the ADI examination to become an approved driving instructor comes in 3 parts, and they are, I believe:
1] a formal written examination with multi answer tick boxes, on road practice, safety and all things written in the Driver's manual handbook and highway code. where one has, or had to gain over an 84% pass in each of 3 sections, with an overall pass on the combined 3 sections of over 90%.
2] a driving test with an examiner from the DVSA.
3 A practical test upon the ADI's ability to actually teach a lesson to a "student" correctly and confidently, and these tests are regularly repeated to make sure that the tutor isn't allowing sloppy working practices.