I cannot be 100% sure, but it is from 2 sources, one from who works for openreach, the other from a database that used to be publicly accessible that showed lengths of D and E sides, but it also showed gauge of the cables.
Yes you will hear odd comments on forums suggesting there is barely any ali in use and that all copper is at least 0.4, but I think they just paint a rosier picture than the reality, my area was built up during the days of copper shortages.