After you guys mentioned it, I searched at youtube just to remember the scenes (watched the movie a long, long time ago).
Noticed two things: the Spits were armed with 20mm cannons, so probably IIb or Vb. The other thing... How the hell was that American flying with an Eagle Squadron badge on the plane and using 303 Squadron's lettering? I'm sorry, but he didn't looked very Polish to me
And, by the way, the speech seen at those scenes were the worst radio chatter I've ever heard in my whole life...