MCE on the 777W out of JFK is fantastic, especially if you can get in the MCE-only cabin section. Additionally, if you route via JFK, you miss the almighty hell that is terminal E CBP upon arrival at BOS from LHR.

However, the BOS-LHR routing is extremely short, so you'll save a decent amount of time in that regard. Add in time fighting down 93 South traffic to get to Logan anyway (regardless of where you route), you may be better off with the direct on BA.