We all had to start at the beginning so, when you chose a driving instructor you can either have a:
1. PDI Provisional Driving Instructor: which means they are still training under a Pink Licence, which is triangular. That means they have passed Part I and Part II, but they have yet to pass Part III test, which is the final exam all Would-Be driving instructors have to go through to become fully qualified before they can be entered in the Driving Standards Agency Register. Part III exam entails: being able to teach pupils on the move which directly affects you as a pupil, as in effect you would be taught by a trainee.
Beware: there are driving schools out there, some quite famous house hold names driving schools, that charge you premium rates whilst they use trainees (e.g. PDIs) driving instructors without first informing you, the pupils and / or their parents!
Make sure you always ask if you are being taught by a fully qualified instructor.
2. ADI Driving Standards Agency Approved Driving Instructor (e.g. DSA approved ADI): which means they have passed the final Part III exam - stringent test - set out by the Driving Standards Agency (DSA); have more teaching experience, therefore it means better value for your money.