Icelandair has low round trip fares at $636 for Washington Dulles IAD to London Gatwick with departure dates in September and October. These are probably a reaction to low fares from WOW Airlines in nearby Baltimore BWI. Icelandair tickets have major advantages to WOW Airlines tickets in that there are no seat assignment, carry-on or checked bag fees for two bags with Icelandair. You can also stopover in Iceland in one or both directions. These fares work out to be a better round trip ticket price than most WOW Airlines round trip fares.
Icelandair Washington IAD – Reykjavik, Iceland KEF – London Gatwick LGW
$651.59 Sep 10 departure to Iceland and stay two nights before traveling to London.
Sep 13 KEF-LGW
Sep 23 LGW-KEF-IAD
Icelandair Economy Booking Code N earns 50% flight miles with FinnAir. This is not a miles earning fare for Alaska Mileage Plan. This flight looks like it earns 7,200 points in Icelandair Saga Club.
The fare for IAD-LGW Sep 20-27 without a stopover is $635.61. Stopovers in Iceland are supposedly free, but I did not play around with dates to look for a lower fare than $652 with Iceland stopover. $16 for a stopover is little change compared to the tourist fare to get to and from Keflavik Airport starting around 4,000 ISK ($34 USD) round trip. http://www.kefairport.is/english/to-and-from-the-airport/buses/
420 ISK ($3.40 USD) fare for the public bus ride to get from Keflavik airport to Reykjavik is the cheapest way to travel.