If you want to use the canals in England then you will need to be much smaller than the Freycinet dimensions Dave quoted. England has canals for Narrowboats (7' from memory) but also has some broadbeam canals. For these, one again from memory, the limitation is lock size: max boat 60' x 12'6".
So staying under the broadbeam dimension, plus getting shallow water draft and being able to get low airdraft ( by lowering pilothouse structure if necessary) will give you maximum flexibility as to where you can go. There are quite a few builders in UK (Piper, Branson, Stoke, Wolstonholme) who design these kind of boats, and some can do both canals and open water (Cat B). Google UK canal barges or something like that and you will likely find many options for design, build as well as some sites where such craft are for sale.