Actually, the 2 possibilities are the player who filled out the form and the retailer who entered it onto the system. But the one with the power to change it is the site, unless of course you are a member of a site who also runs a UKARA retail operation and entered your form.
Call the retailer up, tell them the mistake. If the difference is a substantial mistake (you wrote AB12 4PJ and they entered OP4 1GC) then they'll need proof that you're not just someone else trying to buy without UKARA