A key component to the new charging systems is that providers are not able to mix charging models, i.e. "If a Customer has opted for Usage Based Charging, then Usage Based Charging prices will apply to all of the components of their BT IPstream Service. It is not permissible for a Customer to mix Usage Based Charging, Capacity Charging or Standard Charging.". Interestingly though, the BT Central Plus product as used by BT Retail, does not have a Usage Based Charging option. In the original announcements about UBC and CBC charging, it was mentioned that the Standard model would be removed, but we have not seen any mention of this in the current notifications.
So it seems the original pricing for wholesale ADSL may be here to stay.
I guess all the ISP's with heavy users switching early to CBC may have made a mistake.