Regardless if the items were sent back by free post the Post Office should still have given you a Proof of Postage receipt so that you had a record that the items were given to the Postal system.
It will now be difficult for you to prove that you have returned the items.
I would suggest that you go back to the Post Office to ask what can be done.
In any event BT are taking about four weeks to credit accounts after they have received returned items so if that time scale has not passed you should wait until then to see if BT do receive them.
