If you are the brand owner you have priority and can copy the listing, click “go back and edit”, change the SKU (this is vital), save, and wait 2 mins. Both listings of the same item with different SKU’s will update and you can then delete the one you just created. If this does not work, try any of the following:
a. Use the Amazon Seller phone App to change the details as sometimes this works;
b. Change EAN to UPC or GTIN and see if that works (GS1 UK is a 13 digit GTIN but Amazon only recognises GTINs as a USA 14 digit). UPC is a universal equivalent and this may work;
c. Never list by searching using the ASIN but only the barcode number EAN;
d. You can literally delete the product, wait 24 hours for the system to clear, and relist using the EAN;
e. You can request Seller Support to change it for you, but you may have to jump through all sorts of hoops such as providing your own website page, GS1 Certificate, and be registered on the Amazon Global Brand Registry (holding a Trademark Certificate or similar).
My final tip - persevere, do not give up. It is a nightmare but all learning is useful.