2 ways to do static IP on cable, 1 is semi-static the other 'truly' static that I can think of anyway!
First way semi-static is to do fixed entry in the DHCP dbase and treat this IP as reserved for x customer and only usable from that cable modem. Downside is that certain things can break this IP and it no longer routes properly.
The truly static way is to use PPPoE so that an IP is taken from a central pool rather than local DHCP servers. Think this is the way that Telewest are doing it going by the way their static IPs route.
No idea how ntl are doing it, may be doing it a completely different way, I'm not an expert on these things
