If you can’t go and get your item yourself you can arrange for someone to collect it on your behalf. They’ll need you to bring the 'Something for you' card that was left, along with your photo ID and proof of your address (like a bank statement or utility bill).
Alternatively you can speak to your seller who might be able to re-arrange a delivery for you.
Remember! Royal Mail will hold your item for 18 days before sending it back to the seller.