A WebmasterWorld member says that his .com, which has a UK IP address (and is for the United Kingdom audience), is not being recognized as a UK-specific site even though he updated Google Webmaster Tools and informed Google of the geographic location of the site. He ranks #5 in Google.com but #149 in Google.co.uk. (For reasons like this, I just wish .co.uk was the default domain in Google and that there wasn't a landrush for .com domains out there. I want tamar.com one day, and they're a UK company.) ;)
Why is this ranking occurring? There are a few reasons. While the site is UK centric, the links to the site may be from other foreign countries. The idea, then, is to get more UK links.
One forum members suspects that it may be something else, because in the past, Google has misidentified some UK IP addresses on generic .com TLDs. Furthermore, some UK hosting companies are actually not hosting in the UK.
While some forum members think that WHOIS information is at fault, that's not the case when these domains are protected by WhoisGuard or Domains by Proxy. There must be other factors in play here.
One final suggestion from a forum member comes from checking DMOZ rankings (if applicable) and ticking off the checkbox that says "this is a UK listing." Since Google uses DMOZ for data, this can't hurt at all.
Forum discussion continues at WebmasterWorld.