Maybe best to start it as a one -off bake off and see how it goes. if people enjoy it they will be back for more. This seems to be working with the North-East bake offs.
Agree car parking is an issue - There are a lot of crappy private hotels in central London may be possible to rent a room in one - if it is an old place with ground floor room should be possible to get a decent sound going and some of these places have car parking facilities.
Current Lash Up:
TEAC VRDS 701T > Sony TAE1000ESD > Krell KSA50S > JM Labs Focal Electra 926.