IAM or ROSPA, I did both tests, no real difference, so go with the group that is nearest to you. ROSPA you have to be retested every 3 years, IAM can be for life, but they are now encouraging you to join the fellowship, which means a retest every 3 years, which is a good thing as people lose their skills over years.
Or if you fancy doing it all in one go, have a look at 1st Class Rider Training in Cardiff. This school is run by an ex police traffic guy and all his instructors are either ex or serving officers. Twice a year they run a 3 countries advanced course, where you start in Wales and ride up to Scotland. At the end you can take the IAM and ROSPA tests, all included in the cost. Costs a bit, but it does include training and Dinner B&B. Good course I did it 3 years ago and am going back in May to re-do my ROSPA, its more like a biking holiday.