I too am unhappy with the price increases for broadband and BT sport. I phoned to commence changing providers and was quoted £538 to cancel plus a further cancellation fee for the sports package. I have been with BT since 1991;
and am more than angry and feel ripped off.
If you cancelled within the 30 day notice period from when you received the notification about the price increase, then there should have been no cancellation charges, provided you told BT at the time, that it was the reason for cancelling.
The notifications were sent out at the beginning of December 2017.