Thanks for all the great replies,
The petrol vs. diesel debate pretty much confirmed what I was thinking. Doable in a petrol but easier in a diesel. The question is can I afford the extra boat cost vs. the extra hassle of finding petrol.
Wilk, I think you hit the nail on the head with your post. I would have the cabin as a back up and not live out of the rib, but I like the thought of having the option to kip on the boat rather than hunt around for campsites/B&Bs if they are hard to come by. There are plenty here who do the charity thing better than I ever could so this trip would be purely for my enjoyment.
Polly, the idea of lots of smaller trips to interesting places does appeal, rather than spending along time on legs with not much to see. I will look into that further although something in my mind likes he idea of saying I have been around Britain.
I am abit of a sun worshipper too, so I am very tempted to go somewhere warm. Island hoping in the med appeals, and could trailer my own boat down there. Also it would remove the need for a cabin to some extent as camping would be an easier option. Due to the rubbish British summer, the only sure thing for a sunny trip would be to rig up a sunbed canopy.
I think I have abit more investigation to do as to what to go for.