I have recently upgraded my broadband which involved the installation of a new router. What do I do with the old one?
If you call BT 0330.1234.150 Billing you can request that BT send a returns bag out to you.
Do not just dispose of the old equipment as you may get charged for it.
You should also retain your Proof Of Sending. It may be worth taking a photograph of it per chance you misplace the paper copy.