Flight times from London to Seattle will vary depending on flights. United has a flight with one change of planes with a total flight time of 17 hours and 5 minutes. A long layover when changing planes can increase flight time to 30 hours. If you want to see flights, check http://www.kayak.com/flights#/LON-SEA/2012-04-08/2012-04-15 Air Canada has a flight that is just under 14 hours.
It will take approximately take 11 hours for a flight to fly from London UK to Seattle. The time is inclusive of the checking in time and picking the luggage upon arrival. However, the actual flight time is approximately 9 hours and 45 minutes.