Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 140. The Principle of Reciprocity
The UK Military Manual (1958) provides: “A belligerent is not justified in declaring itself freed altogether from the obligation to observe the laws of war or any of them on account of their suspected or ascertained violation by his adversary.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 121.
On 21 January 1991, in the context of the Gulf War, the UK Minister of Foreign Affairs summoned the Iraqi ambassador to discuss Iraq’s obligations under international law. According to a statement by an Foreign and Commonwealth Office spokesperson following the meeting, “the Ambassador said that Iraq would abide by the Convention and treat POWs [prisoners of war] well if the Allies avoided civilian targets. Mr Hogg said that we expected unconditional observance of the requirements of the Convention.” 
United Kingdom, Statement by Foreign and Commonwealth Office spokesperson, 21 January 1991, BYIL, Vol. 62, 1991, p. 680.