Hi, I promised I would come back. This is all based on DPs experience and doesn't necessarily realte to others experiences.
DP completed the tests about 6 years ago. He learnt with a national ADI training organisation, that wasn't at the time linked to a specific driving school (but I think they have gone bust now). He paid an upfront set of fees for the whole course (which he funded with a 'career development loan' - worth considering if you will be looking to finance this). He was sent a pack of books and mock papers which he studied and passed his part 1 first time.
Because he paid for his course upfront - his part 2 was taught by having a set number of hours lessons. It is basically an extended driving test. After failing first time - he was 'allowed' another X hours of lessons. I think this was the downside of paying upfront - they were inflexible when it came to tayloring the number of lessons to the pupil. He passed second time.
Part 3 was again another X hours of tuition. He failed is first 2 tests (on the morning of his second he was told that DTD2 who was in neonatal intensive care at the time had taken a turn for the worse overnight - so it is not surprising he failed that one). Agree with TM - the examiners are pedantic and want you to tell them stupid things. YOu have to assume they have sat in their house their whole life and never seen a car or road!!!.
Anyway after failing 2 tests - he got a trainee license, meaning he could teach to gain experience. They only last 6m tho. He also sought extra pay as you go training - which he says is the best thing he did. Because it is all based on role play by your trainer you get used to your trainer. You know the 'mistakes' they are going to make before you make them. BY having a new instructor he saw new 'mistakes'. He passed 3rd time (thank god as you only get 3 attempts).
He worked for a couple of years for a semi-national company (including trainee license). They were a rip off. I think he paid £220ish a week to them (it was a long time ago and I may be wrong there) - although that included car hire, maintenance etc. He then brought his own car and reduce the fees to about £120 a week - but when lessons were only £16/hr you do have to work a lot of hours just to pay your franchise. He also only wanted to work part time and there was no flexibility in the fees he paid compared to those that wanted to work full time.
After a couple of years he was barely getting any leads with them - so he moved to an expanding local company. The fees dropped to £60 a week (he had his own car) - partly as they acknowledged he only wanted to work part time. Work flow with this local (but well recognised and established company) was much better. He was rarely left short of hte hours he wanted to work. In the last year he has set up an independent school on his own. It was a nervous decision - but seems to be working OK.
So tips:
TRaining can all be done alongside current job until you get a trainee license.
Trainee license recommended (some trainee instituions try and tell you don't need to/ shouldn't do this but ignore them - it is invaluable). They only last for 6m - so do the amount of training you need to do before applying.
Look at pay as you go schemes and compare them to what you get with pre-pay schemes. You could always use the former to supplement the latter if you need to. In fact DP would recommend looking to gain part 3 training from at least 2 trainers to see a variety of methods and faults. One can be a main trainer - but it is worth having a few hours with someone else IYSWIM.
If DP was doing it again he would probably look for tuition from a smaller - but well recognised local school. Look for Ordit membership (anecdotal evidence they have better pass rates) and ask how many pupils they have successfully got thro Part 3. This gives an idea of how long and how much experience they have of teaching new instructors. It may not be many if it is a small school - but they should have had at least 3-4. Ask how many instructors within the school can train instructors (if it is more than 1 it will be easy to get tuition with multiple trainers).
I know that is a mammoth post. If you could possibly want more info give me a shout and I will give you my email addy.