Just as an update, there are still plans for this to go ahead, it's just going to be a one day rather than two day event, on the Saturday rather than both days.
It'd be really good if we could get lots of participation from Wikimedians. Newbies definitely welcome.
If you are interested, please sign up on Lanyrd:
http://lanyrd.com/2012/cowboy-london/
Or sign up on the OpenStreetMap wiki:
https://wiki.openstreetmap.org/wiki/London_OPC2012
Hopefully in a day or so, there'll be a full announcement with venue details and so on.