think on DVSA you can find the grades if the instructors agree to share them, this is the grade they got at their last inspection
check Google and fb for reviews... before you read decide what you actually want, obviously there are clear red flags like always late or uses a phone during lesson... but my instructor for example could be considered slow or talking to much to some but I really wanted that thoroughness as I was nervous, my big priority was someone whose reviews stated they were patient and relaxing