The aluminium wires date back to political upheaval in Rhodesia (as it was then) following which the price of copper went through the roof. The terrible legacy was its use for telephone cables. IIRC this was of the pure variety and quickly became brittle, leading to a circus act of engineer following engineer following engineer etc. Subsequently an alloyed version was introduced which was much better. Further problems were caused by corrosion between this and existing copper wires. I left BT in '96 and I thought that most of these cables had been replaced, but I suppose there may be a few lengths about.
A possible reason for slow ADSL performance would be a poorly made connection, or changing from one conductor size to another. These can cause impedance mis-matches which will attenuate the higher frequencies used.