As a post at Paul Carpenter's blog notes Google seems to have a preference for US site listings for people searching from the UK. This situation has been discussed before (examples can be found here and here) - and there is even a video from Matt Cutts in the blog post.
There are even US sites in the Google Maps listed for UK based searches.
This is not too much of a stretch given Google's history of problems with languages. BlogScoped pointed out one of many examples where all 10 results for a French search list pages from one site. Seems in Google's world it is the President's language.
Last Week to Save on SES London Tickets!
Learn to engage customers and increase ROI by distributing your online marketing efforts across paid, owned & earned media. Join the leaders of today's digital marketing & advertising industry at SES London. Find out more ››
*Saver Rates expire this Friday, Dec 13.