Ok here is what I know and think.
BT and all ADSL providers.
Currently ADSL is over copper on the local loop and whilst the figure for those who can get a 8mbit synch is below half I dont think it is as low as7%, 21cn network has recently been leaked to include fibre to the cabinet rollout meaning the % that can get high speed synchs will jump right up and of course includes adsl2+ rollout, the rollout is staggered between 2008 and 2011 depending on where you live and their is a checker for the voice side of upgrades here.
http://switchedonuk.org/home/when/ Their is no checker for the broadband side yet.
Ntl.
Ntl for a while have had a big technological advatange over BT allowing them to offer triple play services as well as superior broadband services but have failed to capitilise on it and make the profits needed for expansion, without expansion ntl have a continously shrinking footprint, they also have the disadvantage that their services arent universal across their userbase so different billing systems and different services digital/analogue. The analogue services are typing up a lot of bandwidth and reducing the bandwidth to other services. In short they have a strangled potential with the low coverage and weak technology in some areas.