You might find that a single ticket LHR-YVR (BA) YVR-SFO (?) SFO-LHR (BA) is cheaper than two separate tickets even with stops in YVR and SFO. Last year Kayak sold me a LHR-LAX-BOS-YYZ-LHR ticket (UA+AC) with stops where I wanted for less than the cost of a LHR-BOS round trip on the same dates ^
If YVR has a US immigration pre-clearance checkpoint then going UK-Canada-USA might involve slightly less queuing than going UK-USA-Canada but I don't think there is much in it.
Having a longer overnight flight home with plenty of time to sleep is also nice.