I'd consider other providers as well as many of them offer far more flexible packages than NTL do. Check out adslguide.org.uk for all things ADSL. There are plenty of reviews and comments on ISPs and what features and packages they offer.
Also have a look at
www.samknows.com and see what your exchange supports. You might find you can get a LLU service which opens up a lot of possibilities without going through BT.
Also worth keeping an eye on UK Online's new 8Mbps offering (
www.ukonline.co.uk/8000), and then there's NTL's own LLU plans.