The vast majority of London's Jewish Community doesn't support Quds Day. It's like saying "Hey, some black people attended this Trump rally. How dare you say Trump is bad".
Quds Day is a day that was created by the Iranian Ayatollahs to basically claim that Iran will destroy Israel.
This is a day where people wave pictures of Ayatollah Khomenei, Ayatollah Khamenei and scream "death to Israel". That's the entire original point of this day. Claiming Iran and its militias will turn the land muslim.