Most inland waterways are governed by the British waterways board you can contact them at :-
You do need a licence, not sure how much they are these days, But the maximum width of boat iin a lock is 7 feet 2 inches some bridge holes are not much wider than this, so you will probably still have a problem even if you let the tubes down!!!!!!! Also if you a large A frame you may not pass under some bridges
Maximum speed is 4MPH I did a lot of canalling 20 years ago with a small 2 stroke and oiling up of the plugs was a problem as you are running at almost tickover hours on end. Perhaps 2 stroke technology has improved these days
But don't let that put you off there are plenty of rivers that are navigable and also large canals eg Bridgwater and even Manchester Ship Canal both i think governed by the Manchester Ship Canal Company. If you intend passing thro locks you will need a windlasss of the appropriate type for the canal.