If they were British, they would be aircraft or aeroplanes, not airplanes.

Yep, the shot of fighters taking off looked like Spitfires to me.
The plane that crashed near the car Greer Garson and Teresa Wright were in looked like a Messerschmitt Bf 110.
I was surprised when Teresa Wright said that she thought that was Vin's plane, as it was clearly a 2-engine plane and he would be flying a Spitfire or Hurricane, single-engine fighters. She didn't know much about what he flew, I guess, which is understandable.
Like a bird on the wire, like a drunk in a midnight choir, I have tried in my way to be free

All of the British fighters in 'Mrs. Miniver' are Supermarine Spitfires, shown in one actual newsreel takeoff shot and in other instances by miniature models and one full-size prop mock-up. The only exception may be in the single, brief combat scene in which two models on wires criss-cross the screen - one of these mdoels may have been of a Hurricane (I just watched the film on TV and this combat shot was so brief that I hadn't time to fix recognition of either one, or the other, of the two aircraft models).

I think they were Hurricanes. I'll have to watch it again.
"Whenever Mrs. Kissell breaks wind, we beat the dog."

I wondered about that myself. Could that scene of the group taking off have been shot in UK?