I literally had the same concern about the entertainment box occupying legroom so I paid extra attention the last time I flew their 77W and located those annoying boxes. However it was a few years ago, and I just forget the details after I posted my comments onto seatguru (as I believed it would be a reliable permanent source that I can always go back and check). Unfortunately they seem to wash out all records after BR changed 77W configurations several times.
One thing for sure is that A and H will have those boxes, as they are aisle seats and BR assume pax do get extra space to stretch. I think it was D that has entertainment boxes located for the center rows, so that means G will be the best choice since you get the aisle and no box. But then again it's just my unreliable memory and needs to be verified.
Last month I was in Elite, BKK to LHR, seat 23F and this seat had a entertainment box in the foot well, the location of this box does stop you from stretching your left leg straight out.