Can anyone recommend an airline who flies LHR/LGW - YYZ at a relatively good price (one-way)?

Can anyone recommend an airline who flies LHR/LGW - YYZ at a relatively good price (one-way)? I know BA and AC do however pretty expensive compared to AirTransat, Thompson Air, Iceland Air and Air Lingus.

One way is alwais problematic. You can use cheaper carriers, though you getting what you are paying for. Also you can try 2 way and discard one ticket - Air Canada doing 2 way for very much the same price as IcelandAir one way (cheapest what I found). Or may be they even give you some refund.

Partially utiltised tickets in most cases are non refundable, and the cancellation/refund penaltis will often obliterate any part of the fare that can be refunded. And if they're refundable, the airline will deduce the one way fare, which no suprise here, comes in higher than the return fare.

What you're being hit with the most UK's APD. Take a LCC or any airline to either DUB or FRA or AMS and start your trip there.
or take Norwegian Air Shuttle to NYC (from either LGW or ARN or CPH) then overnight in NY and take any airline the next morning to YTZ (not just YYZ ;) ) 

I did a quick search on a random day in September and found Iceland Air for $509. That's pretty hard to beat; it seems to be half their round-trip fare. I've never flown them but by all accounts they're a perfectly respectable airline, so I'd just go with that. 

I'll be flying this route in August. I did look at alternatives from CDG which turned out to be $300 cheaper then leaving from LHR on the same day. The issue is I'll be in Paris the week before London and the money and time spent just to go back to Paris for a cheaper flight will work out to be the same cost than leaving from LHR/LGW. I did look at Norwegian however they were pretty expensive for the times.

I've flown Air Transat booked through Canadian Affair and they were fine!