I would avoid the early/morning departures to JFK - you will get good work done on the flight (but not as good as on the ground) and it's a really long tiring afternoon to get through on arrival

I would also avoid the horrible 5/6/7 pm "red-eye" departures from JFK on the way home - it's way too early to sleep then, you won't get any rest and get dumped at LHR with a long Friday to get through and basically no sleep because you land at the equivalent of midnight/1am New York time (just when you are most tired) - that is grim and I avoid that at all costs: day flight, or later evening departure from JFK