Just seen this thread...
If you are still looking for a venue central to the UK, then might I suggest Arrow Mill in Warwickshire?
Sure it looks a bit fancy for biking types, but actually the owner Denis Woodhams is a total bike nut, has a weekly meet there on a Sunday evening and is more than happy to host events...
I'm also a member of the UK (Ducati) Monster Owners CLub who have our annual weekender rally there each May bank holiday (this year it's the first weekend of May btw).
The venue is in a lovely setting, has accomodation (of course) and they are quite happy for people to camp in the grounds/by the river too during this sort of event.
The only thing I would say is their diary does tend to get full during summer weekends with wedding and anniversary type parties in the main hotel/restaurant, but if anyone is passing, pop in and check the place out - I think you'll be impressed, and I'm sure if they can accomodate another event like the Monster weekend, they would be happy to host a HU event too?