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Basically when you talk about freedom of speech you are talking about tolerance and the idea is that if people say things we don't agree with then we should be tolerant of their views even if they are diametrically opposed to our own. But the freedom of speech is not absolute and legal systems, and society at large, recognize limits on the freedom of speech, particularly when freedom of speech conflicts with other values or rights.
Limitations to freedom of speech may follow the "harm principle" or the "offence principle", for example in the case of pornography or "hate speech" critics say that society need not be tolerant of the intolerance of others such as those who advocate great harm, such as genocide, and preventing such harms is claimed to be much more important than being tolerant of those who argue for them.
An example is the Westboro Baptist Church who have been specifically banned from entering Canada because of hate speech. So there is a specific group that has had the freedom of speech taken away from them. If you wanted to have a "Speakers Corner" on Fargate you would have to obtain permission from the council to hold a meeting or an assembly, so again the freedom of speech has been removed, or at least permission has to be sought.
Coming down to our neck of the woods if someone on here was advocating violence against others or defending the use of violence and saying the London Bombings were acceptable and people should be able to do these methods out of desperation, this could fall under "Hate Speech" and should not be tolerated.
Further to your question about shouting "Fire" in a crowded building I don't think there is a specific law [it is an example of a principle] but if people were killed in the rush to get out in the ensuing crush then I am pretty certain you would face manslaughter charges.
(Some material from Wikipiedia)
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