Hence the banning of Baa Baa Black Sheep to protect people that are not offended by it in the first place. I see PC as a thin end of a wedge that will erradicate ANY free speech.
But there is no ban on baa baa black sheep, you keep on saying it but it simply is not the case.
You mentioned health and safety earlier, which has a similar problem. The people who make the sort of decisions that end up in the pages of the Sun are not health and safety legislators, they are ordinary people interpreting a set of rules. Nobody from the government has gone to a school and banned conkers, it is simply one person reacting or being overzealous.
In exactly the same way that nobody in government has banned baa baa black sheep, all it takes is one person to say we should sing something else and suddenly we are all on the highway to hell because political correctness has gone mad. Political correctness is in an even worse position because it is merely opinion, nobody goes to jail because they were not sufficiently PC.
You claim that political correctness is a slippery slope but generally it is used to describe societal peer pressure, if society is changing and becoming more PC that is a result of a change in social attitudes. Nobody is forced to comply, complaining about peer pressure is a bit like complaining that we are hairless apes.