When you are making a purchase online, the system will automatically check the submitted information against that stored by the card's issuing bank. 


Therefore, since the postal code check is done by the issuing bank and you should contact their bank directly to determine which postal code you have registered with your card. 

If the card was issued outside of the UK and is linked to an address outside of the UK the address and postal code to use would likely be the original one for the bank account.