I have checked the BA website but could not find an answer. Does anyone know if there is wifi on the BA direct flight from Seattle to Heathrow?
Generally, no. BA flies 747's from Seattle to London that have not been equipped with Wifi. You might get lucky if they were to happen to switch aircraft, but that's rather unlikely. Here's a link to SeatGuru with extended info about BA aircraft.
According to the BA website, they don't have it on the 777 either. But the inflight entertainment system with on demand video offers lots of choices.