David said this is brand new behavior from Yahoo. 24 hours earlier, if a UK internet user went to Yahoo.com, it would keep them on Yahoo.com. Now, Yahoo is sending UK users off to Yahoo UK.
I honestly have a feeling this has to do with Yahoo Now Showing Geo-Location Under Search Ads from two days ago. Yahoo told me that they rolled out that feature last "week with the roll out of a new “geo labels” feature." I would not be surprised if the same geo-technology was applied to this.
I am going to post screen captures of this at Search Engine Land, with the behavior of how Yahoo, Google and Live handle UK users.
Forum discussion at DigitalPoint Forums.