I'd be very surprised if your contract guaranteed you 8 meg, 5 meg, or any specific amount at all... if you want that kind of assurance, you'd need to get a leased line, and pay accordingly for the privilege.
I think if you examine the small print (and probably the large print for that matter), you'll see you signed a contract for "
up to 8 meg", which could of course mean much less in practice.
You could of course change to another provider, but I'm afraid as things stand 02 are well within their rights to insist you pay for the full period you signed up for.