Icelandair flies out of Washington Dulles IAD and there is a sale to Europe for winter travel being matched to a large degree by Delta, United and their Alliance partners from D.C. airports IAD/DCA and BWI. Lowest fares are usually Icelandair, but Delta and United offer far more low fare dates on routes they discount. Oneworld does not appear to be participating at all in these sales to Europe at time of this post 6:45am Eastern Sep 26.
Departure Dates: January – March 31
London Gatwick LGW
$448-$470 Icelandair IAD-LGW (Google Flight shows $383 fare, but these re-priced at $448 for dates I checked).
$560 Turkish Airlines IAD-LGW
Manchester UK MAN
$504 Delta/SkyTeam Air France IAD – CDG Nonstop
Sofia, Bulgaria (depart by May 15)
$517 United/LH/LX/OS IAD – SOF Sep-May 15 all dates, except week around Dec 25
Athens, Greece (depart by May 15)
Air France Washington Dulles IAD – Paris, France CDG