Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 33. Personnel and Objects Involved in a Peacekeeping Mission
The UK LOAC Manual (2004) states:
[W]here a United Nations force or other PSO [peace support operation] force is not engaged as a party to an armed conflict, its personnel and equipment would not constitute a military objective and attacks on them will therefore be unlawful. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 14.15.
With regard to internal armed conflict, the manual states:
It is prohibited to attack the “personnel, installations, material, units or vehicles involved in a humanitarian assistance or peacekeeping mission in accordance with the Charter of the United Nations, so long as they are entitled to the protection given to civilians or civilian objects under the international law of armed conflict”. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 15.27.
The UK UN Personnel Act (1997) provides:
If a person does outside the UK, any act to or in relation to a UN worker which, if he had done it in any part of the UK, would have made him guilty of [murder, manslaughter, culpable homicide, rape, assault causing injury, kidnapping, abduction or false imprisonment], he shall in that part of the UK be guilty of that offence. 
United Kingdom, UN Personnel Act, 1997, Article 1.
The Act contains a similar provision for the prosecution of threats of attacks against UN workers, within or outside the United Kingdom; attacks against UN vehicles and premises committed outside the United Kingdom; and threats of attacks against UN vehicles and premises, within or outside the United Kingdom. 
United Kingdom, UN Personnel Act, 1997, Articles 2 and 3.
Within the framework of this Act, members of the military component of a UN operation engaged or deployed by the UN Secretary-General are UN workers. 
United Kingdom, UN Personnel Act, 1997, Article 4(1)(a).
Under the UK ICC Act (2001), it is a punishable offence to commit a war crime as defined in Article 8(2)(b)(iii) and (e)(iii) 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).
In 1995, during a debate in the UN Security Council, the United Kingdom condemned attacks against UNPROFOR. 
United Kingdom, Statement before the UN Security Council, UN Doc. S/PV.3553, 12 July 1995, p. 11.
In 1996, during a debate in the UN Security Council concerning the situation in Liberia, the United Kingdom expressed deep regret at the loss of life among ECOMOG forces and outrage that peacekeeping forces were subjected to attacks. 
United Kingdom, Statement before the UN Security Council, UN Doc. S/PV.3621, 25 January 1996, p. 19.