The last two times I've been flying LHR-YVR in J they have been selling upgrades for £399 + tax at check in and a few took this up and I saw at least two SWUs being used each time. Cabin full both ways in J. I've done YYC-LHR and YYZ- LHR both with loads of space in J and upgrades for SWU and SSWU in evidence. These flights were taked in May, July, Feb and Oct respectively.
From my experience YYZ has a better chance as more flights.