If you aren't an NTL customer yet, then the install tech who brings you your STB will put the main wall box where you want it, running cables as ncecessary. This won't cost you any more than if you wanted the install done with the wall box left sitting where it now is.
Technically you shouldn't do cabling work yourself, although seeing as you aren't an NTL customer yet I'm not sure what they can do about it. They can't accuse you of braking terms and conditions you've not signed up for!
On the whole I'd suggest you order from them and let the tech do the wiring.
to the forum by the way!