A number of ducks have to be in a row for autoland, or Cat IIIA/B as we now call it. Both crew qualified and current, aircraft systems on line, all required ground equipment and lighting serviceable. Take any one duck out and it cannot happen. Not sure of the state of play of Heathrow's kit back in 1975.
Incidentally, Luton was the first airport where the technical services were not operated by NATS to go CAT III in 1989 or 90. We sweated blood.
Edit: cj beat me to it. Wiki suggests the Trident fleet went from Cat IIIA to IIIB in 1975 so we are talking of 75m RVR touchdown.