Just to support others (including, in particular, Eddie51):
The definition of a 'ferry' usually refers to a service where passengers can easily book either single or return fares. As others have indicated, there are no such trans-Atlantic services. The QE2 was the last Atlantic 'liner' offering a comparable service.
Unless you can find a cruise company which allows you to book just part of a cruise twice (once for your outbound journey and once for the return), your only option would be to seek passage on a freighter. Only a small number of cargo ships accept passengers, with fares typically around �200 per day. Container ships typically cover about 600 nautical miles in a day so, even if you could find a ship going directly from the UK to Canada, you're probably looking at around �2500 for the round trip. However a container ship from the UK might well go via Rotterdam (or even via the Mediterranean) en route to Canada, so you might have to travel for (and pay for) 10 or 20 days in each direction.
This company
might be able to provide some suggestions but, unless you're super-rich, I fear that what you propose is simply impossible:
http://www.cruisepeople.co.uk/freighters.htm
Chris