Taken from the Yahoo help pages:
<<YA redirects users located in countries other than the United States from Yahoo.com to one of their country-specific sites (e.g. kr.yahoo.com for Yahoo! Korea) to provide them with the most locally relevant Yahoo! experience. The computer's IP address is used for this, which is usually the internet address of the respective Internet Service Provider (ISP), to determine the country location and corresponding Yahoo! site.
If you'd prefer to visit Yahoo.com instead, click on the Yahoo.com link located directly below the Yahoo! logo.
If cookies are enabled, your browser will navigate directly to Yahoo.com on all subsequent visits. For more information on cookies, please visit our privacy page
If cookies are disabled, you'll continue to be redirected each time you visit Yahoo! You can prevent this either by enabling cookies or directly typing http://us.yahoo.com into your browser. Visitors to http://us.yahoo.com will be shown the same site as Yahoo.com.
Finally, if you feel that we're detecting your IP address incorrectly, please use the Feedback link below to send us the following information:
1. Your IP address.
If you don't know your IP address, please visit http://whatismyipaddress.com/ and send us the IP that is listed on the top of the page. Your IP address will look something like this: 217.148.184.7
2. Your location (e.g. Seoul, Korea).
3. The Yahoo! site you're being redirected to (e.g. au.yahoo.com).
If we're able to fix the problem, we'll do so as soon as possible.>>
http://help.yahoo.com/l/uk/yahoo/homepage/homepage/whatsnew/homepage-90.html