Usually .co.uk will rank high in uk-based google search results. But if your IBLs are relevant and powerful, you will rank high in global Search Results. There are options in the webmaster tools where you can set the preferred country or region you are trying to optimize for.
If what you offer is the same or the difference is minimal for both markets, you can promote using your existing site. It it differs greatly, then a separate domain for the US would be a good idea.
