Unfortunately, when there was a copper shortage, BT used Aluminium cables to save on costs. Aluminium cables had next to no effect on telephone usage in most cases, so it seemed like a good idea at the time. Unfortunately, because Aluminium isn't as good at conducting electricity as copper, it attenuates the higher frequencies much faster than copper would.
The upshot of which is that if you have any amount of Aluminium cable on your line to the exchange, your broadband speed will be slower than what it would be if provided over copper.
However there is an anomaly in that 0.7mm Aluminium is better to have in your line than 0.32mm Copper; due to its thickness, the poor qualities of Aluminium are not so detrimental to the ADSL signal.
Affected areas are typically those that had been wired or re-wired by BT in the Early 1970's. There is a lot more aluminium in the D-Side (Green cab to pole) than E-Side (Green cab to exchange).
BT aluminium deployment
