BT is an ambiguous term really.
AFAIK it would be better to think of two regulated separations.
Openreach who have to make/sell products equally to LLU providers like Sky/Talk Talk and treat BT Wholesale just the same as them.
If Sky have kit/handover in an exchange then they would have their own BRAS elsewhere.
BT Wholesale have products they sell to other ISPs and have to treat BT retail the same as the other ISPs.
ISPs using BTW products would be via BTW BRAS servers, I don't really know anything in detail, but BTW ISPs are subject to the BTW ipprofile/BRAS rate.
Their handover is via expensive interconnects and their are different products with different pricings (WBC WBMC - look on sinet). The ISPs will likely have their own auth servers and kit that in s/w terminates each users ppp/whatever other tunnel (L2TP?) and also limits their connection rate to BTW bras rate.
It is also possible that "other" ISPs like Andrews and Arnold buy products from both BTW and LLU (Talk Talk).
This is all vague really as I don't know the true detail - search for the BT sins/openreach product guides for more detail about ISP - BTW interconnects. As for the internals of BTW I guess there is less published about that.