Note there is a sticky in this section of the forums.
http://wordpress.org/support/topic/welcome-to-the-multisite-forum
It contains a LOT of important information.
How do I turn on WordPress Multisite?
http://codex.wordpress.org/Create_A_Network
Specifically, I need to enable my "Category" sections to essentially become the sub-domains/subfolders.
Thus each main category will become an sub section of the main domain accordingly thus:
(http://uk.somesite.com) or (http://trinidad.somesite.com)....so it is ideal that each subdomain be treated as separate entities!
When you use multisite, you are setting up your own personal version of wordpress.COM where you run the show.
Why are sub-folders 'best' over sub-domains [SEO purposes]?
One is not "better" than the other just different. Subfolders are good if you want the main domain to have the PR authority.
Subdomains are "better" if you want them to be seen as separate sites. Which you just said you did.
Do I have to set up seperate files for this or a separate index page [per sub-domain] or can I aggregate Categories to make the content on the sub-domains?
As noted in the stick, all blogs are virtual. you do not need to set up folders or other files, but you *do* need to do your homework and learn about the server.
Ultimately, you're the one who has to decide if this is how you want to manage your content. Each subdomain will have to be visited to post content to that section.
unless you use plugins to push or pull the content around the site...