BT will use thicker cable for properties a long way from the cabinet. About 800m of my D-side is 0.9mm copper and the last 500m or so is 0.6mm (I tried to push for 0.9mm, but they said it couldn't be run overhead). However, the E-side was likely aluminium, which is why the figure was so woeful. I think I read somewhere that BT tried to ensure that all of their network was no longer than the equivalent of 6.5km @ 0.5mm, but I could be pulling that out of my rear.
14dB/km must be for thinner and possibly more degraded lengths of copper, or maybe a very conservative average. On my old routing I was 5.9-6.0km from the exchange and my attenuation on all 3 lines was between 56-57dB on ADSL1, which wouldn't correspond very well to those measurements. Realistically once it hits that maximum of 63.5dB, it probably would be pretty difficult to estimate what the attenuation figure actually is.