deems it necessary to impose limitations, the number permitted must not be less than two letters and four cards each month. This is in addition to the internment card. Letters and cards must be conveyed with reasonable despatch and must not be held up as a disciplinary measure. Internees, in urgent cases, or who have been a long time without news or cannot obtain news from their relatives as well as those who are a long distance from their homes, must be allowed to send telegrams at their own expense. States are under a duty to reduce telegram charges as much as possible. Correspondence is normally to be in the internee’s native language unless the detaining power authorizes correspondence in another language.
9.62. Internees must be allowed to receive all letters and cards addressed to them. Restrictions may only be imposed by the power on which they depend though this can be at the request of the detaining power. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 9.45 and 9.61–9.62.
On prisoners of war, the manual states:
Immediately on capture, or within one week of arrival at a camp, each prisoner of war must be allowed to write one card to his family and another to the Central Prisoners of War Agency informing them of his capture, address and state of health, see the illustration in [appendix]. This also applies in cases of sickness, removal to hospital or transfer to another camp. The cards must be forwarded as rapidly as possible. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 8.42.
The manual adds:
Prisoners of war must be allowed to send, in addition to the capture cards, at least two letters and four postcards every month. This quota does not include complaints, correspondence between prisoners’ representatives at labour detachments and those in the main camp, and official correspondence by medical officers and chaplains … Prisoners of war must be allowed to receive all letters and cards addressed to them … All correspondence must be conveyed as rapidly as possible and must not be delayed or retained for disciplinary reasons. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 8.62.–8.64.
The manual further states: “All prisoners of war sentenced to confinement have the rights to … receive and send correspondence.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 8.144.
With regard to internal armed conflicts in which the 1977 Additional Protocol II is applicable, the manual provides:
Those responsible for the internment or detention are also placed under obligation “within the limits of their capabilities” to “respect” some further provisions relating to persons interned or detained for reasons relating to the armed conflict. These are:
b. that all persons whose liberty has been restricted in any way whatsoever for reasons related to the armed conflict shall be allowed to send and receive letters and cards, the number of which may be limited by the competent authority if it deems necessary. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 15.41.
In 2003, in a reply to a written question in the House of Commons, the UK Minister of State for the Armed Forces, Ministry of Defence, wrote:
In accordance with Article 70 of the Geneva Convention relative to the treatment of prisoners of war, upon capture, and subsequently following any change in location, every prisoner of war is given the opportunity to write direct to his or her family, and through the completion of a Capture Card, to the Central Prisoners of War Agency in Geneva. Prisoner of war details are given to the International Committee for the Red Cross who have the responsibility for the distribution of letters, parcels and Capture Cards. 
United Kingdom, House of Commons, Written answer by the Minister of State for the Armed Forces, Ministry of Defence, Hansard, 14 April 2003, Vol. 403, Written Answers, col. 572W.
In 2003, during a debate in the House of Lords, the UK Minister of State, Foreign and Commonwealth Office, stated:
We have made representations to the United States Government in regard to the physical conditions under which the individuals are held – for example, the inadequate facilities for exercise and the inadequate facilities for contact with their families. Representatives of the International Red Cross have visited Guantanamo Bay and are giving their advice to the United States Government. 
United Kingdom, House of Lords, Statement by the Minister of State, Foreign and Commonwealth Office, Hansard, 17 July 2003, Vol. 651, Debates, col. 967.