The back up option is to check your carrier's agreements with AC to see if they have Zed with you. AC run 3 x daily LHR-YVR nonstop and BA also run LHR-YVR nonstop. YVR is not all that far from SEA - if friends can't pick you up, you have a heap of YVR-SEA shuttles on both AC and AS (Horizon) to use as well.

The BA nonstop to SEA leaves Thiefrow at 1445, but the AC's to YVR leave at 0840, 1425 & 1615 with the BA to YVR leaving LHR at 1715, so some sort of hard decisions would need to be made on the day of or day before departure, pending the loads on the nonstop BA to SEA.