Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 25. Medical Personnel
Section B. Equipment of medical personnel with light individual weapons
The UK Military Manual (1958) lists among the conditions not depriving hospitals and mobile medical units of their protection the fact that “the personnel are armed, and use their arms for their own defence or for the defence of the wounded and sick”. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 352.
The UK LOAC Pamphlet (1981) provides: “Medical personnel may carry and use small arms for their self-defence and for the defence of the wounded and sick in their care.” 
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, § 9(b).
The UK LOAC Manual (2004) states:
7.15. Medical personnel may be equipped with “light individual weapons for their own defence or for that of the wounded and sick in their charge.”
7.15.1. Light individual weapons are those that can be handled and fired by one person and primarily intended for personnel targets. It follows that medical personnel may be armed with sub-machine guns, self-loading rifles and handguns. It should be stressed that the provision and use of these arms must be merely for defensive purposes as outlined above. Medical personnel (and chaplains) are non-combatants so they are not otherwise entitled to take part in hostilities. Medical personnel may use their weapons only if they, or those in their care, are attacked. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 7.15–7.15.1.
The UK LOAC Manual (2004), as amended in 2010, states: “Medical personnel do not forfeit their protection under Geneva Convention I 1949 by being armed with light individual weapons and by using those arms in their own defence or in the defence of the wounded and sick in their charge.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, as amended by Amendment 3, Ministry of Defence, September 2010, § 4.2.3.
The Report on UK Practice refers to a letter from an army lawyer who, after consultation with the medical-legal department, confirmed that medical personnel may carry a weapon for the purposes of self-defence and defence of their patients only. He also noted that, during the Gulf War, a certain commander of a field hospital would not allow any weapons at all within the hospital confines, even for self-defence. 
Report on UK Practice, 1997, Letter from an army lawyer, 3 March 1998, Chapter 2.7.