Norwegian Airlines (DY) operates nonstop service between Boston and London Gatwick LGW. From September through March 2017 there are several flights most weeks priced at $195 one way for Boston to London and $184 one way for London to Boston, making it possible to book a round trip ticket for $379.
Norwegian charges $45 for one checked bag and $45 for a seat reservation and $45 for meals. You can carry on one bag up to 10kg/22lb for free and if you are fine taking your chances on a non sold out flight and bring your own food, you can take your chances on a free seat with intent to move inflight if you get stuck with middle seat. You can buy a lot of carry-on food for $45.
Final ticket price will range from $379 to $559 round trip.
Google Flights Calendar Norwegian Boston to London LGW one way Oct-Nov
Google Flights Calendar Norwegian London LGW to Boston one way Oct-Nov
London Gatwick LGW drops from $244 to $184 on Oct 31 and remains at $184 for most dates through March 2017.
Google Flights Calendar Norwegian London LGW to Boston one way Dec-Jan
Personally I have taken advantage of several low fare Norwegian flights from California to Europe this past year. They have a wonderful in flight entertainment system for films and games (no music) on their 787 Dreamliner. This is a great deal to London to check out their service if you have not flown Norwegian Airlines before.