A common but difficult question was asked at HighRanking Forums about the issue of targeting different countries.
My advice for when it comes to targeting different countries is almost always to have two different sites on TLDs (top level domains) that match that country. So if you want to target the US and the UK, have one US site on the .com and one UK site on the co.uk and make sure your content is localized for that market. It might help to also host each site in that region, plus it is always good to set the geographic target for the UK site in Google Webmaster Tools (I would leave the US version untargeted in webmaster tools).
High Rankings administrator, Randy, adds:
When you're using a .com domain name you're not really targeting the US market. You're targeting the entire world. It's one of those double-edged swords we Murcans get/have to deal with. In that .com is a Worldwide competition. Not a US one.
The good edge of the sword being that since pretty much all US based sites use .com (as opposed to the almost never used .us) you end up with decent worldwide rankings if you get good rankings you can see from your PC from a US location. The bad edge of the sword being that if you can't compete in a worldwide market, you're basically screwed. Then the only thing left to you is your very, very localized market.
Forum discussion at HighRanking Forums.