I notice that some hyperlinks when I click on them detect that I am in a certain part of the world ( SE Asia ) and they return a 'sorry not available in your area' page which then redirects me to a page in my area. How is that accomplished? I would like to appear to be UK based and only allow UK people to access certain parts of the website- areas of interest only for people living in the UK for example. I know proxy browsers can overcome this but I don't really care if someone is that determined to get into one of these areas.
This is done using a Geo-ip database. It is a database tracking all the IP numbers back to the probable country of the user. they are about 99% accurate these says but you do need to consider keeping the database current. Various subscriptions available from your friendly online supplier for you to choose from!
So - when a request hits your server, you check the IP against the database. Then you can deliver content based on the location of the user.
There's a downside. Whilst Geo-targetting per-se is seen as a legitimate use of cloaking (for that is what it is), you may find that you inadvertantly block search engine spiders, as these may be coming to your site from all sorts of countries. You could aim to get around this by finding a list of IPs they Googlebot, Slurp or other bots use, but be prepared for the possibilty of unexpected results. Keep at eye on Google's sitemaps reports and react if you suddenly get your site blocked or unspiderable.