Oops. Stubby little fingers are typing too fast again. Thanks for the correction of the web address.
http://www.peakridertraining.com/ is correct.
Re: IAM v RoSPA. A RoSPA test is a higher level than an IAM test. Not only are you expected to "make good progess" using The System and the principles of Roadcraft but you will need to demonstrate that you are actively planning your progress at every stage of your ride. The line between restraint and progress becomes much finer. An examiner will look for your positioning and speed to be nigh on perfect at all times for a Gold level pass. The examiner will almost certainly look for safe and effective filtering. Every turn of your head for observation will be duly noted. They are looking for visual evidence that every move is considered. If you've ever watched a well-ridden police bike slip effortlessly through traffic without causing other road users to alter speed or position then that's the kind of riding they want to see. There should also be a written test, not too long, based on Roadcraft and the Highway Code, with some multiple choice questions and some which require a written answer. They may want to see a POWER check or equivalent and it will earn you kudos if you know how to do a good one and introduce it yourself. Finally, RoSPA demands a re-test every three years, even for instructors, so there's good incentive to keep your skills and knowledge sharp.