Not sure on ROSPA but for IAM just go into www.iamroadsmart.com/local-groups enter your postcode and it will come up with your local IAM motorcycle group. Contact them and go from there. Almost definitely you will be able to get a free (you may be asked to put a tenner for fuel) observed ride to give you a flavour.
Advanced rider course is currently £149 and covers membership for first year, all training and test when ready, only other cost is membership of your local group (mine is £22) and fuel for your observer and copious cups of tea