Looking at the flight schedules for tomorrow, there are 6 JFK/LHR flights! Four of those are 747-400s.

However, the earliest flight leaves LHR at 8:55am and arriving at 11:45am. However, the next return flight (JFK-LHR) is not until 8pm (with a 747-400). So again, there is a 8 hour wait on the ground in JFK. There doesn't seem to be a way to use the HKG/LHR planes on the JFK/LHR routes efficiently.

As for BOS/LHR, they only operate 747-200 and 777 planes.

As a side note, I didn't know they had that many JFK-LHR flights now. I suppose it is partly due to the suspension of concorde. My parents just took the LHR-JFK flight and they had a full first cabin. My sister was also on the flight and had an empty economy cabin with 4 seats to herself!