The airframe looks like a heavily modified Aero Commander or a Fokker F.27... Still, national pride would probably have not allowed the use of a foreign airframe when there were so many being designed in Britain at the time. Both the F.27 and the Handley Page Herald had those big oval windows and sat low to the ground. If it's a Herald, it's been pretty well disguised. As the plane is being rolled out in the early seconds of the film, you can clearly see the tail which looks to have been constructed from wood and paper mache. I should imagine the windows would be hard to redesign off an existing airframe, though hardly impossible. The Herald would be my best guess, and it is most definitely a guess.