The Royal Mail website says this:
"Was your mail sent to the UK from outside the UK? Although we deliver the mail once it arrives in the UK, you still need to go back to the original sending company for help if your mail has not arrived on time.
Contact the postal service or authority used to send the mail. If necessary, they will contact us to make enquiries on your behalf".
So, assuming that the item was sent via USPS (and not via an independent courier firm) it looks as if this is the link you'll need:
https://prd2faq.usps.com/faq/iq/usps/request.do?forward=emailUs