On the one-page lister:
1. Make sure you have ebay Site: United Kingdom seleted at the top of page.
2. In the Shipping Tools section (use Maximize) check the Zip/Postal code and your location info is correct. You can also use the Maximize All at the top of the one-page to view ALL the possible settings and look for any unfilled or conflicts.
Use the ERROR CHECK button at the bottom of the one-page to see if there are any obvious conflicts that it can detect before listing.
Danno