Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 28. Medical Units
The UK Military Manual (1958) restates Article 27 of the 1907 Hague Regulations. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, §§ 290 and 300.
The manual notes, however:
Accusations have frequently been made that the rule concerning immunity of hospitals has been deliberately disregarded during a siege. The complaints were often due to the fact that buildings used for medical purposes were scattered over the town and that they were thus liable to be struck by chance of erratic shots. It is therefore desirable that the sick and wounded should, if possible, be concentrated in one quarter, remote from the defences and the defending troops, or by arrangement with the besieger, in neutralised grounds. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 304.
The manual further states: “It is forbidden to attack civilian hospitals.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 30.
The manual also states: “Fixed medical establishments (hospitals) and mobile units of the medical service may in no circumstances be attacked. They must at all times be respected and protected.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 351.
The manual states: “In addition to the ‘grave breaches’ of the 1949 [Geneva] Conventions … the following are examples of punishable violations of the laws of war, or war crimes: … (o) bombardment of hospitals and other privileged buildings”. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 626(o).
The manual restates the rules on loss of protection of medical units and civilian hospitals set out in Articles 21–22 of the 1949 Geneva Convention I and Article 19 of the 1949 Geneva Convention IV respectively. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, §§ 31 and 352.
The manual further states: “In addition to the ‘grave breaches’ of the 1949 [Geneva] Conventions … the following are examples of punishable violations of the laws of war, or war crimes: … (h) improper use of a privileged building for military purposes”. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 626(h).
The UK LOAC Pamphlet (1981) provides: “Protection from attack is given to fixed and mobile medical units … Medical units can be military or civilian and include medical depots and pharmaceutical stores as well as hospitals and treatment centres.” 
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 6, p. 23, § 8.
The pamphlet further states that medical units and transports “must not take part in hostilities and if they do it may result in protection being forfeited”. 
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 6, p. 23, § 8.
The UK LOAC Manual (2004) states:
7.10. Medical units are “establishments and other units, whether military or civilian, organized for medical purposes, namely the search for, collection, transportation, diagnosis or treatment – including first-aid treatment – of the wounded, sick and shipwrecked, or for the prevention of disease. The term includes, for example, hospitals and other similar units, blood transfusion centres, preventive medicine centres and institutes, medical depots and the medical and pharmaceutical stores of such units. Medical units may be fixed or mobile, permanent or temporary.”
7.10.1. It follows that the definition of medical units is sufficiently comprehensive to include those on land, sea and in the air. It includes civilian medical units provided that they (a) belong to one of the parties to the conflict; (b) are recognized and authorized by the competent authority of one of the parties to the conflict; or (c) while belonging to a neutral or other state not a party to the conflict, or a recognized and authorized aid society of such a state or an impartial international humanitarian organization, have been made available to a party to the conflict for humanitarian purposes.
Protection of medical units
7.13. Medical units are to be “respected and protected at all times and shall not be the object of attack.” Medical units, personnel and transport must not be used for non-medical purposes, otherwise their protection will be jeopardized. In addition, medical units must not “be used in an attempt to shield military objectives from attack” and, where possible, should be so sited that attacks against military objectives do not imperil their safety. The improper use of medical units to kill, injure or capture the enemy amounts to the war crime of perfidy.
7.13.1 Medical units are given this general protection to enable them to perform their humanitarian functions. Thus, the protection given to medical units ceases if “they are used to commit, outside their humanitarian function, acts harmful to the enemy”. Protection may only be withdrawn, however, after due warning has been given, setting, whenever appropriate, a reasonable time limit and after such warning has remained unheeded. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 7.10–7.10.1 and 7.13–7.13.1.
With regard to internal armed conflicts, the manual states:
It is prohibited to attack … medical buildings, including hospitals and places where the wounded and sick are cared for, medical material, medical units, and transport … Medical units, personnel and transports must be protected from attack and are entitled to display the protective emblem of the red cross or red crescent on a white background.
Medical units and transports “shall be respected and protected at all times and shall not be the object of attack”. However this protection may cease if “they are used to commit hostile acts, outside their humanitarian function”. Even then, the protection will only cease “after a warning has been given setting, whenever appropriate, a reasonable time limit, and after such warning has remained unheeded”. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 15.29–15.29.1 and 15.47.
Lastly, in its chapter on enforcement of the law of armed conflict, the manual refers to the “use of a privileged building for improper purposes” and “attacking a privileged or protected building” as war crimes “traditionally recognized by the customary law of armed conflict”. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.29.
Under the UK ICC Act (2001), it is a punishable offence to commit a war crime as defined in Article 8(2)(b)(ix) and (e)(iv) of the 1998 ICC Statute. 
United Kingdom, ICC Act, 2001, Sections 50(1) and 51(1) (England and Wales) and Section 58(1) (Northern Ireland).
At the CDDH, the United Kingdom welcomed “the humanitarian advances made in such fields as … the extension of protection to a wider group of medical units”. 
United Kingdom, Statement at the CDDH, Official Records, Vol. VII, CDDH/SR.58, 9 June 1977, p. 302, § 114.