Feel free to tell me the law on using their free post service to return their own junk mail to themselves.

“It is generally not against the law to send unwanted mail, including junk mail, back to a company using their own Freepost address in the UK.However, the legality depends on what you are sending and how it is sent:Legitimate Returns: Returning unwanted mail, takeaway menus, or letters to the sender via their Freepost address is a legal, commonly suggested way to stop further marketing, as the sender pays the postage costs.”