What cc bike can I ride?

[chooky536]

i thought it went by what license you have like l's to 600 or sumthing or power to weight ratio

 

[Lyall]

I would like to know what cc motorbike I can ride when I am 18?
Is it 400cc or 600cc?
Thanks
I live in the UK, and I am nearly 17, but I want to save up loads for a really good bike, and I want to get either a 400cc or 600cc when I am 18, but I don't know which one is legal.
Also, what tests do I have to take to ride a 400/600cc bike when I am 18?
Thanks