Flying economy via Dublin (DUB > SEA, then SEA > LON) knocks down the cost to £568 with UA, US Airways for £600, Delta for £626... you'd just need to get a flight to Dublin of course and make the timings work with bonus of clearing US immigration there and onward connection will be domestic. Similarly, MAN > SEA, then SEA > LON also has similar prices but without the benefit of US border.