Avon is still an RDA but also does good able bodied lessons.
Urchinwood is where all the beginner/intermediate Bristol uni riding groups go, and they're fab for people who've been out of the saddle for a while.
Summerhouse is about 40mins away, but big and large selection of horses.
Hope...
The Avon RDA centre run regular lessons along side their other ones. Not sure what they're like, but its in North Bristol.
If not Summerhouse would be your closest (up the M5).
Hey, thought I'd register to reply to you!
Barton End are fab, we've used them all year for our 3rd team uni comps and got on well with all the horses and people.