LHR-JFK - treat it as a very long day because of the time zone change. I prefer evening departure flights around 6:00 pm, which get you in around 9:00 pm and mean you can collapse into bed when you arrive at your destination.
JFK-LHR - all overnight flights with the exception of one each on BA and AA which leave 8:30 am and arrive LHR 8:30 pm (referred to as the Pensioner's Flights where they can avoid an overnight). Regardless, be prepared for jet lag as the time difference works against you.