You should be aware that the JFK-LHR flights starting with the 8pm flight are more often than not delayed. Usually the 10:30pm flight is delayed longer than the 8pm and 8:30pm flights. Of course, that is not always the case, but I've found it to be generally true in my experience (I do that route once or twice per month and if I want to count on and on-time departure/arrival, I don't book on a flight later than the 7:30pm one).