I would have thought that BOS to LHR would be hurt by the new additions. Although we haven't seen too much of a downturn.
There's great data on Boston-Heathrow Traffic here along with anything else UK (Manchester-Boston, Providence-Belfast etc):
https://www.caa.co.uk/Data-and-analysis/UK-aviation-market/Airports/Datasets/UK-airport-data/
You want to take a look at Table 12 for any month.
It goes by month and will give YOY change for the month for airport-airport (examples: Boston-Gatwick, Newark-Heathrow, Oakland-Gatwick etc.)
Boston-London (Heathrow and Gatwick) its easy to get an idea how an airline is doing pax wise since there's only one airline on each route.
For December 2017 vs December 2016
Boston-Heathrow = 61872 vs 56488 about 10% increase
Boston-Gatwick = 13620 vs 10709 about 27% increase