I find the easiest way to find out is to either do a search on Google or go into the nearest tackle shop to the canal in question. You may have to pay in many ways, sometimes you pay on the bank, sometimes in the tackle shop or in my case the local canal is run by a club and you can either get a yearly membership or just pay a dayticket on the bank.
The easiest thing to do would be to contact British waterway, tell the what the bridge numbers are either side of the stretch you want to fish & they will be able to tell you who has the fishing rights on it.
That way you don't have to worry about falling foul of the law or getting known as a "guesting" angler (ieoaching).
There's a monster outside my window..can i have a glass of water?