I would advise against an East Coast transit because no lounge is worth trading 4.5-5 hours of BA 744/777 time for AA/AS 738/757 time. If AA offered 767s SEA-JFK then things might be different. If you can't get SFO then I'd go for YVR, LAX or, if flying J, SAN (since you'll be in the highest class on the plane).

LAX may be your best bet since it has by far the greatest number of LHR flights each day, though one is AA-operated (which is still quite nice in F if you're not used to BA F; avoid in J because the seats suck, though).