Although I really doubt they show up as such in the timetable, another very regular 720 operator into Heathrow by the summer of 1972 would be Aer Lingus. They still had their two original 720 turbojets until the end of the year, they were regarded somewhat as spare aircraft but could turn up whenever, and while the Dublin to Heathrow service was likely shown as all 737, if there was any disorganisation - fog, strikes, pre-Christmas rush, etc, and a passenger backlog developed on the frequent service, then a 720 would be assigned to work maybe multiple round trips to London and back to help clear things up.
It's still like that nearly 50 years on, and if these same events disrupt things nowadays it's common for an Aer Lingsus A330 to do a couple of round trips to Heathrow. In the intervening years it was even their 747s on occasion.
For decades now Dublin has been London's busiest short haul destination by passenger numbers, and New York the long haul one. I believe Dublin retains it's position, but for long haul, this year, amazingly Dubai has actually overtaken New York.