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.