You don't have to pay back either.
You agreed to a contract price for 12/18 months. BT could have honoured that price and only put up pricing on out of contact customers but decide to apply to all, triggering the get out clause for in contract customers. I guess the loss from those who leave early is lower than the increased revenue from those who remain.
Recently BT lost some TV channels (fox) and so had to let people leave their TV service. In doing so I was released from my TV contract and didn't have to return the box.
I'm now considering leaving my phone/broadband contract as the discounted price is nearly ended.
Ashington (Northumberland) Exchange