Hello Scott, and welcome.
Your drive with Hugh sounds like a good introduction to the world of driving improvement, and your plans sound very sensible too. The IAM / RoSPA choice generally comes down to the quality of your local group - they should be happy to answer questions, so do your research before deciding.
As you'll have discovered by reading the forums, there are more opinions and options around advanced driving than are likely to be covered by either organisation. Find out as much as you can from as many people as possible, and think about what works best for you.
The ADUK driving days are a great way to get that kind of input, but tend to be a bit thinner on the ground at this time of year. Have a look at the mentoring thread for people who may be available for one to one drives in the meantime:
viewtopic.php?f=13&t=3187
Do less, better.