Can't help you for any around Essex, but here's a rough guide
Don't immediately be put off by small establishments - sometimes they can be as good as larger ones.
Ask if you can have a look round and check if the horses look reasonably happy and in good condition, and the stables look clean.
Observe a lesson, and see what the instructor is like.
For a list of ones in your area, have a look on here
Hope I have helped