ntl haven't had a chance to finish implementing the traffic shaping and throttling hardware that'll slow your connection down when you hit your limit for the month then offer to charge you per GB yet, and there are the details of per protocol shaping and caching that are going to need to be done. They are struggling with the existing bandwidth demand going by recent entertainment so are either going to need more bandwidth or a smarter network.
To take all tiers to 10Mbit with shaping, etc is optimistic, without it is suicidal