You don't have to have a phone line for ntl cable broadband - the modem (or STB) connects to the cable feed, not the phone line.
There's also no need to have to subscribe to the ntl phone service if all you want is broadband - the only service for which you *must* have an ntl phone line is the TV (as they bundle the phone & DTV together).
I don't see why a SACM couldn't be used. The "STB broadband" areas did used to only have broadband via STB, but they can now use SACMs too (well, some parts can, certainly).
Best thing to do, just phone up ntl & ask them