The requirements of inland boats and sea-going boats are very, very different, but there are a few that sort of meet in the middle (and will be a bit of a compromise; they won't be best of breed in either scenario, but may get you through). The Peter Nichols Hufflers come to mind - they can go pretty much anywhere and do most things with a good degree of competence. They won't get you on a narrow canal (as above) but should be okay on the wider ones (the manufacturer is based in Braunston). Peter Nichols also make some sturdy sea-going barges, as well as narrow-boats and inland boats.
Also mentioned above, the sea is a big, dangerous and rightly scary place, but every round-the-world-yachtsman was a beginner once. Don't be afraid to learn, but don't think you can just hop into a boat and point it at the horizon (at least not if you're planning on coming back...)
Maybe also try to watch "all at sea" with Timothy and Shane Spall on their sea-going barge Princess Matilda. Again, not a (UK) canal boat, but she is very at home on bigger rivers (I have seen her on the Thames a few times). Also, maybe also consider the inland waterways of mainland Europe which tend to be designed for bigger traffic than ours.
One final also - big is tempting, small is better. A boating lifestyle is generally about learning to make do with the least you can get away with, not trying to replicate a luxurious land-based lifestyle. Many 30-35' yachts spend years circumnavigating the globe...