One of the most anticipated contemporary art moments of the year arrived this week in the UK, when Marc Quinn’s much-ballyhooed sculpture of supermodel Kate Moss was unveiled. And that, says Jonathan Jones, should tell us all we need to know about the state of contemporary art in Britain. “After all the sensations, after the brilliant careers and after the fire, we have arrived by some cyclical divine joke in 18th-century London, where portraiture is god and the leading artists of the day compete to depict [celebrities.]”