I think the navigable waterways in Britain are some of the narrowest in Europe. The maximum 'go anywhere' size is roughly 7' beam, 60' length, 2' draft and 6' above the waterline. Yes very long as the canals here were designed for our traditional horse-drawn narrow boats 300 years ago.
Even smaller would open up some lovely rivers and backwaters that are not considered navigable by most, but you can still get all over the country on the larger navigable canals and rivers.
There's no limit - the smaller and shallower the more places you can pass through.
I too want a small boat - the best I've seen so far are the micro-cruisers designed and built by Matt Layden of Florida or Sven Yrvind of Sweden. See
www.yrvind.com and
www.microcruising.com. They average about 15' length, 4' beam, 3' air draft (without mast) and only 6-10" water draft. But you'd have to build it yourself. The only plans currently available are for the Paradox by Matt Layden, in plywood.
The next size up you can buy very cheaply second-hand - a small yacht with a lifting keel or with twin bilge keels, for example a Hurley or Westerly about 20' long, which you can buy in England in good condition ridiculously cheap from £1000 up. Maybe 1' draft for a lifting keel and 3' for a bilge keel yacht. You must find one with the mast in a tabernacle, or fit a tabernacle which is basically a hinge enabling you to drop the mast down to deck level to go under a bridge. These are seaworthy boats and some types have gone right around the world. Search
www.apolloduck.co.uk and specify 'Sailing Yacht'.
It's really a compromise between living space/comfort, and the versatility and range of your boat. More living space, headroom, and stability means more weight, deeper draft, higher freeboard and greater beam (all or a combination of these). How simple can you live?
(All measurements are in feet and inches sorry about that.)