American Airlines codeshare tickets or Virgin Atlantic nonstop flights from Washington Dulles IAD to London LHR are $558 round trip for travel departures January 10 to March 31. British Airways nonstop flights can be ticketed with AA.com, Iberia and Finnair. American Airlines has one stop flights at $536 from Washington National DCA. Lowest fares on American may require a change of airport in New York and/or overnight.
Washington Dulles IAD to London Gatwick is as low as $404 round trip on Icelandair for travel departures January to March.
American Airlines codeshare ticket for British Airways operated nonstop flights
$557.76 round trip Washington IAD – London LHR Thu March 24 – Thu March 31
Delta codeshare ticket for VS operated nonstop flights
$557.76 round trip Washington IAD – London LHR Thu March 24 – Thu March 31
Icelandair (Orbitz ticket) Washington IAD – Reykjavik, Iceland KEF – London Gatwick LGW
$404.32 round trip Wed March 8 – Thu March 16
Google Flights shows several dates with $342 fares, however, I was unable to price any dates on Icelandair in the $300s.
Loyalty Traveler curated Airfare Deals and Google Flights and Kayak Explore tutorials.
