Icelandair has Seattle to London Gatwick at a low $303 round trip for some April departure dates. The lowest fares look like they require a one day transit layover in Iceland on the return flight. There are tickets priced at $369 round trip with no overnight layover in Iceland.
Most dates March to May are low to mid $400s with some upper $300s dates.
Icelandair Seattle SEA – Keflavik, Iceland KEF – London Gatwick LGW – Keflavik (26 hr layover) – SEA
$302.73 round trip (Priceline) Sat April 27 – Thu May 9 (arrive in Seattle May 10)
Icelandair Seattle SEA – Keflavik, Iceland KEF – London Gatwick LGW – Keflavik KEF – SEA
I flew Icelandair SFO to London in October 2018 with an overnight layover on the way back.