They all use the 'BT Wholesale' core infrastructure.
But they have differing capacity pipes into it from their own networks.
'BT Retail' (who you buy from) also uses 'BT Wholesale' infrastructure.
BT Retail is (in theory at least) completely seperate from BT Wholesale.
As such, it has to compete on equal terms with the other non BT isp's.
In fact, BT Retail are looking at local loop unbundling just like the others.
So, could end up with BT Retail putting their own kit in BT Wholesale's exchanges :duh: