I could spend an hour writing a post ripping some of the nonsense posted already on here to shreds, but I can't be bothered, so let's try this:
I passed my Direct Access five years ago. After three years of riding I decided that a) I knew I could ride better but b) I didn't know *how*, so I signed up with Solent Advanced Motorcyclists.
Since then I've passed my Advanced Test and got my Green Badge, however I've not bought a BMW, nor a pipe and slippers. What I *have* done is learned much more about how to position the bike, how to judge the speed of entry into a corner, how to extend my observation to as far as I can see and many other useful techniques which have improved my riding beyond recognition.
I don't give a toss whether people do IAM, Bikesafe, RoSPA or whatever, but if you think you know it all even if (or especially if!) you've been riding for X many years and never had an accident, then you're either a) a Riding God or b) deluding yourself with a false sense of your own abilities.
Go to somewhere that gives Advanced Training, they will probably give you a free (or cheap) introductory opinion of your skills according to what they consider to be good practice. Maybe you won't agree, maybe you are actually a Riding God or maybe, just maybe, you'll actually pick up a useful bit of information that you'd not thought about in the past.
Do you *really* have anything to lose by doing this? Or are you actually just afraid of criticism and scared that you might not be as good as you thought?