Your eBay account (https://www.ebay.co.uk) and the eBay Delivery account (https://delivery.ebay.co.uk/) are 2 different platforms connected to each other in order to provide the shipping functionality. This allows to import effortlessly in ebay delivery your items sold on ebay. As you can see the URL to access eBay Delivery and eBay are different.
eBay Delivery uses your settings in your eBay account, for example if you have already default addresses on your eBay account, these addresses will be your default addresses on eBay Delivery as well. If you have added different default address on eBay Delivery, the eBay account address will always prevail.
In such case every time you import the shipments you will have to manually change the delivery address of the shipments imported to the Manager. Check that you have correctly deleted all the addresses in your eBay account from the menu: My eBay - My Account - Addresses.
Alternatively when you create your eBay listing go to item location and change the postal code within item location: Sell item > Listing Details > Item location (located at the very end of your ebay listing):