Where else to hold Champions than the best city in the world?
On the real though, London has lots of students including me who are massive esport fans and a diverse demographic of people that are into gaming. Where I live we have gaming cafes full of people on the weekends and great infrastructure to host such an event with fans. Fnatic is also based in London with useful resources to help Riot setup a good event.
TLDR: London PogChamp!