Many different formats also have international tournaments and world championships, albeit they are not always as big as WUDC. For example, the World Schools format also has the World Schools Debating Championships every year. Additionally, famous debaters from many other formats are also just as well known.
Many videos of debate rounds in the Policy format have tens of thousands of views as well. Other debate formats also have notable alumni. Josh Gad, for example, participated in Original Oratory.
Therefore, although the number of people who participate in British Parliamentary debate each year is impressive, it is by no means exceptional.