Mark
I'm a commercial boat driver and drive a dive rib for a North Sea technical services company and I don't think that Southampton or the Isle of Wight will provide you with testing enough conditions to have your skills honed.
This is a bummer because it's where our school is based also
Three of the best Boat drivers, and one of the best personalities I have ever met are based in a company called Safety Boat Services. James, Iain and Jon are all North Sea Veterans and commercial and sports divers.
http://www.safety-boat-services.co.uk/fleet.php and it's close to the Dutch ferries
Dave Mallett Wavelength on this forum also has a good and I am told is a very sound guy.
http://www.powerboat-training.co.uk/...Wavelength.htm
Also as I recall theres a school in Banff . 58 North I think they are called which has a good training seahttp://www.north58.co.uk/ That's right up the top of scotland where they let moose roam the streets. I don't know this school but I saw the place when I was up there in a tug a few weeks ago and it looked like a good set up