Some UK Google users have noticed that search results pages are showing more results from .com sites these days, than in the past. They are used to .co.uk sites getting better rank, and assuming that they are more relevant to their geographic location.
Certainly in some cases the .co.uk site would be more relevant to a UK searcher, but that is not always the case. Google’s Matt Cutts has posted a video in which he answers a question on this subject from a user. The question was: