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Good points from Lee. Adding on that, in general: if you get more .co.uk links, you'll be better found in the UK than in the US. If you get a lot of .com links, your findability will be less in the UK and higher in the US. Google will determine that, considering the things Lee said, you might be relevant to US searchers. And then there is the IP. A .com domain can be hosted in the UK. Adding more to your UK relevance than your US relevance. And than you have the links the .com domain receives... Same thing... So not so straight forward to answer.