Don't worry; if you leave your new provider there are plenty of options that will be better than BT.
But if you do go back to BT you can bet your life they'll make you pay.
And that'll be after they've totally screwed you up, and only after you have then gone through the necessary ordeal of speaking very slowly and clearly to at least 6 call-centres in India, none of whom will have any knowledge about your case.
TalkTalk, Utility Warehouse, and others all offer excellent prices and service.
Edit; sorry, I forgot TalkTalk is now Opal, for no very good reason.