I'm not sure if this is the right circle for this but I thought as this is where activists are they would most likely know the law.
We are increasingly seeing rainbow crossings being painted on our streets by local governments, is it illegal to chalk on them?
children chalk on pavements all the time, it just washes off in the rain, if we were to go out at night and write womens rights slogans on the crossings with chalk would that be illegal when its not a permanent defacement?