Also, regarding the practicalities of flying LHR-JFK-LHR, one has to remember that there were two airframes involved - the first return flight having nightstopped JFK. So the early JFK-LHR service was just taxying out as the morning LHR-JFK landed.
It was possible to operate the early JFK-LHR (BA002) and then turn round at LHR to operate the late LHR-JFK (BA003) and this was done occasionally, generally at short notice to cover illness or crew shortages.
I only did it once and you certainly knew you'd done it afterwards....