Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 117. Accounting for Missing Persons
Section D. Right of the families to know the fate of their relatives
The UK Military Manual (1958) states: “Belligerents must facilitate enquiries by members of families dispersed as a result of war, with the object of renewing contact between them.” 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 38.
At the CDDH in 1975, the representative of the United Kingdom stated that:
He did not consider, for instance, that it could be said that it was a fundamental right of families to know what had happened to their relatives, although it was a basic need. To go further than that would not be wise. 
United Kingdom, Statement at the CDDH, Official Records, Vol. XI, CDDH/II/SR.35, 13 March 1975, p. 371, § 49.