As an Elite, it all depends on the fare basis you chose to LHR or wherever. Since there is only one flight to London from YVR, you might be best to book via YYZ or another gateway such as Calgary so you could always fall back to the direct flight if space comes available. I often use this strategy but as I am a SE usually never have to. On the otherhand, you could book with UAL and try your luck getting their SWU certs.