Aha right, on the site where im based we have large facilities for provision of things like this, we currently hire out to services like slimming world etc, and with it being health-related the exec probably would have gone for it; but atm over half of the community facility area is undergoing refurbishment/building work, so im not sure it would be very comfortable to hold 100 people at present.
Ill have a word with the exec and see what the legal capacity is of the current available space and insurance status for charitable events, as he loves supporting anything health-related - he's a doctor lol. If you get stuck, im sure he would love to help if at all possible.
Would it be on a weekday or a weekend, if the latter, it expands the options a bit further.