You've got me curious and a little confused on what you are attempting. It sounds like you might be a US seller and want to sell one item on the UK site.
If you are a US seller and list on US site with auction or BIN open to bidders/buyers and shipping to UK, your auction/BIN will automatically appear on both US and UK ebay sites.
For instance..... I ship and have auctions open right now to both US and Canada. My current 11 auctions (see my store) are appearing on both US ebay.com and the Canada ebay.ca sites. If you go to ebay.ca, you can quickly find all 11 by searching the term LtdEdn with "Search title and descripton" checked. That text seems to be unique to my subtitles on my books and is a quick way for me to see if they are showing in search.
If on the other hand, you are truely trying to list on UK ONLY as a US seller, I can see a conflict if you're trying to ship from US to UK for sales. That's because where you register and list assumes that's HOME PORT for shipping.
My best suggestion for that situation is an explanation in the description with no shipping info. Whether ebay allows that situation to occur is another good question I can't readily answer.
Dan