If you shouted it and there was a stampede where people died then you could be found guilty of manslaughter if proven you shouted it with ill intent.
Even if no one was hurt, you could still find yourself in legal trouble if the police decide to charge you with variations of "Disturbing the Peace" or "Causing disorderly conduct". Nobby would probably have an idea of which law they would charge someone if the shout out caused panic.
None of the charges of disturbing the police or manslaughter directly impacts the right of free speech,
As it happens, this, in itself, is a very interesting legal arena. The thought of someone doing such an act and providing sufficient evidence of their intent so to do, and what consequences they were attempting to incite is one that I doubt many have ever come across.
Anybody shouting "fire" (the crowded theatre scenario), assuming they were not mentally unwell etc., would still have an enormous amount of space (no pun intended) to find, or be provided with, mitigation sufficient to ward off a prosecution.
That`s not to say that they wouldn`t be arrested in the first instance (arrest and prosecution are two very different things). Plenty of grounds to arrest but, plenty of grounds also to avoid a prosecution. . Not least of which is that, as far as I can think at this moment, I don`t believe there is any specific legislation to cover that situation.
I can`t cover all the scenarios because much cleverer people than I invest huge amounts of time and study pursuing this sort of legal precedent and associated inchoate liability.
I might well be wrong, but, my guess is that if such an act, culminating in a number of deaths, was committed by a person shouting "fire" and in relation to whom there was an intent to cause some sort of mayhem, it might fall to a High Court or Govt Inquiry to determine and identify responsibility for the consequences. "Responsibility" would be key.
Can`t see any subsequent prosecution alluding to "disturbing the peace" or "causing disorderly conduct" existing or being appropriate, though a situation relating to breach of the peace might be an initial tool to arrest the utterer !
Edited as I meant to say that a) There is a criminal law that would cover "Fire" if shouted on an plane b) Gunners on HM naval ships would get into deep bother if they shouted anything other than "Shoot" !