Norwegian has nonstop flights from Seattle to London Gatwick LGW for $433 round trip. But add another $90 each way, if you want a checked bag, assigned seat and meals or $45 if you want any single one of these paid extras.
Icelandair has Seattle to LGW at $484 round trip or $551 to Frankfurt for travel October to December 2017. Icelandair tickets include checked bag and free self-selected seat and non-alcohol beverages.
Food on Icelandair flights is extra and can be pre-ordered online or purchased on board.
A stopover in Iceland can be added for as little as $10 to $20 more.
Icelandair Seattle SEA – Keflavik KEF – London Gatwick LGW – KEF – SEA
Icelandair Seattle SEA – Keflavik KEF – Frankfurt, Germany FRA – KEF – SEA