Apparently they are changing the entire UK test regulations in 2012:
at 16 you can ride a 50cc at 17 you can ride a 125cc but if you pass your test on this you can only ride up to a 125cc at 18 you can ride a 250cc once you passed a test on a 250cc but can only stay on a 250cc at 19 you can ride whatever you want but it has to be restricted to 33hp for a few years
The age when you can do the test a straight off and ride whatever you want is to be raised to 24yrs old.
These could be changed, but I don't really see the point - the only thing it seems to be doing is trying to put people off from riding motorcycles. Are they going to be doing the same thing for cars???
So basically someone could end up taking their test more than twice for the same result. If you're young, and want a bigger engined bike, you're going to have to fork out for it, again and again.