In my view freedom of speech is an "illusion" only if it is defined so broadly and artificially that the concept loses all rational meaning.
First, the classic example often given of a kind of freedom of speech that doesn't exist is that of someone yelling "Fire!" in a crowded theater when there actually isn't any fire. (Even if there were a fire, yelling this out of nowhere would likely cause a stampede and conceivably more harm than the fire would cause.) The reason this is a bogus example of "freedom" is that 1) it's based on a falsehood, and 2) the intention of shouting "fire" is to harm people. People don't have an unlimited right of any kind of "freedom" if the result is that people would be hurt by it. But the absence of such an unlimited right doesn't destroy or invalidate the concept of free speech.
Second, the fact that freedom of speech is a genuine principle can perhaps paradoxically be illustrated best through negative instances, ones in which the valid exercise of it has been prohibited. In the Soviet Union, one could end up in jail or killed by criticizing the Communist Party or the Premier. Alexander Solzhenitsyn was thrown in the gulag for having made an offhand remark about Stalin. Those who have read The Joke by the Czech writer Milan Kundera will know that in a totalitarian country, even a comical statement that appears subversive but is meant ironically could land someone in prison. Whatever our real faults in democracies such as the US and the UK, people are not imprisoned for criticizing the President or the Prime Minister. If freedom of speech were merely an illusion, this would not be the case.
Last, it's worth mentioning that in recent years, an idea has been repeated in some quarters that people who are liberals or progressives "don't believe in free speech." The only thing this assertion can possibly refer to is that those considered on the political left are often highly critical of bigoted or demeaning speech, especially when directed against groups that have historically been marginalized or oppressed. But this, like the "Fire!" example, is another form of harmful "free speech." And in the instances of it that conservatives cite, so far as I know, no one advocates criminalizing such speech unless it has led to people being physically harmed. If, for instance, a radio host makes bigoted and racially insensitive remarks, it's legitimate for him to lose his job over it. But this, in my view, doesn't represent a denial of free speech, unless that host were also to be thrown in jail, and again, so far as I'm aware, this hasn't happened. The same people who believe that censuring and firing someone for hateful speech is a denial of "freedom" would themselves just as strongly censure a left-wing person who said the same kinds of things against conservatives. But neither of these instances would indicate that genuine freedom of speech is an "illusion."