You could try posting on the Lost Family & Friends/UK board. Post your cousins' full names and the last location (but not the specific street address) where they were known to live.
If you have any idea of where your cousins may be living you could post a notice in the local newspaper. Likewise, with the administrator's permission, you could post on the RootsWeb mailing list for the appropriate location.
Check BT phone book.
The Salvation Army will sometimes help locate living relatives, so contact them.