Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 70. Weapons of a Nature to Cause Superfluous Injury or Unnecessary Suffering
The UK Military Manual (1958) stresses:
It is expressly forbidden to employ arms, projectiles or material calculated to cause unnecessary suffering. Under this heading may be included such weapons as lances with a barbed head, irregularly-shaped bullets, projectiles filled with broken glass, and the like. The scoring of the surface of bullets, the filing off of the end of their hard case, and the smearing on them of any substance likely to inflame a wound, are also prohibited.
The prohibition is not, however, intended to apply to the use of explosives contained in mines, aerial torpedoes and hand-grenades. The use of flame throwers and napalm bombs when directed against military targets is lawful. However, their use against personnel is contrary to the law of war in so far as it is calculated to cause unnecessary suffering. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 110 and footnote 1.
The UK LOAC Pamphlet (1981) states: “The following are prohibited in international armed conflict: … d. arms, projectiles or material intended to cause excessive injury or suffering.” 
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(d).
[emphasis in original]
In its chapter on weapons, the UK LOAC Manual (2004) states the following as a guiding principle: “It is prohibited to employ weapons, projectiles and material and methods of warfare of a nature to cause superfluous injury or unnecessary suffering.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 6.1; see also § 13.28 (maritime warfare).
On the application of the guiding principle, the manual states:
6.2. The correct criterion is whether the use of a weapon is of a nature to cause injury or suffering greater than that required for its military purpose.
6.2.1. In deciding the legality of use of a specific weapon, therefore, it is necessary to assess:
a. its effects in battle,
b. the military task it is required to perform, and
c. the proportionality between factors (a) and (b).
6.2.2. However, even if the use of a weapon is considered under this test to be generally lawful, its use in certain ways, or in certain circumstances, may still be unlawful. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 6.2–6.2.2
The manual further states:
Weapons use is restricted by international conventions, protocols and declarations and by the customary law of armed conflict. Moreover, there are the compelling dictates of humanity, morality and civilization to be taken into account. The idea of humanity is expressed in the preamble to the St Petersburg Declaration thus: “this object would be exceeded by the employment of arms which uselessly aggravate the sufferings of disabled men, or render their death inevitable.” It has been said that “commanders are quite ready to admit the claims of humanity to the extent of forgoing the use of any engine of war whose military effect is disproportionate to the suffering it entails”. Any weapon that goes beyond what is needed to achieve the military object of disabling the enemy combatant would be difficult to justify on the grounds of military necessity. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 6.1.1.
With regard to bayonets and swords, the manual states:
Stabbing or cutting weapons such as lances, swords, bayonets or knives are lawful provided they are not of a nature to cause superfluous injury or unnecessary suffering, for example, because they have barbed heads or serrated edges or are spread with substances designed to inflame wounds. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 6.6.
In addition, the manual states:
6.11. The general rule prohibiting the infliction of unnecessary suffering or superfluous injury does not preclude the use of hand-grenades and other fragmentation weapons.
6.11.1. The general rule must be taken as banning the use of weapons or projectiles that discharge broken glass, nails and the like. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 6.11–6.11.1.
On the use of non-lethal weapons, the manual states:
6.18.1. Non-lethal weapons are weapons that are explicitly designed and developed to incapacitate or repel personnel, with a low probability of fatality or permanent injury, or to disable equipment, with minimal undesired damage or impact on the environment.
6.18.2. Devices such as water cannon, plastic bullets, CS gas, stun grenades, electronic jammers and laser weapons would fall within this category. So would acoustic devices or those causing metal embrittlement, or entanglement …
6.18.3. Generally speaking, devices that temporarily incapacitate combatants or that have only anti-materiel applications are, from the legal point of view, to be preferred to lethal weapons or those that cause permanent harm to individuals. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 6.18.1–6.18.3.
With regard to internal armed conflict, the manual prohibits the use of “weapons of a nature to cause unnecessary suffering or superfluous injury”. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 15.28.
