You need to understand the difference between BT Wholesale and Openreach.
BT Wholesale has competition for backhaul in exchanges that represent 96% of UK households.
The current issues are may not be specifically the capacity in the BT Wholesale backhaul from your exchange, but where it aggregates with others in the area.
has a little more for how you can spot if it is affecting you and if its bad you have the choice on your exchange to switch to TalkTalk or one of the providers who uses their wholesale network.
Capacity issues like this usually hit one or more of the providers each year, just that this is the first major one on the WBC network that BT Wholesale run.