Trident into Edinburgh :
I recall that the Edinburgh route stayed with the Vanguard until the end of its service, because BA said the old runway was too short, and the wrong orientation for the prevailing wind, to use for Trident operations. This was a significant part of the discussions in the mid-1970s about the new runway.
Once the new runway was approved, it seemed the Vanguards steadily disappeared, and the route became 100% Trident quite some time before the new runway was commissioned (1977 ?).