Complex sentence: Initially, only British riders were allowed to compete (although women were allowed, despite being banned from riding in the Olympics), but the competition is now an international open to all riders from around the world who have qualified for this level of competition.
Split into simpler sentences: Initially, only British riders were allowed to compete (although women were allowed, despite being banned from riding in the Olympics), but the competition is now an international open. That is, open to all riders from around the world who have qualified for this level of competition.