Ethernet cables aren't co-ax they're twisted pair cables. In fact they're actually bundles of 4 twisted pairs so 8 cores in total - the pairs are twisted around each other to reduce crosstalk and interference. Co-ax on the other hand only contains two conductors, a (usually) solid single core and a grounded braided shield with a plastic separator. The signal doesn't actually travel down the centre conductor, rather it travels as an electromagnetic wave down the space between the shield and the central core.
Not sure of the details of the NTL network but I imagine it will be fibre optic in the main (I think they're trunk backhauls are actually leased from BT) with co-ax from the street to the set top boxes.