Originally Posted by
gglave
It wouldn't be that complex. You'd enter Canada from the USA, using Nexus, then go up one level on the escalator, head to US departures and pass security and US CBP for your flight to ORD and on to LHR.
I just did a dummy booking at United.com and it allowed me to do just that.
What's not clear to me is what would happen with your luggage - Would they transfer it or would you have to claim it and re-check it?
Right now, it seems that I can't book a SFO-YVR-ORD-LHR ticket without the fare becoming MUCH more expensive. So I am planning the RT SFO-YVR on a separate itinerary. Maybe we will do carry on only.