Hi Vincent.
I'm just starting out too, I'm 19 and I joined the IAM in June. I have yet to officially have an observer though. Unfortunately the waiting list in my area is quite long so please be prepared for a long wait.
I say advanced driving's great, and you should go for it. Personally I wouldn't count too much on getting much of an insurance discount...I think some companies don't give one at all. However, there is a good article about this on the left hand side..."Car Insurance Tips".
There are a few options you can take with advanced driving. I went for the IAM (Institute of Advanced Motorists), but there is also RoSPA (Royal Society for the Prevention of Accidents also known as RoADA - RoSPA Advanced Driver's Association) and HPC (High Performance Club).
Those are the 3 options that I know of...but as I said I'm just starting out, so I'm not so sure that there aren't any more.
There are others here who can give you much more help, I just happened to come across your post at this time of the morning.
Good luck!
jibberjabber25.