Hi Dave (Clarky)
I know that some of the Severn further North is navigable but I've never tried it.
We go on the river South of Worcester. You can launch from Kempsey or Upton-on-Severn. See www.boatlaunch.co.uk
for details of the slipways.
There is a speed limit on the river (
honest ) and you need a day licence at a couple of pounds. However if you do a search and see previous threads, you will see that it can be a bit difficult finding somewhere to get one.
The Severn is a beautiful river and is well worth trying out. It is navigable from Worcester out into the Bristol channel. We will let you know when we go down next. I understand that David Manning goes out there quite often.