Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 100. Fair Trial Guarantees
Section B. Trial by an independent, impartial and regularly constituted court
The UK Military Manual (1958) states: “In no circumstances whatsoever may [prisoners of war] be tried by a court which does not afford the essential guarantees of independence and impartiality.” It explains that prisoners of war, under certain conditions, may be tried by civil courts and that “such courts must in any case comply with the requirement of independence and impartiality”. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 202.
The UK LOAC Manual (2004) states in its chapter on the protection of civilians in the hands of a party to the conflict:
No sentence may be passed and no penalty executed “except pursuant to a conviction pronounced by an impartial and regularly constituted court respecting the generally recognized principles of regular judicial procedure”. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 9.6.
In its discussion on punishment of prisoners of war, the manual states:
Where there are to be judicial proceedings, prisoners of war are normally to be tried by military courts. If the law of the detaining power permits the trial by civil court of members of its own armed forces for particular offences, the civil courts may try prisoners of war under the same conditions. However, trial may only take place if the court is independent and impartial … 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 8.118.
In its chapter on internal armed conflict, the manual restates the provisions of common Article 3 of the 1949 Geneva Conventions:
Under the terms of Common Article 3, the parties to a non-international armed conflict occurring in the territory of a party to the Conventions are obliged to apply “as a minimum”, the following provisions:
(1) Persons taking no active part in the hostilities, including members of armed forces who have laid down their arms and those placed hors de combat by sickness, wounds, detention, or any other cause, shall in all circumstances be treated humanely, without any adverse distinction founded on race, colour, religion or faith, sex, birth or wealth, or any other similar criteria.
To this end, the following acts are and shall remain prohibited at any time and in any place whatsoever with respect to the above-mentioned persons:
(d) the passing of sentences and the carrying out of executions without previous judgment pronounced by a regularly constituted court affording all the judicial guarantees which are recognized as indispensable by civilized peoples. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 15.4.
In 2004, during a debate in the House of Commons, the UK Parliamentary Under-Secretary of State for Foreign and Commonwealth Affairs stated regarding the trial of UK citizens before a US military commission:
[T]he view of the Attorney-General was that the military commission, as constituted, would not provide the type of process that we would afford to British nationals. We have been attempting to ensure that the legal process will respect the human rights of the detainees, including the right to a fair and public hearing by a competent, independent and impartial tribunal, established by law. 
United Kingdom, House of Commons, Statement by the Parliamentary Under-Secretary of State for Foreign and Commonwealth Affairs, Hansard, 24 March 2004, Vol. 419, Debates, col. 307WH.
In 2010, in its closing submissions to the public inquiry into the circumstances surrounding the death of Baha Mousa and the treatment of those detained with him by UK armed forces in Iraq in 2003, the UK Ministry of Defence stated regarding common Article 3 of the 1949 Geneva Conventions: “On its face this protection is restricted to armed conflicts not of an international character. However, it is understood to apply in all forms of armed conflict as part of customary international law to set out the irreducible minimum standard.” 
United Kingdom, Ministry of Defence, Closing Submissions to the Baha Mousa Public Inquiry on Modules 1–3, 25 June 2010, § 10.2, p. 10.