I'm sure someone will answer this in more detail, but a disconnection is not always down to the poker room - in fact often it isn't at all.
Sometimes it is your ISP that has a problem, sometimes it's a connection problem somewhere between you, your ISP and the ISP and server of the poker room. PokerStars has a trace connection facility and this will often show you where the problem is.
You'll also suffer from 'disconnection' problems if your computer is pushed to its limits, either in terms of memory or processor. For example, if you run a graphics package such as Photoshop, it can use up all the memory on your computer, causing the poker software to hang. The same if you are running video or something else that requires a lot of your computer's resources.
Because there are so many reasons that your connection can drop with the poker room, it's difficult to say what each one is down to unless you are there at the time. Often I'll be on Blonde and PokerStars and one of them will hang, whereas the other one will be fine. When both of them hang, I know that it's a problem with my computer or with the connection at my end.
I too am on ntl, and although I'm paying more than others the connection is very stable and only twice have I ever had problems with the connection that was ntl's fault (as in beyond my control and therefore my own fault).
You can have an Internet connection, but be unable to get data through certain ports (similar to TV channels I guess). Not sure what ports the poker rooms use, but your web browser generally uses port 80. So it could also be the case that you can access the web, but not your poker room, or vice versa.
Now someone who knows what they're talking about might answer your question...
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