“Aren't case management hearings reserved solely for Crown Court cases?”
No. Case Management hearings are held in Magistrates’ Courts both for trials to be held there and, on occasions, for trials which eventually end up in the Crown Court. The hearing seeks to clarify matters for the trial (court time needed, witnesses to attend, etc.). Among other things, an important undertaking is for the defence to outline the issues involved (i.e. why a Not Guilty plea has been entered). As I said, they cannot be compelled to do so, but if (say) the prosecution presents its case and the defence opens up by saying “Well actually I wasn’t there at the time. I was 200 miles away and I have alibi evidence. I call Fred Bloggs” the prosecution will get an adjournment to check out Mr Bloggs and his evidence. The “Preparation for Trial” form which you mention is indeed mainly completed by the prosecutor but some information (such as the ”issues” I outlined above) are completed at the CM hearing. The form is then made available to the trial Bench before the trial begins.
I’ve chosen a simple example (which you mention specifically) which is unlikely to happen in practice (particularly in view of the usual “PACE” warning “…but it may harm your defence if you do not mention when questioned something you later rely on in court”). But the principle holds good. A defendant is entitled to hold his cards close to his chest up to a point but he cannot simply reveal them at the trial without the prosecution having had the opportunity to examine them beforehand. Very often the prosecution may be content to have some unannounced evidence admitted, but they have the right to object. The question for the court would then be whether to grant an adjournment, whether to decide that it was the fault of the defence that the evidence was not disclosed earlier and so refuse its admission or whether to admit it nonetheless.