the type of, and condition of, cable feeding your premises can make a major difference.
One local trading estate here is fed by an aluminium cable around 1.5km long from the nearest cabinet and the busineeses there are lucky if they get better than 0.5Mb. Total cable length is around 3.5km to the exchange (which should be OK), but the aluminium section simply carries too much noise.
However I got BT Infitiy installed at one of the companies last week and their speed went from 200kb/s to 15Mb/s. It looks - based on that one case - that the Infinity system may cope with aluminium better.
The flip side is that I know of one site, in a remote valley near Coniston in the lake district, fed by an overhead cable across the fell tops leading to an exchange something like 6km away, which get a 5Mb/s service. I can only assume that it uses a decent quality copper cable