Same problem here. Mistakenly threw returns bag away with new packaging. Found bt.com advice to order another but the links go nowhere. Called phone number recommended but menus again go round in circles and nothing to help. Don't like this cos I think BT will charge my account if I can't return the old hub!!!
If you call BT 0330.1234.150 Billing you can request that they send one out to you.
Or,
Select the 'message now' option on this link https://www.bt.com/help/contact-bt/account-and-billing/broadband to have a live chat with a billing guide, they'll get you sorted.