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But you can change the SERPS by overwriting the ".co.uk" with ".com" in the address bar.
I'm not saying it gives you the US SERPS (as viewed by someone in the US), but it does change the SERPS.
Of course, you can also go through a US proxy and get the real Google US SERPS ;-)
The google.com I see in the Netherlands and Kazakhstan is different from the google.com I see in the US. Especially sites from outside the US seem to rank somewhat lower on the US google.com than on the international accesible google.com. I have not noticed any difference between the SERPs from google.com when I query from the Netherlands (Europe) or Kazakhstan (Asia). So it seems to me that the international google.com SERPs are independent of the location outside the US where you request them.
So if you want to know your actual ranking in the US, you need to access the search engine via a proxy.