// I, and I suspect many others, have no problem with defending the right to free speech. But speech is done with your mouth ... //
This isn't really correct. "Free speech" as a concept has long been understood to extend to *any* form of self-expression that communicates an opinion, and isn't just limited to literal speech. Most obviously, if instead of shouting "Just Stop Oil", the protesters have it written on their clothing, this is still (independent, at this point, of whatever else they get up to) an exercise of their right to free speech; as would be the distribution of written material. Even not using words (in either spoken or written form) is still an exercise of "free speech", as long as the symbolic statement communicates the meaning. Going back to JSO for now, the colour orange, in the form of powder or confetti, is rapidly getting associated with the movement to the extent that we need only see that colour scattered all over the place to understand the message and who's sending it.
The freedom also extends, at least somewhat, to acts of self-expression that are disruptive to others. If "free speech" is limited only to acts of expression which cause no harm whatsoever, then it's barely free. None of this is to say that there are no limitations, but if your definition of harm is so broad as to include "I was annoyed by this", then that's a restriction on free speech that renders the term meaningless.
Also, free speech is a protection from *criminal* restrictions (or from restrictions imposed by the State, where those restrictions are unreasonable), rather than from civil ones. So crowds can freely boo and jeer, or private organisations like Wimbledon can eject JSO members after (or before) their protest, and that isn't per se a violation of the right to freedom of speech (and of assembly, which combined implies a right to protest).
I'm personally unconvinced that JSO's campaign is helpful or effective, or even particularly well-targeted. Still, a certain amount of disruption is necessary if the aim is to be noticed, and to be listened to, which is what a protest is about.