Does it have to be LHR & full-J lie flat?
If not, Air Transat can fly you LGW-YVR in "Club Class" for ~$1700 CAD return.
They won't interline to SEA though, so you'll need a separate ticket for that leg, or a rental car.
You could also look at Icelandair - But again, not lie-flat.
Last edited by gglave; Oct 11, 2018 at 11:52 am