They don’t really exist, it’s something they themselves made up to paint the working class as all savages. Like with dragons, bigfoot, etc there’s tons of anecdotes about run ins with “feral chavs” that always seem way too cheesy to be true (why would 15 people randomly attack an old woman just for looking at them like a pack of frenzied piranhas?), but no one can actually prove that chavs exist. At the same time the media hysteria about “chavs” died down, so did the “chav” encounter stories. The middle and upper classes are fearful of “chavs” when in reality the people they’re fearful of are just normal people.