glengara is right here, the other methods do not show what you are wanting, just closer to what you want than .co.uk searches.
the true way to see your us results is to use a US proxy.
Search for proxy list in google, then set the proxy in your browser, in IE its Tools/Connections then the Lan Settings button.
then if your browser redirects to .co.uk when you do a search, change it too .com and do the search you are wishing to see again.
This is particularly relevant if you have a .com domain on US hosting and want to know what penalty a site maybe incurring in google.co.uk.
I use this method to see where a term will rank before moving to uk hosting for a site that is penialised because of its US hosting or TLD ( penialised in .co.uk).
An example is:
no5 for a term in: google.co.uk
no3 for a term in google.com
no1 for a term in google.com via a US proxy
this is for a .com site with US hosting.
Hope this helps.