I read it as a little more than 15 hours for the fast eastbound trip!
As i read the Stratocruiser to use about 16 hour for the same trip, I'm puzzled: the gain by the 707 jet shouldnt be just one hour
What timetable are you reading? On the Pan Am timetable the times are:- depart New York 20.00, arrive London 07.35. With 5-hour time difference (ie 20.00 in NY is 01.00 in London) that gives a journey time of 6hr 35m.
Similarly, for the BA510 (BOAC Strat) the flying time is 12hr 45min.