Yes it is limited, that limit varies depending on the capabilities of the networks in each area.
No the internet services don't take any bandwidth away from the TV when used, the two both have seperate quotas of bandwidth and cannot take bandwidth from each other.
http://www.digitalspy.co.uk/cable/howitworks/ is 'kinda' right.
You receive a lot more data than you are actually using at any one time.
As a quick rough and ready guesstimate it's not impossible for Telewest / NTL to send over 4Gbit/s to your home.