Was there ever a timetable that permitted that kind of day trip, as they took off and landed in pretty civilized hours?
A schedule I found was:
BA002 dep JFK 08:30 -> arr LHR 17:15
BA003 dep LHR 18.25 -> arr JFK 17:00
Still, certainly allowed "oh crap, need to get to london to do some very important same day business (and return the next day)"