(2-part post):
Some relatively minor offences are 'summary only', which means that only a Magistrates Court can deal with them.
Very serious offences are 'indictable only', which means that magistrates must pass the case to the Crown Court.
'ABH' is an 'either way' offence, which means that it can be dealt with solely by magistrates or passed to a Crown Court. Unless the defendant opts for trial by jury (in a Crown court), the case will remain in the Magistrates Court as long as the magistrates are convinced that they can deal with any legal complexities which might arise and (importantly) that they also believe that their sentencing powers (if the defendant is convicted) will be great enough. If the magistrates think that a sentence of greater than 6 months might be appropriate (if the defendant is convicted), they'll adjourn the case until such time as a 'committal hearing' can take place. At that hearing the defendant (or, more usually, his solicitor) has the right to oppose committal but most cases are still transferred to the Crown Court.