I dont know when light aircraft were banned from LHR, I know AVGAS became unavailable many many years ago just after the days of the lonely Kar Air DC6 which left its mournful tones in the air long after its early morning departure for Helsinki as the last 'big prop' in the LHR .
Way before that ,in my spotting days at Cains Lane, Bedfont, in the early 1960s light twins were not uncommon, several regulars and often the opportunity for something exotic, all those years ago i remember one on of our band excitingly point out an Austrian Twin Comanche which had just landed on 23R.
I n the same era , on a bank holiday weekend when little was happening and it was time to shut up shop and go home when again another spotter peering down (or rather up) finals for 28L exclaimed 'Spitfire' ! indeed there was a spit rapidly approaching with no gear down . A quick increase in volume of the Air band radio (Scanner ha ha) heard tower clear spitfire XXX to land. A slightly amused sounding response along the lines of 'touch and go ok' ? and duly cleared we saw what is, obviously to me unforgettable, a Spit, Merlin roaring hurtle low over the A30 and far too fast to land and after what was probably a couple of thousand feet down the runway pull up into a rolling climb , roll off the top and vanish into the evening sky before we had really taken it in- never see that again CV19 or not.