By the way, "record numbers" of youths in custody doesn't necessarily mean there are more yobs than ever - it could just be that there are more of them in custody.
It's convenient (and in a way comforting) to want to pin it on one thing, e.g. to take your suggestion, 40 years of liberal thinking. And I'm not even entirely sure what that means anyway. And if it counts as one thing. But the phenomenon of yobs and "anti-social" youths is never going to be neatly ascribed to one cause. There are all sorts of possible factors, such as: no sense of community therefore no sense of belonging, respect or responsibility (and if so, what are the causes of that?), progressive stunting of basic social skills, bad parenting, progressively greater emphasis on material gain and therefore higher expectations, lack of discipline at school, genetics... etc etc etc - the list could continue with dozens more possibilities, each with its own set of possible causes.
Bear in mind, some of these yobs were born and raised during the Thatcher years - not exactly a period of wet liberalism, in terms of the tone of government, were they? It's complex, and if it's due to anything that "went wrong" it might be a lot of things.... or maybe even due to stuff we thought was right.