and will BT be in default on the contract through failing to provide 24mb+???
Of course not. Read the service spec and the small print of your contract. 24Mb is the headline (as in advertising headline) rate. BT employ all the tricks in the book on their network to cram as many subscribers onto as smaller pipes as possible.
I know many places slap bang in the middle of London which have a cats chance in hell of achieving the headline rate due to the wonderfully antique structure of the BT network. So I rate your chances even lower than that of the London based entities to whom I refer.
(and before you say "oh but they can get fibre in London" .... nope. They might as well be located out in the sticks as far as BT is concerned).
I maintain that people should not be entering into contracts with BT Retail if they can possibly help it (and avoid indirect contracts that stick you on the BT Wholesale white-label ADSL service.... you should go LLU all the way !). BT are nothing but clever marketing... incredibly poor track record on delivery.