At risk of throwing an unwanted curveball, have you considered SEA-YVR-LHR? SEA-YVR is economy-only, but I'd happily take that in exchange for a longer TATL flight with a decent chance of getting a good night's sleep. Other upsides are guaranteed 777 or 787 with the larger J seats, and likely a less crowded Signature Suite with excellent views over the tarmac.