Working estimates for venue space requirements are 4200-4500sqft. A fair bit more than the pure guesstimates.
This means the budget estimates for rent are about £1000 too low.
The cheaper properties are warehouse units as it is, so one option is to buy a used mezzanine floor and put some areas above others. It would increase setup cost & operating complexity, but decrease long-term running costs.
Sunbeam City is a anticapitalist, antifascist solarpunk instance that is run collectively.