I dont think there's any proper hierachy in place as all the respective organisations compete with each other, and so do not concede readily who is better!
As commuting is your thing, probably the best start you could do is the
BikeSafe course run by the Police. A lot of the guys here have done it, and I dont think I've seen one criticism of it. It's not proper training - its too short for that. But they will give you a very good introduction in how to be a safer rider, as well as clear up a few myths.
If you enjoy that, then you've then got a choice as to where to go next: IAM, ROSPA or other (paid) tuition.
If you do a search for all three here you'll find plenty of recommendations and past experiences. This is my experience, having done all three:
IAM. Based on the Police System of Motorcycle Control (AKA the Blue Book, pretty much all advanced road riding is based on the Police System of Motorcycle Control). Tuition and passing with the IAM is certainly a very good thing, but they do have a few drawbacks. Firstly, standards vary. For every good group or tutor, they'll be a disappointing one too. Secondly, you are only required to pass your test once for life - so standards could fall. Thirdly, the pass level isn't all that high really. Which brings me on to...
ROSPA (aka
ROADA). Still based on the System of Motorcycle control, but the quality of instruction is kept consistent by a structure of senior instructors with ROSPA diplomas and national standards. They have three levels of pass: Bronze, Silver and Gold. Its generally accepted that a good IAM pass falls between Bronze and Silver. Having done both, I agree. A pass at Gold level affirms an extremely professional and capable rider; one who would do well if they were to take the Police Class One riders course. The Gold Award is the highest test pass available to the general public - it is not easy to get! ROSPA have retests every three years.
Paid Tuition is your remaining option if you dont suit the IAM or ROSPA. Obviously you pay your money and you take your choice! I'd go for an instructor who has served as a police class one rider/examiner and is now offering you the wealth of his experience and training - but at a price! It will be a lot more than either IAM or ROSPA, but the quality should be very good and obviously tailored to you as an individual.
HTH,
Mike