Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 39. Use of Cultural Property for Military Purposes
The UK Military Manual (1958) 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 Manual (2004) states:
5.25. It is prohibited:
b. to use [historic monuments, works of art or places of worship which constitute the cultural or spiritual heritage of peoples] in support of the military effort;
5.25.1 In general, all civilian objects are protected. However, the protection of cultural property and places of worship is given special emphasis in the law of armed conflict, both in respect of protection from attack and in respect of discouraging their use for military purposes, so that the need to attack such property does not arise.
5.25.2. Because of its wording, the prohibition in paragraph 5.25 only applies to very important cultural property of international stature. It is not clear whether “works of art” includes scientific collections or libraries, but the prohibition certainly applies to more than just buildings and would cover cultural or archaeological sites. Property loses its protection if it is used for military purposes. The protection is against all acts of hostility, not just “attacks”. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 5.25–5.25.2.
The manual further states:
The prohibition in the Protocol of the use of cultural property for military purposes is important because if it is not so used, there is no need to attack it. Thus a church tower or mosque minaret should not be used as a military observation post. There may be rare cases where it is essential to use cultural property for military purposes, for example, an historic bridge that is the only available river crossing. These cases only occur where there is no choice possible between the use of cultural property for military purposes and another feasible method for obtaining a similar military advantage. The main advantage of having special rules for cultural property is in making attacking commanders more aware of the existence of cultural property, especially if it is marked or contained in a published list which is available to them. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.25.3.
[emphasis in original]
With regard to the 1954 Hague Convention for the Protection of Cultural Property, the manual states: “The detailed rules of the Convention do not apply to states not party to it, but the general principles of immunity of cultural property … do apply to those states.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, p. 71, fn. 114.
In relation to internal armed conflict, the manual states: “[T]he law also prohibits … the use of cultural property for purposes which are likely to expose it to destruction or damage in armed conflict, unless there is no feasible alternative to such use.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 15.18.
With regard to internal armed conflicts in which the 1977 Additional Protocol II is applicable, the manual further specifies: “It is prohibited ‘…to use [historic monuments, works of art or places of worship which constitute the cultural or spiritual heritage of peoples] in support of the military effort’.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 15.51.–15.51.1.
In its chapter on enforcement of the law of armed conflict, the manual refers to the “use of a privileged building for improper purposes” 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.
At the CDDH, the United Kingdom declared, with respect to Article 47 bis of the draft Additional Protocol I (now Article 53): “If these objects are unlawfully used for military purposes, they will thereby lose effective protection as a result of attacks directed against such unlawful military uses.” 
United Kingdom, Statement at the CDDH, Official Records, Vol. VI, CDDH/SR.42, 27 May 1977, p. 238.
(emphasis added)
Upon signature and upon ratification of the 1977 Additional Protocol I, the United Kingdom stated, in relation to Article 53: “If the objects protected by this Article are unlawfully used for military purposes they will thereby lose protection from attacks directed against such unlawful military uses.” 
United Kingdom, Declarations made upon signature of the 1977 Additional Protocol I, 12 December 1977, § g; Reservations and declarations made upon ratification of the 1977 Additional Protocol I, 28 January 1998, § k.
(emphasis added)
In 2003, during a debate in the House of Commons, the UK Secretary of State for Defence stated:
Despite its protestations to the contrary, the Iraqi regime shows no greater respect for the country’s cultural wealth than for its people. The coalition is taking every precaution to avoid damage to the holy sites in Najaf and Karbala. By contrast, we know that Saddam Hussein has plans to damage the sites and blame the coalition. Indeed, his forces have used the site at Najaf as a defensive position, firing on United States forces, who commendably did not return fire. 
United Kingdom, House of Commons, Statement by the Secretary of State for Defence, Hansard, 3 April 2003, Vol. 402, Debates, cols. 1069–1070.