I've actually had a similar discussion with a US seller on Auctiva that wanted to list their items on the UK-ebay. That's not really a good method to do International, if that's your intention.
If you list on ebay.co.uk and select International ship to USA, your auction will concurrently post on both ebay.com and ebay.co.uk, and you will have "home-port" shipping from UK with Royal Post, etc. as shippers.
If I'm not understanding exactly what you want to do, give me more details.
ebay has a pseudo-tutorial on International selling:
http://offer.ebay.com/ws/eBayISAPI.dll?GlobalTradeHub&hubType=1Danno