Polly gets her hair cut in the original version of Macra (though it had grown again by Faceless Ones!
). That scene was cut out of the animated version for time/budget reasons.
That being said, I don't know why they didn't just have her with long hair. As it stands, watching the DVD she inexplicably goes from long in Moonbase to short in Macra to long again in Faceless for no apparent reason.