SSR was introduced to a small number of UK Area units in June, 1963 and arrived at Heathrow in the late 60s. Initially, responses were two slashes with a third for "ident". Carriage of 4096 code transponders became mandatory in the UK above FL100 in CAS in January, 1972. Radars at West Drayton were always able to display the four-figure codes plus height readout from the time the radar controllers moved there. First and second digits indicated route and the last two for individual aircraft.
(Info from the book Heathrow ATC - the First 50 Years).
All I can say is "Thank God" as I would never have validated without SSR. One watch at Heathrow viewed SSR as black magic and always switched it off!