You don't think that maybe different societies naturally reach a point of relative racial harmony at different times? By your logic if you have two children, one who is 8 and can read and write, and one who is 3 and cannot, you should severely punish the younger child to encourage them to read and write.
Face it, Britain is much more racially mixed than Serbia, because of our colonialist history. That's why we have encountered this problem before them and dealt with it already, not because we are inherently better and more progressive than they are.
Within the last 20 years Serbia was involved in a war which was based entirely on the various races and, to a lesser extent, the religions of the peoples of that region of Europe. You would think that the various sanctions imposed on Serbia by the UN following that awful conflict would have given them a hint that the rest of the world won't put up with that sort of thing any more. Racist chanting at football matches may not be on the same level of bad behaviour but it points to the fact that they haven't completely learnt the lesson.