I went to a TKD school which was part of the GBTA, and the instructer was rubbish.
To be honest is really doesn't matter what martial art you do, as a good instructer will teach you all the basics. Then, when you know a little about the arts you may choose a diffrent style.
when looking for a class look for...
people you think you will get on with,
-a friendly atmosphere,
-an instructer who doesnt hesitate when demonstrating new techniques,
-lessons over an hour,
-schools which have more than one or two classes a week,
-an instructer who is a full time instructer (not just a part time job)
-classes with lots of pairs work (training with a partner is essential)
bad points to look out for...
-classes who do lots of exercise, (when u pay for a lesson you want to be learning not sweating)
-instructers who keep accidentally hitting students when demonstrating.