Great Britain was involved in total war between 1939 and 1945, and everybody was recorded under a National Registration scheme (a wartime census) that took place on 29 September 1939 (i.e. after most evacuation was deemed to have happened). People were issued with an individual national identity card that had to be carried at all times (until 1952) and ration books for food and scarce commodities (rationing continued until 1954). Information about people from England and Wales who have now died can be obtained through the commercial website Find My Past.
Records for the 1939 Register in Scotland are available from the General Register Office for Scotland.
Northern Ireland records can be applied for through a Freedom of Information request via the Public Record Office of Northern Ireland.