Zoom
Airlines does 1-way tickets for a bit cheaper than return. In general they're one of the cheapest UK-Canada connections anyway.
I came here on a one-way ticket and a working holiday visa, with no problems whatsoever. The point of showing a bank statement with a minimum balance was so that I could afford to support myself for a month AND be able to return home if necessary. Here it is 2 years later and I still haven't booked a
flight home. Of course, all this was 2 years ago, so if the rules have changed (with regards to return flights), then my apologies.