Northolt was used only for short haul flights. All long haul services used Heathrow (or rather London Airport as it was known) from its opening, using the temporary "facilities" on the North Side adjacent to the A4. When the new buildings in the central area at Heathrow opened in 1955 (the control tower, Queens Building and Europa building, which later became Terminal 2), short haul services transferred from Northolt. There was an Ealing Studios film made just after this called Out of the Clouds, which featured the new terminal (the interior was the largest set ever built at Ealing, I believe), but they had both long haul and short haul operating from it which was incorrect as long haul continued to use the North Side until the Oceanic building (now Terminal 3) opened in 1961.