going back in time, to the very first wam show (or months before it, we gave brief thought to using a School during the holiday period, and in particular, using a large independent school, because:
They are located all over the country, nearly always in or close to a decent sized town and train station
They are quite empty during the holidays
they have limitless car parking
they have a canteen whose staff will doubtless be ok coming in for a day or two
they are often housed in huge old buildings with both large rooms and small, but solid walled
They will cost less than a hotel
they are keen to be used
they wont be shabby
there will be caretaking staff on hand to help move furniture and prepare stuff
schoolroom carpets are used to hard knocks
and etc etc.
Pick a town central in the UK. Find the nearest School. Phone the bursar and ask.