Do you want to go direct or via?
Direct flights...Air Canada, BA, Transat, Thomas Cook (although not sure if they have a xmas schedule)
Via flights
Air France, KLM, Lufthansa, Icelandair (pricey last time I checked but they include a stopover in Iceland if you wish), and all the American carriers via their respective hubs.
I agree that Canadianaffair have a good selection. My sister managed to get an Air transat flight last year at xmas for 400 pounds return. Not bad I though..although she did book it EARLY. I think you would be doing well to get that price for xmas. Try all the major travel sites and see
Flyglobespan.....how have people found them? It seems to me that they NEVER seem to arrive at the same time each day...are they early, late, unreliable??