Just an update: I visited the venue today. It's all going to be going ahead on Saturday!
It's going on at Top Office Machines, 133 Bethnal Green Road, London. We're starting at 10am on Saturday, and going until about 5pm-ish, then going for some food or drinks afterwards. Wikimedia UK are kindly supporting lunch.
The wiki page and Lanyrd event page have been updated.
I'd love to see lots of Wikimedians come along and learn another thing they can get ridiculously obsessive about. I'll certainly be showing people how to link objects in OpenStreetMap to Wikipedia and Commons too.
If you are in London on Saturday, do pop in, it'll be fun!