The CPS != "the crown" (at least not today). The CPS is the UK equivalent of the DA's office. It's a political bureaucracy (I could literally complain for days about how much of a bureaucracy it is, but you'd get bored of me :)).

The judge, theoretically, is more of the expert on the law and whether it should be tried - but the CPS have to decide to bring the prosecution to him/her. Which is why you regularly get cases that should be seen in court, but which a suit decides is statistically unlikely to prosecute favourably.