if you're under 21, you can do a 'restricted' test, which means you can ride bikes bigger than 125cc but not more power than 33bhp. the idea is to stop young people from jumping straight on a big bike and not having the skill to control it. after riding for two years you can ride anything.
if you're over 21 you can skip this and go straight to any bike (called Direct Access or DAS).
HTH