No, it is data , pure and simple , data !! It should be up to the USER to decide how it is used, not the operator to impose draconian charges on one way of internet connection.
Well if that's how you feel, you really shouldn't have signed a contract with O2 saying you agree otherwise.
Seems kind of lame to sign a contract saying you agree with that, then turn around and demand legal action against it. Why did you agree to those terms in the first place if it bothered you so much?
EDIT: And according to google that's the same as $24.20 here in the U.S. I guess this thread is just a small preview of the one we'll get when AT&T announces their $30 tethering plan, hm?