I don't think that you'll be able to find the answer on my site, at least not yet but you could ask in the forums of course.
The 1968 summer timetable has a 2:00 departure and 8:40 arrival for the NY to London flight listed (times in GMT). As to how long it took on a good day... that remains the question.
Edit: Figured I might as well add it myself:
http://www.vc10.net/forum/viewtopic.php?f=3&t=261