There are three approaches. The first is to use the "lang=uk" boot options when you first boot the antiX LiveCD. Three other boot options are also available for fine tuning:
If you don't want to go that route then you can trying editing /etc/default/keyboard and change KBLAYOUT. You will at least have to restart X and you may need to reboot for this to take effect.
Finally, you could edit your /etc/X11/xorg.conf directly and add a section like this:
You then have to restart X which you can do by logging out and then logging in again through the Slim login manager. I have not tested this. It may need some (hopefully) slight modification.