Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 129. The Act of Displacement
Section B. Evacuation of the civilian population
The UK Military Manual (1958) provides:
The Occupant … is permitted to undertake total or partial evacuation of a given area, but only if the security of the population or imperative military reasons so require. Except when any other course is materially impossible, such evacuation must not involve the transfer of protected persons outside the limits of occupied territory. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 560.
The UK LOAC Pamphlet (1981) provides:
A local cease-fire may be arranged for the removal from besieged or encircled areas of the wounded and sick, … old persons and maternity cases. Evacuations can also be ordered for military reasons or for the security of the population. 
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 9, p. 34, § 3.
The UK LOAC Manual (2004) states in its chapter on occupied territory:
11.55. The occupying power is forbidden to transfer forcibly or deport protected persons from an occupied country either to its own territory or to that of any other state … An area may be totally or partially evacuated by the occupying power if:
a. such evacuation is required either for the security of the population or for reasons of imperative military necessity; and
b. protected persons are not moved outside occupied territory, unless there is no alternative; and
c. the evacuees are returned to their homes as soon as hostilities in the area have ceased; and
d. to the greatest extent practicable:
(1) proper accommodation is provided, and
(2) movement takes place under satisfactory conditions of hygiene, health, safety and nutrition, and
(3) members of the same family are not separated; and
e. the protecting power is informed of transfers and evacuations as soon as they have taken place.
11.55.1. Unlawful deportation or transfer is a grave breach of the [1949 Fourth Geneva] Convention. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 11.55–11.55.1 11 .
In its chapter on internal armed conflict, the manual states:
15.14. It is prohibited to order the displacement of the civilian population for reasons related to the conflict, unless the security of the civilians involved or imperative military reasons so demand.
5.14.1. It may be necessary to evacuate civilians temporarily from areas of danger, from encircled areas or for the better conduct of military operations. It is prohibited to move them for reasons based on race, colour, religion or faith, sex, birth or wealth or any similar criteria or in order to shield military targets from attack. 
United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 15.14.–15.14.1.
Under the UK ICC Act (2001), it is a punishable offence to commit a war crime as defined in Article 8(2)(e)(viii) 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).