So, blame Ofcom for not allowing BT to provide lower wholesale prices in Market A.
Ofcom set charge controls on market A wholesale pricing to stop BT Wholesale from over-charging.
Ofcom do both.
The prevent BTW from over-charging, to protect consumers, and prevent BTW from under-charging, to encourage LLU providers to consider the exchange.
The latter control stays in place even after LLU companies turn up, so that the LLU provider can be assured to have competitive advantage for a while, so as to generate some RoI.