LON-BOS - isn't comfortable due to lack of fuel capacity in moderate to strong headwinds. Even the legendary intercontinental 738 struggles with this and you'll end up wearing the arm-floaties on too many crossings. It'll haul 156 pax and some cargo (think incontinence pads with 7.5 hour flight in the narrow-body) but the tankage is just too small to get there reliably. No problems on the return trip with the wind behind you. NYC and YYC are worse than BOS.
That's why Gander was upgraded so many years ago - to cater for exactly this class of intercontinental aircraft.