Relative to me you are not far away from the National collection in Cornwall, perhaps you could consider that!

Here is the link to their website;

http://www.national-dahlia-collection.co.uk/en/

Alternatively email them and ask them the question you asked here.

Sorry I can't be of more help!

If you want some pictures see here;

http://www.thegardenersalmanac.co.uk/Data/Dahlia/Dahlia.htm

I did the blog on Ayletts this year and last year too. Although its a good way from Swindon I dont think there is anything similar nearer you. Sandwich in Kent has a dahlia festival and there is one in north Wales but these are even further away. And a NGS garden in Cheshire has about 300 dahlias but thats a long way too.

Good luck with your hunt.

There's one near me called Withypitts Dahlias and they have (weather permitting) an open day on 6th October 10 till 4. It is just outside Crawley in Sussex in a village called Turners Hill. They have their own web site and a facebook link. I think its only about an hour and a half from Swindon up the motorway to M25/M23.