I agree with White Knights. I’ve paxed many times from Western Canada to the UK and back. About half the trip is over Canada and most of it is desolate country until you get to within an hour of your destination.
I’ve flown into Whitehorse, Inuvik, Yellowknife, Churchill and Iqaluit (Frobisher Bay) and you wouldn’t want to take a widebody into those airports unless you had to.
Edmonton and Winnipeg are the only appropriate choices, although if a bit earlier in the trip, Keflavik would be an option.