Hi. The onus is on the airline to prove in Court why its reason was extraodinary and beyond its control.
If you were told at BOS that the delay was due to operational issues then case law has found that is not extraordinary.
I would advise you have a look on flightaware to see if other flights landed/took off from BOS and LHR at those times and if they suffered delays.
I currently have 2 MCOL's in play against airlines for claiming extraordinary whilst I do not believe them.