Check your local newspaper. Quite often there are adverts for extras on location in and around London. Register with an entertainment agency (see Stage magazine) and make sure they've got a recent photo and all your uptodate details on file.
Best advice is to follow your instinct when it comes to registering with an agency. The best agencies do not advertise and you would only find their details in industry manuals like The Knowledge, Contacts and Kays Catalogue. If you cannot find any of these or would like further information on how to become a film extra check out The UK Film Extra's Guide at www.hiddenextra.com
Good Luck
The Hidden Extra