I couldn't find a retailer in London, but I did find that the Lomographic Society Intl. has an "embassy" in London:
Linda Scott
The Mews Cottage
28a Grafton Square
London SW4 0DB
Tel: 0044 (0)20 7498 0500
Fax: 0044 (0)20 7498 2737
[email protected]
I bet she'd be quite helpful in answering your question.
Also, Here's a lomography society event going on in London July 15th. On the webpage there is also some contact info. I think they would be helpful too:
http://www.lomography.com/events/?sub=0&evid=354