The beta is currently not accepting more pariticpants. When it opens back up, it is technically available to any English operating system, even though it reports that it is not available in the UK.
The Not Available message should appear if you click on Get it Now. The link for more information should bring you here:
http://www.microsoft.com/security_essentials/resources.aspxThere isn't a whole lot of information there, but you are welcome to browse the MSE forums:
http://social.answers.microsoft.com/Forums/en-US/category/mseWeb searches for MSE should bring back some reviews that include screen shots and descriptions that were published when the beta was initially released.
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator