There was Hazel Blears in A Taste of Honey (though that photo is not a still from the film) and Stephen Pound in Here We Go Round the Mulberry Bush.
Now Amber Elliott on Total Politics has come up with another example. A very young John Cryer, now MP for Leyton and Wanstead, appeared in The Railway Children - and she has a photo to prove it.
John's father Bob Cryer, himself a Labour MP, was a great supporter of the Keighley and Worth Valley Railway - the preserved line where the film was shot.
When discussing this enthusiasm of mine in my New Statesman column I added:
I have also been told that Glenda Jackson, an obscure backbencher who was briefly a transport minister, appeared in a film once. It sounds unlikely to me.And if you are interested in future (or past) Liberal MPs in films, see this post.