So you want to get to Halifax? That's a different story, and a fair distance from Newfoundland. AC has the only non-stop flight to/from LHR. Westjet likely has connections via ??? UA or DL might offer connections via the US, but they will be on commuter type planes in/out of YHZ - if they have business class it wont be their ling haul business product. You probably don't want to travel that way as ...
Depending on direction it's a 6~7 hour flight from Heathrow, but they do offer their longhaul J on these flights.