Icelandair has round-trip flights from Seattle (SEA) to Dublin, Glasgow, Barcelona, Helsinki, Munich, Berlin, London, Oslo, Copenhagen, Manchester, or Prague (DUB, GLA, BCN, HEL, MUC, BER, LGW, OSL, CPH, MAN, PRG) for $365-$470 with a connection. Flights November to March. Extended weekends available. No Christmas dates available.
Dublin is the cheapest fare in this list; about $365.
London and Manchester are the most expensive fare in this list; about $470.
Prague availability is very limited.
Some small OTAs on Skyscanner and Google have this for cheaper than the airline. If you plan on booking there, beware of any site that charges a fee to cancel a ticket within the 24-hour “free” period. Use unknown booking sites at your own risk.
A Carry on bag (20-lbs @ 21.6 x 15.7 x 7.8 inches) and a personal item (15.7 x 11.8 x 5.9 inches) is allowed for free on Icelandair. You can avoid seat selection fees by accepting the randomly assigned seat at the gate.
Icelandair offers free stopovers, which good if you want spend a few nights in Reykjavik. More details are on their website (link).