BLackberrys are odd things, they require a proper blackberry data plan on the account for several of the features to work, if you are using a standard O2 account then the normal WAP settings will let you browse the internet using a thrid party browser (like opera mini) but not using the blackberry browser (that requires a blackberry account to work)
You also don't get the benefit of push email or being able to download and install extra apps.
You can use the full features while connected using wi-fi, but otherwise, without a proper account they just become fairly normal (if slightly large) phone :(