Thanks for posting your question here. I am pleased to inform you that you can use Auctiva to post listings to the eBay UK website. When you open up the Create an eBay Listing page within Auctiva, you will just need to select "United Kingdom" from the "eBay Site" drop down menu in the upper right part of the page to load the eBay UK lister.
I hope this helps. If you need any further assistance with your account, please feel free to contact our customer support team by mousing over the "Help" tab and clicking the "File Support Case" link.