Looking at random midweek travel dates in mid-June via Expedia, Air Canada will do LHR-YVR-LHR for US$830. They'll do LHR-YVR-SEA-YVR-LHR for $931. So the connecting tag by air to Sea-Tac is only another $101. Considering that Quick charges $59 return for the bus from Vancouver Airport down to Seattle, I'd fly.