It has already been slow tonight on NTHELL....which would seem to indicate that capping is necessary to keep the bandwidth at reasonable levels as there is too much usage.
NTL have been victims of their own success and are running out of bahdwidth. They can rectify this by increasing capacity but this would be prohibitive as they are technically bankrupt and more importantly, they would have to increase their subscription costs significantly. As cost is more important, to the majority of users, this would mean a desertion to other ISPs which would drop NTL firther in the mire.
Naturally you have the right to express your disatisfaction and sign up with another ISP but I suspect many more will follow suit and the ones that are left will attract the 'power users' and consequently their service will be dire.