If you look carefully most services are advertised at "up to 8meg". Although many suppliers previously left out the "up to", but they aren’t now.
A recent Ofcom investigation into ADSL Max (up to 8meg service) found that the nationwide average download speed was 3.6meg.
This is basically the speed as it leaves the box in the exchange, but its not usually possible to get more than about 7meg due to data losses and other stuff. The speed of your service is dependant on several factors (this applies to BT copper lines, what 90% of people have).
1) Length of the copper line into your house from the exchange.
2) The quality of your copper line.
3) The contention (number of users all using the same ‘box’ in the exchange.
There is a finite length of line before the broadband (DSL) signal degrades to the extent that its not possible for it work reliably, taking into account the other factors also. Its also very difficult to predict accurately what speed you should get, hence the "up to".
Im about 600m from my local exchange and usually get 6-6.5meg download speeds.
If you want to get a decent idea of what you can expect to get, go to http://www.samknows.com/broadband/checker2.php
and you’ll get a reasonably accurate idea of your lines capabilities.
Clearer now?