A "right of centre" think tank called Policy Exchange has backed dropping fuel duty and replacing it with a pay per mile charge.
They aren't only prposing to make you pay by how far you go though, they want you to pay more if you drive at busy times like rush hour.
To make this work though, it would seem that every car would have to be equipped with gps, tracking your every journey.
It's too much like 1984 for me I'm afraid.