Yes OP is having to connect at either LHR or FRA (or possibly MUC?) regardless - but given LH's ground service for F passengers (car to and from the aircraft, and excellent F lounge and even F Terminal at FRA) I'd say the connection on LH will be way better than on BA travelling in F, before you even get on the plane.

I agree it's not really worth back-tracking from LHR to Europe in order to get to Boston, as a general rule!