As I understand it, becoming an ADI (Approved Driving Instructor) is not an easy matter, and by no means is it a short straight road to great wealth. I know somebody who went in for that and reached the stage where he started teaching leaners. Initially it all seemed quite good, but unfortunately he failed Part 3 of the examinations, which he said meant that he could continue teaching for a further six months, but would then have to pass part 3 within that period, otherwise it all came to nought. In the event he went for a different career option and is now doing very well at that. From what I've heard, even if one passes all the examinations and becomes fully approved, the income level can be a bit uncertain, and it might take a lot of hours per week to make the money.
On the other hand if ADI means Advanced Driving Instructor this is clearly a very different matter, and a pretty good licence to print fivers at a cracking production rate, or so I've heard.
Some hold the view that it's money for old rope - but I wouldn't know about that. I think it's best if I keep quiet; I get into enough trouble as it is.
If I were you, I would take a little while to absorb all the relevant information from those in the know, but if you decide to go ahead I wish you every success. I can see that it would be a very satisfying career if you're keen on promoting good driving standards and getting new drivers off to a good start.
Best wishes all,
Dave.