Direct Flights departing Heathrow on Friday 10th March
Returning from Boston with flight departing on the night of Friday 24th March, arriving Saturday morning.
Add another £40 if you want to start journey from Manchester to connect to above flights. or just £305 flying with KLM from Manchester
There will likely be other dates at this price from Jan -March. Shoot up from April unfortunately.
Norwegian Air also fly on same dates from Gatwick, direct to Boston and that's £258
If you want even cheaper on above dates then Wow air are £252 flying from Gatwick and include a 1hr30 stop off/plan change each way. They may charge extra for luggage though