Have you checked the address on the UK postcode address service? Jamal ? Because maybe if this address was not EXACTLY the same as on the UK computers then it may have been rejected by COMPUTERS ! Put in the exact address you gave the seller and search, even one or two letters or numbers can cause a rejection. Please search online, and with Royal Mail UK.
If your address is EXACT as on the UK postal data bases, THEN it could be that the address was slightly wrong on the package, for instance a wrong letter on the postcode or a wrong number in the state code etc. This then would be human error on the packers part.
But first Jamal, maybe contact Royal Mail and see further why the rejection OR do as I suggest above and see who is to blame FIRST so that you know who to turn to so they can fix it. !
Hope this helps :)