Many companies including BT are keen to offer new customers any number of incentives to join but offer nothing to longstanding customers who stay despite everything.
Why not offer those longstanding customers some form of thankyou, maybe a number free films on BT Vision or free HD Sport. The incentive value could be proportional to the length of time as a customer.
I recently received such an incentive from Microsoft for being a Windows phone user, specifically 20GB of extra Skydrive storage. It cost them very little but I felt it was a nice gesture and good for customer relations. BT should do something similar.