I remember the FAA Convairs which I think might have also done calibration work for European USAF/USN bases.
Also the CAA Doves which might well have done the job between the princes and the 748s but not sure on that.
I also remember a NASA DC6-so long ago it might have been NACA doing a whole series of flights out of LHR for some reason that was supposed to be to do with future ATC issues but that really was along time ago
PB