LHR-YYC is a daytime flight, so not the end of the world to do it in WT. No question the extra space of a WT+ seat is nice, and it will probably enable you to sleep a bit if you wish, which is much harder to do in WT. That's a typical price for the upgrade, I doubt you'd see it much cheaper.

I just find those long day flights boring, there are only so much I can read and watch the IFE before I get a headache. That being said, the scenery is spectacular flying over the arctic islands.