From what I understood, not enough of the aircraft have been 'modified' with the Kevlar lined fuel tanks to allow them to be used for charters.
This means that there are just enough available to operate 1 daily round trip to New York, hence the reason for the 2nd daily LHR-JFK flight having not resumed yet.
I believe that 3 aircraft are required for the daily flight, one at LHR, one at JFK and a standby.