There are about 400 microlights registered for the event, which counts as a busy day at Sandown I'd say. See
www.wightparty.org for details.
I'm not aware of any GA ban. In fact, as a (temporarily I hope) ex-microlight pilot thanks to the Redhill vandals, I am intending to fly myself there in a C152. Perhaps I'll have to saw a few feet off the wings to get it under the 450Kg limit
And by the way, the event is not next month, it's next week - June 20th to 22nd.