The reason it’s a smaller number is because pro-Trump rallies were banned by Khan. Anti-Trump rallies are OK though.
https://www.breitbart.com/london/2018/07/14/police-khans-london-ban-pro-trump-rally-u-s-embassy/
One of the speakers said that the police there told him that if they arrive with more than 3 in a group they could be arrested, or moved away.