It’s not for you to determine whether a public nuisance, or disturbance happened. That’s the role of our police. I saw no assault. If the police witnessed one then for sure they would have intervened. If the guy canvassing thought an offence had been committed then he has the right to press charges.
Exactly the same applies to anyone protesting anything. If you believe “anti-migrant” protesters get treated differently it’s perhaps because they have previous. Having previous means the police take preventative action early.