If you have a .co.uk domain, it’s obvious to search engines that you are a UK site. But what if you have a .com domain? A .com domain is international, so search engines have to look at other factors to determine which results you should appear in (i.e. Google.com, Google UK, Google Japan, etc.).
Some major factors include where your hosting is located, a UK address on your site and where your backlinks are from. When you work with clients, there are often stipulations on projects. On a project I recently worked on, I was to have a site display in UK search engines that had a .com domain and hosting that was in the U.S.
You can read about it more in a YOUmoz post I wrote on SEOmoz: UK Indexing Case Study
