Hi,
The guidance on returned goods relief states:
"Goods imported by post from outside Great Britain
Ask the sender to write ‘Returned goods — relief claimed’ clearly on the package and its accompanying import declaration. The declaration must include a complete and accurate description of the goods, their quantity and value. If you’re registered for VAT, make sure your VAT number is quoted on the declaration (CN 22 or CN23)."
You can speak to the courier who will submit import declarations and explain what documents are required.
Getting someone else to complete your import declaration
Thank you.