Before people go off the idea, London can be a cheap place to stay. Easyhotel
(like Easyjet, but in a hotel!) costs about £20-25 a night and have 3 or 4 sites in Central London, plus theres loads of B&Bs across town. Even the Travelodge group are cheap if you book early enough, and a group booking at most of the bigger hotel chains would reduce the cost as well.
Also, are there any other groups who would be interested in getting together for a joint meeting? Sort of like a combined RETRO show?
First things first tho, Venue location? Size? Budget? Once you have that nailed, you can start looking at local hotel accommodation.
I live fairly close to London, so access to the city isnt a problem for me and Im more than happy to help out where I can.