Heathrow was once LON and I'm pretty sure flying back for Mahon to East MIdlands in 1975 our bags were tagged CDD or CDN for Castle Donington.
Yes, EMA replaced CDD (indicating Castle Donington/Derby, presumably).
LON preceding LHR must have been before my time, though I do recall in my youth when the road signs for Heathrow said simply "London Airport".