There is a big difference between how sites are ranked on google.co.uk and google.com. When most people in the UK visit Google, they are automatically forwarded to google.co.uk. The listings on Google UK have more of a UK slant, whilst .com Google tries to be all-inclusive, with varying degrees of success!
Google wants to deliver the most relevant results as possible, especially when it comes to geographical location. Therefore, if you access the Internet in the United States your results will likely differ still. If you are sat at your computer in the UK, even google.com listings will be tailored - at least to some extent. In the future we will no doubt see more of this.
Depending on the audience you want to target, you can make changes to begin appearing in various Google country indexes. The benefits obviously being you will connect with your main target audience.
1) Country-specific domains – if you use a .co.uk domain name, this guarantees you automatic entry into the Google UK index. Likewise, if you register a .in domain, this is right of passage into the Indian Google, and so on.
2) Where you are physically hosted – this is important because your Web host may have its head office in the UK, but the servers could be housed in Germany. This is why the physical aspect cannot be reiterated enough. The country your site is hosted in effects which Google country your appear in. Sites hosted within the UK but using a “.com” domain will still appear in the Google UK index on account of the server location. See how it works?
3) Inclusion by association – If most of the sites linking to you are German and based in Germany, it’s a fair bet your site must be interesting to German people. There is a school of thought that believes links from various areas of the world could effect your rank in country-specific indexes. There is little direct proof of this but it’s certainly something to keep in mind.
4) In Google Webmaster Tools, you can tell Google which area of the world your site is targeting. This means Google doesn’t have to work it out for itself!