Given the time of year, and therefore the uncertainly of the weather, some sort of motoring related activity seems to point, like last year, to indoor karting.
Last year we went to Reading, a great facility, but a bit of a hassle to get to with a track of 450m and electric karts.
Looking around the Greater London area for other indoor venues seems to point towards the Docklands Raceway, a 650m track with 10hp LPG karts.
The facilities look better than at Reading.
Here is the link...
http://www.theraceway.com/default.aspx
I've made an enquiry to see whether they're okay for the 10th and what the options / costs are.
I'll let you know in due course, but if anyone has any other suggestions in the meantime?...........
Cheers, Em.