Heritage Centre is Mucho money (and next door to where i work), i wonder is the National Motorcycle Museum might be a better bet?
http://www.nationalmotorcyclemuseum.co.uk/
Out of interest why would it need to be a CCC meet? My understanding is that the best Citroen meets have nothing to do with the CCC, some of the regional events are quite good, but the national offers little by way of entertainment or space to party. 2CVGB meets are legendary and much more the sort of thing that would interest me rather than sitting in a field polishing cars and discussing the finer points of Dosseur valve design.
My understanding is that rallys normally build up, often comprising multiple sub groups? I think asking for 100 cars is pointless, you might get 100 cars but if they just drive up park for a while and leave, what is the point? I'd rather have a weekend event with cracking interesting company with only 10 cars, then look at a car park of BX's.
The way to build good meets is to provide something with entertainment (not needs be karaoke, but points of local interest), plenty of room, reasonable organisation, and a great atmosphere. I've worked some really big events before which started much smaller, and they get progressively more dull as time goes by and they get too big. Equally i've been to close packed events where there is nothing to do locally and you come away thinking, 'why did i bother?'
I would also be tempted to link the event into something bigger, like the ICCCR though pehaps that is a long way away yet? And invite other BX groups and even non BX groups. I imagine the Dutch boys and girls would love an invite, and i've been invited to the 2cvgb meets multiple times.
You might even consider having the event near a race circuit such as Mallory Park and have it coincide with a low budget series championship such as the tin snail racing or ginetta cup, or even a saxo competition? Most circuits have campsites, many with good entertainment, and the middle of the table circuits always love to have a good turn out to any event! Infatc there is no reason why it has to be circuit racing, there are a lot of hill climb events accross the country which the keen BXer could happily enter, if we had sufficient entrys we might be able to have a sub class. (hill climb is simply driving down an access road, there is not needs be a hill involved, but it is single car on course, and you race the clock!).
I'm just throwing my thoughts in, a BX Rally would be good, but i feel thought needs to be given into sustainability of a annual/bi-annual event.