Openreach operate as a seprate company to BT Retail, they provide the infrastructure work to ALL ADSL providers including BT BT Retail. Only Openreach can work on their equioment asd its their equipment so nobody else can touch it. As part of the Ofcom regulastions to encourage competition their charges are fixed and published to all ISP's.
I guess what im tryinh to say is that although owned by the BT Group Openreach are a completely seperate company to the BT retail and BT Wholesale.