Use the availability checker !
There is some laws of physics stuff in here which non-techies might find difficult to swallow. Essentially the further from the exchange you are, the lower the speed you'll get.
The people at BT have provided a line-checker which gives you a much more accurate view, based on your phone number - check the link here: http://www.productsandservices.bt.com/consumerProducts/displayTopic.do?topicId=16738
This tells me I should get 2.5 (Megabits per second) - in fact I get nearer 4.
Perhaps the answer is for ISPs to point to their version of this link - that way you know before you sign up what you're likely to get.