Under the Consumer Rights Act 2015 the product must be 'as described', but then I am not a lawyer. If you were to ask a lawyer, then it might be said that the ISP has absolutely no chance of enforcing a contract where they are only delivering half the performance described unless in special circumstances (multiple TCP connections). Or they might not. In any case the ISP could do without the bad publicity. I suspect that you do have options. It is up to you.
Although I must stress that I know absolutely zero about the details of the affair, it does seems clear that TalkTalk have hurt a lot of their customers with this demonstration of incompetence. Even if it is not directly their fault, they have taken an outrageous amount of time to debug the issue and were the TT staff concerned working under me, then, well, errm, they wouldn't be. TT's customers have their own reputation to maintain.
Luckily for me, I wasn't affected, because I am on a BTW service.