My vets in Cheltenham (Woodlands Veterinary Clinic 01242 255133) are excellent having nursed several of my animals back to health. Mike Daley is their top horse vet and he's very experienced. If you're after specialists then either Wilesley or Bushy Equine in Dursley are good.
Three Counties Equine Hospital just outside Tewkesbury -used to be Kearns & Rea - have amazing facilites inc their own lab and operating theatre. They have about 10 vets so cover every speciality