You should know, all these ISP's use the same BT exchanges. If you are a fair distance from it, you will suffer from an unreliable and slower connection than most others. This is thanks to LLU (or local loop unbundling) which has opened a bandwagon that every company and their dogs are jumping on to. So no matter what ADSL provider you go with, in general you will get the same or similar service. The only difference being the customer support. Also remember this service is running through very old cabling in some areas, and old exchanges.
In short, ADSL is the worst of the 2 types of broadband available. If you want a broadband service AS ADVERTISED, you will need to go with ntl or Telewest (which are now the same company). They use cable/DOCSIS for their broadband services, and you will recieve the full line speed and reliability no matter where you are. Although it does mean uprooting driveways and drilling holes through houses to get it installed if you haven't got a hook up point already.
I have missed ntl and my 10mb connection from the day I moved out of my old flat. ADSL just isnt the same experience in any way shape or form. I have 8mb ADSL with virgin at the moment and hate it. Never get near 8mb - and I'm close to the ancient exchange we have in my town as I live in the town centre.