My hunch is that the JTR to LHR flight is on a separate ticket from the LHR to JFK flight. That would explain the 62.5K miles which would be 40K for Premium Economy LHR to JFK, and 22.5K JTR to LHR.

if that is the case, when BA cancelled the LHR to JFK flight they were obligated to rebook that flight without the need for award space. If the JTR to LHR flight was on a separate ticket they had no obligation to rebook that flight. Is it possible that you were rebooked into a different JTR to LHR flight in order to connect with the new LHR to JFK flight? If so, that would explain why they needed award seats to be available. If you are on the same LHR to JTR flight as originally booked then they should have left that alone. If you needed to be on a different JTR to LHR flight then you had to have award space available for them to rebook, and it sounds like that was the case.