Freedom of speech in the UK versus United States of America
It is said that there is no constitutional clause ensuring the freedom of speech to UK citizens. Despite that, the press in the UK and United States have similar freedoms.
How does the US contrast freedom of speech with illegality of certain words and ideas? How are certain ideas made illegal despite freedom of speech, for example holocaust denial, defamation, rude words, protection of legal information, of corporate secrets, etc.?
There seem to be two questions here. Are you asking about the differences between UK freedom of speech and US freedom of speech, or are you asking how and why there are exemptions to US freedom of speech?
What exactly do you expect in an answer which isn't contained in relevant Wiki articles in excruciating detail? http://en.wikipedia.org/wiki/Freedom_of_speech , http://en.wikipedia.org/wiki/Freedom_of_speech_in_the_United_States, http://en.wikipedia.org/wiki/United_States_free_speech_exceptions
Many of the things you mentioned either are legal in the US, or are much less restricted than in the rest of the world (particularly for defamation, which is extremely difficult to prove in the US). Holocaust denial and rude words are generally protected (the latter can be unprotected in a very few cases, but the conditions where they're unprotected are narrow).
There is very little freedom of speech in the UK. For example, it is a criminal offence to deliberately offend someone in public. It seems British people like it that way as there are almost no votes in making it more free.
I don't know enough about the UK to give a complete, contrasting answer. I'll tell you about the US and hopefully someone else will give a better more complete answer later.
Generally, nothing is illegal to say in the US as long as it can be half argued to be an expression of ideas. You can literally walk into a courthouse with the words "fuck the draft" on your shirt, and it's perfectly legal. Cohen v California.
Even disgustingly dishonorable lies, when not perpetrated for the purpose of financial gain, are okay. US v Alvarez.
In fact, fake child pornography may be legal simply because it could fall into the category of art and art is considered speech since it is used to convey ideas. Ashcroft v Free Speech Coalition.
The restrictions come into play when you are not expressing ideas at all. For example libel and slander, lying to defraud people for financial gain, or obscenity with no speech content.
Health and education information are legally protected and can't be disclosed without your consent. HIPAA is the law protecting health related privacy. I don't know the name of the law protecting education information. In general, privacy is protected because the value of privacy outweighs the value of the limited, if any, ideological content that could be conveyed.
Corporate secrets aren't directly protected by the government. One signs a contract with one's employers promising not to disclose secrets and the government only gets involved in civil court if one violates said contract.
This is inaccurate in a number of respects; it doesn't talk about time, place, and manner restrictions, or other exemptions to free speech which is specifically what OP seems to be asking about. And it extrapolates incorrect conclusions because of it; all pornography is considered obscenity, and thus not under free speech protections. For example, even if it were legal, you still couldn't sell it to minors, but if it were protected by free speech, that wouldn't be the case (see Brown v. EMA).
True. I left out a lot because covering every exception would require writing a book. I took a class on individual rights in college that devoted a month or two solely to cover free speech and the various tests they use. Just look at the Wikipedia page on free speech in the US. If you really think it's necessary to cover everything, then I think the best solution is probably to delete my answer. I don't have the effort for that. :P I'm going to leave it long enough for you to see this and delete.
And your exception doesn't break my general statement that speech is protected when it can be argued to be about ideas. Giving porn to children isn't about ideas.
That seems to support my point about obscene speech not being protected. Regardless, I think part of the issue stems from OP's question being unclear. I've asked him to clarify.
You might also want to mention that the US definition of defamation is *much* stricter than elsewhere.
@cpast in particular it's _way_ more strict than in the UK, which has famously broad defamation laws.