GA: Even though 8Mb never really means 8Mb, there are still 'trigger' levels, below which BT consider service to be unacceptable and they have to fix it. Can't remember offhand what they are, but it's worth finding out if you have constant problems.
Oh, and your ISP can't force you to pay for a contract which they themselves are not sticking to.
Oh, and your ISP can't force you to pay for a contract which they themselves are not sticking to.