In Code Geass Britannia maintains a social darwanist society. They believe that everything belongs to the strongest so they feel that they deserve the world because in their nations eyes, they are the greatest nation in the world resulting in a view that all others are inferior. Zero's philosophy is more related to the Code of Hammurabi which depicts a righteous society in the strong shall not harm the weak. He believes in social equality while Britannia believes in heirarchy based on power.
It is not mainly a sense of racism in Code Geass instead it is more of a primitive view of Darwins theories combined with nationalism, democracy, emancipation, and a sense of freedom brought in a world of strife.