Given that BT installers are known to fashion data extension kits from catv with even a couple of pairs removed, what cable should they be using to have the same impedance as BT twisted-pair?
Who knows? It'll depend on what decade your phoneline was installed I'd guess - different gauges/different metals/different cable bundles have been used over the years. I'd be surprised if BT had a clue for a lot of historical plant. Edit - and I'd be astounded if they'd ever had figures for 15/30/60/100MHz insertion losses/xtalk in the first place for that plant.
However what you have to bear in mind is that this is going to become a lot worse if :
a) g.fast is the chosen method to compete with Virgin;
b) Ofcom go full retard & permit sub-loop unbundling on Thursday 25 Feb.
ADSL works up to 2MHz, VDSL2 (FTTC) works up to 15MHz in the UK (30MHz elsewhere), g.fast works up to 110MHz. The higher the frequency, the more problems you have with old copper due to shortening effective antenna lengths. Edit2 - as the effective antenna length shortens then sections of cable with faults/dodgy joints are going to be radiating/receiving a lot more than they do now.
Still I suppose it keeps people in work.