The new (for the UK) 8 MB ADSL is most definitely "up to" -- most people won't even get close to that speed. ISPs who don't state "up to 8 MB" are just asking for trouble...
If you're on cable (rather than DSL), you're sharing a local loop through your neighborhood. If you have a lot of neighbors online doing big downloads, your maximum possible speeds drop as well
This is generally the case for ADSL too -- most UK ISPs run 50-1 or 20-1 contended services (ie. potentially up to 50 lines sharing the same bandwidth, though it's apparently not generally pushed that hard in real life).
Also, the distance from your house to the exchange can be very misleading when it comes to determining the speed you'll get, as the telephone line will almost never be routed in a straight line from your house to the exchange, and the longer the line, the lesser speed you'll get.
Also, BT telephone lines can run past sources of interference, be of poor quality for ADSL (they're only guaranteed to give reasonable quality voice calls at present), etc, etc.
I have 10mbit cable and I usually only get around 250kb/s
Weird, I have 4 MB cable and get up to about 460 KB if downloading from a decent server... I don't believe cable has anything like the same distance limitations since it's a fibre-optic network.