My previous recommendation has been for CJ Hall in London. I have no problems with their treatment of both gliders I have taken to them.
However, when one of my gliders became ill on a Sunday evening, they were closed, and there was no exotic vet on call, so I couldn't take him there.
After a lot of phone calls, I managed to find this vet:
Great Western Exotic Vets
Unit 10, Berkshire House,
County Park Business Park
Shrivenham Road
Swindon
SN1 2NR
01793 603800
http://www.gwev.co.uk/The vet my glider saw was Neil Forbes.
I was really impressed with this place, and even though my glider didn't make it Neil was very helpful and talked everything through with me, even very late on a Sunday.
This vet is a veterinary hospital, meaning that they are available 24/7, and they have 3 exotic specialists, one of whom is always on call.
I am going to be using them in future, because I know I will be able to see a vet quickly if I need to.