Buster - even with my steadily advancing years, I can not remember when an airline totally left a BAA airfield for Luton ... although maybe the years have affected my memory! As you know, it's usually the other way round.
The only thing I can think of would be in the late 1960's or early 1970's when Air Bridge used to run Vanguards/Merchantmens on Heathrow - Brussels for DHL and they moved the whole operation to LHR.
Maybe EGGW Relic will know - maybe ask him monday night before he has too many!!!