BT are correct that the further the distance you are from the exchange the slower the maximum speed of the service becomes.
Ignore anything that says you're x distance away from the exchange (straight line) - BT will be referring to the length of the cable, and cables dont go in a straight line from the exchange to your home - they take longer routes to follow roads, and to allow the cable to serve the maximum number of homes possible to keep costs down.
http://www.predatorconservation.com