In its chapter on enforcement of the law of armed conflict, the manual states:
The Hague Regulations 1907 are now recognized as part of customary law. Those regulations provide that the following acts are “especially forbidden”:
e. to employ arms, projectiles or material calculated to cause unnecessary suffering. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 16.27.
Under the UK ICC Act (1998), it is a punishable offence to commit a war crime as defined in Article 8(2)(b)(xx) 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 1970, during a debate in the Third Committee of the UN General Assembly on Resolution 2677 (XXIV), which emphasized the need to “secure the full observance of human rights applicable in all armed conflicts” and called upon “all parties to any armed conflict to observe the rules laid down in the Hague Conventions of 1899 and 1907, the Geneva [Gas] Protocol of 1925 … and other humanitarian rules applicable in armed conflicts”, the United Kingdom, as one of the sponsors of the resolution (the others were Australia, Belgium, Ceylon, Greece, Ireland, Japan, Luxembourg, Netherlands, New Zealand, Philippines, Singapore and Spain) 
Australia, Belgium, Ceylon, Greece, Ireland, Japan, Luxembourg, Netherlands, New Zealand, Philippines, Singapore, Spain and United Kingdom, Revised draft resolution on respect for human rights in armed conflict submitted to the Third Committee of the UN General Assembly, UN Doc. A/C.3/L.1809/Rev.2, 26 November 1970.
defended the broader expression “all armed conflicts”, instead of India’s proposal “armed conflicts”. The United Kingdom stated: “The fact was that in any armed conflict, whether international or not, certain minimal standards had to be respected, and for that reason the sponsors wished to retain the phrase ‘all armed conflicts’”. 
United Kingdom, Statement before the Third Committee of the UN General Assembly, UN Doc. A/C.3/SR.1799, 26 November 1970, §§ 6 and 9.
A training video on IHL produced by the UK Ministry of Defence emphasizes that the 1907 Hague Regulations prohibit weapons that cause unnecessary suffering. It adds that weapons must not be altered with a view to causing unnecessary suffering. 
United Kingdom, Ministry of Defence, Training Video: The Geneva Conventions, 1986, Report on UK Practice, 1997, Chapter 3.1.
In its written statement submitted to the ICJ in the Nuclear Weapons case in 1995, the United Kingdom stated:
3.63 It has also been argued that the use of nuclear weapons would violate the prohibition on weapons which cause unnecessary suffering. The most recent statement of this principle is contained in Article 35(2) of Additional Protocol I, 1977, … The principle is, however, a long established one.
3.64 The principle prohibits only the use of weapons which cause unnecessary suffering or superfluous injury. It thus requires that a balance be struck between the military advantage which may be derived from the use of a particular weapon and the degree of suffering which the use of that weapon may cause. The more effective the weapon is from the military point of view, the less likely that the suffering which its use causes will be characterized as unnecessary. In particular, it has to be asked whether the same military advantage can be gained by using alternative means of warfare which will cause a lesser degree of suffering. 
United Kingdom, Written statement submitted to the ICJ, Nuclear Weapons case, 16 June 1995, p. 50, §§ 3.63 and 3.64.
[emphasis in original]
In its oral pleadings before the ICJ in the Nuclear Weapons case in 1995, the United Kingdom stated:
The principle that a belligerent must not use methods or means of warfare which cause unnecessary suffering and superfluous injury does not prohibit the use of a weapon which causes extensive suffering unless that suffering is truly unnecessary. What is required, therefore, is a balancing of military necessity and humanity … Consideration of military necessity is an integral part of the unnecessary suffering principle. It is not a case of necessity being invoked to justify the use of an unlawful weapon; the use of that weapon is not unlawful if the injury it causes is necessary to the achievement of a legitimate military goal. 
United Kingdom, Oral pleadings before the ICJ, Nuclear Weapons case, 15 November 1995, Verbatim Record CR 95/34, p. 39.
[emphasis in original]