Working from the visitor lists for the RAeS Garden Parties at Heathrow, KLM flew DC-2s and DC-3s there, the DH91 Albatross visited, an early Vickers Wellington also. The Croydon Airport Society have a record that Lord Forbes DC-1 landed at Heathrow . Various Lockheed twins flew there ,too.
The Empire flying boat Calpurnia displayed there in 1938 but didn't land :-)