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
Found this pic of N428NA at Heathrow dated 1971
Douglas C-118A, N428NA, NASA
I wonder if that's one of the Froggy Falcon 20 calibrators behind it.