Hi there,
Are you searching in Google.com from the UK, or are you actively making sure that you are seeing US-centric Google.com results? I ask because if you search in Google.com from the UK (without modifying your search to specify US results), you will receive UK-targeted results, even though you are not using Google.co.uk.
I also happen to be in the UK at the moment:
In order to see how Google.com ranks sites for a query in the US, you need to specify that you want US results. I use a Firefox / Chrome plug in to easily modify the URL for me: http://www.redflymarketing.com/internet-marketing-tools/google-global/
Thus, if I search for [credit cards] in Google.com, I can get this URL for US results: https://www.google.com/search?q=credit+cards&pws=0&gl=US.
If I just go to Google.com from here in the UK, these are the results I get: https://www.google.com/search?hl=en&source=hp&biw=1177&bih=539&q=credit+cards&gbv=2&oq=credit+cards&aq=f&aqi=g10&aql=&gs_sm=e&gs_upl=770l1989l0l2037l12l8l0l1l1l1l271l903l3.3.1l7l0
This search results page contains a few UK brands, because even though I'm on .com, Google knows I'm in the UK. It's not totally UK-centric, of course, since there are UK brands in there too. Depending on your query, you can get different degrees of relevancy.
Are you seeing true US results, or are you seeing UK-centric results on Google.com? If "&gl=US" in the query string of the Google search URL, like there is in my first example here, then you are still seeing UK-centric results.
It is not clear why Google shows different results in the UK for google.co.uk and google.com queries, but word is that soon, google.com results in the UK will be the same as google.co.uk.
If the site really is ranking a lot better in the US than in the UK, there is a different problem, but I thought this might be more likely.
Also, have you had a look at where most of your traffic comes from in Analytics?
Cheers,
Jane