Icelandair has Seattle to London at $410 round trip with stopover in Iceland for some dates between February and April 2017. There are not a lot of dates at this low $410 fare, and more dates at $472 round trip, but play around with Google Flights and the Icelandair website. You might be able to track down more low $410 fare dates. Most of the $410 dates showing up on Google Flights re-priced to $472 when I clicked through on Orbitz, but I was able to locate a $410 price within about ten minutes when I entered a stopover itinerary on Icelandair website.
Icelandair 3-night stopover ticket $409.89 round trip April 13-22
Thu April 13 Seattle SEA – Reykjavik, Iceland KEF – London Gatwick LGW
Wed April 19 London Gatwick LGW – Reykjavik, Iceland KEF (Stopover 3 nights)
Sat Apr 22 Reykjavik, Iceland KEF – Seattle SEA
Two checked bags and seat assignments are free on Icelandair, unlike low fares to Europe on Norwegian Air and WOW.