Download and run: http://www.ukcs.net/plugins/CSS_UKCS_GUI_Setup.exe
Apart from having a cool official UKCS logo screen in CSS, the on screen join options allows sponsors to join the servers even when full. Eg, if UKCS#2 is on 46/46, still click the "Join UKCS#2" option, and an extra slot is added at the time you join making it 47/46!
To install the GUI on a Mac, all you need to do is to unzip the following file to your ..Steam/steamapps/common/counter-strike source/cstrike/custom/ folder:
http://www.ukcs.net/plugins/CSS_UKCS_GUI_MAC.zip
Note. The extra menu options can be removed at anytime by going in to Windows Remove Programs and de-installing.
Screen Image

There is also a version of the plugin without the background if for example you already have one and do not wish to change it, the link is -> http://www.ukcs.net/plugins/CSS_UKCS_GUI_Mini_Setup.exe .