Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 77. Expanding Bullets
The UK Military Manual (1958) states that the United Kingdom engages “to abstain from the use of bullets with a hard envelope which does not entirely cover the core, or is pierced with incisions”. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 109.
The manual further notes: “It is expressly forbidden to employ arms, projectiles or material calculated to cause unnecessary suffering … such … as … irregularly-shaped bullets.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 110.
The manual also provides: “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: … using expanding bullets.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 626(g).
The UK LOAC Pamphlet (1981) states: “The following are prohibited in international armed conflict: … b. dum-dum bullets”. 
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 5, p. 20, § 1(b).
(emphasis in original)
The UK LOAC Manual (2004) states in its chapter on weapons:
6.9. It is prohibited to use in international armed conflicts “bullets which expand or flatten easily in the human body, such as bullets with a hard envelope which does not entirely cover the core or is pierced with incisions”.
6.9.1. This prohibition is aimed at soft-nosed bullets that mushroom on impact or bullets whose casing is designed to fragment on impact causing, in either case, unnecessarily serious injuries. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 6.9–6.9.1.
Under the UK ICC Act (2001), it is a punishable offence to commit a war crime as defined in Article 8(2)(b)(xix) 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).