Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 30. Persons and Objects Displaying the Distinctive Emblem
The UK LOAC Pamphlet (1981) provides that “persons, units or establishments displaying either sign [red cross or red crescent] are protected from attack”. 
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 4, p. 16, § 9.
The UK LOAC Manual (2004) refers in its chapter on enforcement of the law of armed conflict to “attacking a properly marked hospital ship or medical aircraft” as a war crime “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)(xxiv) and (e)(ii) 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).