I just mocked up a booking on Air Canada from SEA-LHR. I used November 9 - 24th as my test dates, and their flexible option. The price for everything in the grid was the same. After taxes/fees, the cost was $580/pp!
This included a layover in Toronto. If you're inclined to book this trip, you may want to watch the tight layover. On an AirCan flight to Ottawa, we got messed up in Toronto and had to wait for the next flight.
It's a pretty good deal if you had a London trip planned.