Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 45. Causing Serious Damage to the Natural Environment
The UK LOAC Pamphlet (1981) states that it is forbidden “to use methods of warfare which are specifically intended to cause widespread, long-term and severe damage to the natural environment. This rule does not prohibit the use of nuclear weapons against military objectives.” 
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 4, p. 14, § 5(h).
In a subsequent section, the manual states:
The following are prohibited in international armed conflict:
g. weapons (other than nuclear weapons) intended or which may be expected to cause widespread, long-term and severe damage to the natural environment. 
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(g).
[emphasis in original]
The UK LOAC Manual (2004) restates the relevant provision of the 1977 Additional Protocol I: “It is prohibited to employ methods or means of warfare which are intended, or may be expected, to cause widespread, long-term and severe damage to the natural environment.” 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, § 5.29; see also § 6.3 (weapons).
Under the UK ICC Act (2001), it is a punishable offence to commit a war crime as defined in Article 8(2)(b)(iv) 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 its written statement submitted to the ICJ in the Nuclear Weapons case in 1995, the United Kingdom stated:
Articles 35(3) and 55 of Additional Protocol I are broader in scope than the [1976 ENMOD] Convention, in that they are applicable to the incidental effects on the environment of the use of weapons. They were, however, innovative provisions when included in Additional Protocol I, as was made clear in a statement by the Federal Republic of Germany on the adoption of what became Article 35 of the Protocol [see infra]. As new rules, the provisions of Articles 35(3) and 55 are subject to the understanding … that the new provisions created by Additional Protocol I do not apply to the use of nuclear weapons. The view that the environmental provisions of Protocol I are new rules and thus inapplicable to the use of nuclear weapons is confirmed by a number of commentators. 
United Kingdom, Written statement submitted to the ICJ, Nuclear Weapons case, 16 June 1995, § 3.77.
Upon ratification of the 1977 Additional Protocol I, the United Kingdom declared with respect to Articles 35(3) and 55:
The United Kingdom understands both of these provisions to cover the employment of methods and means of warfare and that the risk of environmental damage falling within the scope of these provisions arising from such methods and means of warfare is to be assessed objectively on the basis of the information available at the time. 
United Kingdom, Reservations and declarations made upon ratification of the 1977 Additional Protocol I, 28 January 1998, § e.
In its written statement submitted to the ICJ in the Nuclear Weapons case in 1995, the United Kingdom stated:
The [1976 ENMOD] Convention was designed to deal with the deliberate manipulation of the environment as a method of war … While the use of a nuclear weapon may have considerable effects on the environment, it is unlikely that it would be used for the deliberate manipulation of natural processes. The effect on the environment would normally be a side-effect of the use of a nuclear weapon, just as it would in the case of use of other weapons. 
United Kingdom, Written statement submitted to the ICJ, Nuclear Weapons case, 16 June 1995, § 3.7513–3.116.