I believe that Luxair was the last scheduled operator into Heathrow with turboprops ( Q400s, coincidentally ) until BAA 'suggested' that they release the slots or put something bigger and / or faster on them. They now operate to City.
KLM also had F50s on the Eindhoven service until around the same time.
I'm not so old but I even remember Sheds at Heathrow! Air UK and Manx I think.