So, my suggestion is:
Letīs find (something like) an assembly hall, a gym or a youth centre.
Yes, those should be cheap and large so thats good.
Im doing a search on the net for london venues and this is probably a no go but I thought I would throw it in The Brewery,
It is probably expensive though.. and I dont know if overnight-ers are on but ffs its a brewery! Piss up anyone? (ok so its not a brewery now but still...)