the whole purpose of the IAM, Rospa and others are not to learn how to ride fast but to ride safely and smoothly.
I for one, would much rather learn how to ride safe, smooth and making progress then how to "get a knee down" on a corner just to find a tractor in the road that you now cannot avoid because you were taking it too fast for what you can see.
A lot of bike schools only teach you to pass a bike test, not what you get to know via a Bikewise, IAM/ Rospa course - lines for corners, etc.
Horses for courses, it's depends on what you want to learn and how you wish to ride.
gingerB, as well as IAM also have a look at the Rospa option and see which one suits you best, closer etc.
IAM is a matter of pass or not, Rospa is graded Bronze, Silver and Gold and retested every 3 years to ensure you are still up to the standard or improved.
Good luck on whichever route you take.