[ceedee]

From the article:

Varley continues, “there are many variables that affect broadband speeds, distance from the exchange, contention, line quality and so on. It is a challenge to be able to factor all these into a calculation that gives you a reliable idea of the actual speed an individual connection will achieve.”

So difficult that SamKnows Mapping have cracked it!
(Have a look and see what speeds you'd be getting with ADSL/ADSL2+)

(Thanks for your work, Sam!)