Careful with that legal axe, Eugene.
You need to look at the small print carefully to see
(a) who precisely your contract is with - you have to serve proceedings on the defendant at its registered office- check on the
companies house website for the current address
(b) what if anything it says about a FUP or other restriction
(c) if the usage policy has been changed since you signed up, whether you have been notified about the change in accordance with the contract (which probably says something like "changes will posted on the website and if you don't like them you have 21 days to sod off or you'll be deemed to have accepted them"). Since this is all about the internet it's difficult to argue that posting a T & C change on the website is unreasonable, especially since you probably agreed to it even if you didn't notice.
If in fact the rules have not been changed in accordance with the rule change rules (if you see what I mean) then they haven't been validly changed and you're on stronger ground. You can then say that Demon have not provided what they agreed to provide in which case you're entitled to at least some financial adjustment.
The question ultimately is "What financial loss have you suffered" and how can you prove it. No loss or damage = no claim.
There might still be an OfCom/Trading Standards issue.
Bear in mind also that your T & Cs may provide a dispute resolution procedure. If so, that's also part of your contract. If you go to court a clever defendant could ask the court to "stay" or halt the proceedings until you've gone through the
agreed process first.
The theory of the law of contract is that parties are free to agree anything they want and that in general there should be no interference with that - it is not for the courts to decide that an agreement you've entered into voluntarily is unreasonable. The basic principle is altered somewhat by things like the Sale of Goods Act and the Unfair Contract Terms Act which provide some consumer protection. But apart from those safety nets, the basic principle holds good.
Good luck with it but read all the words before you start.