My answer (which I was composing whilt Chris was posting his) adds a little, but not much.
For each offence that is �either way� (that is, can be dealt with either by the magistrates or the Crown Court) magistrates have guidelines that help them decide whether to retain jurisdiction or not. As Norman and Chris say, these usually relate to the length of sentence the magistrates consider appropriate. If they consider it longer than their maximum (currently six months, but, exceptionally, 12 months for two separate either way offences) they would decline jurisdiction and pass the matter up to the Crown Court. The guidelines describe aggravating features for each offence which, if present, would put the offence beyond the lower court�s maximum.
If the defendant pleads not guilty and the magistrates decide to retain the matter, the defendant has the final say and can opt for a trial before judge and jury. Note, however that he has no right to a trial in the lower court if the magistrates decline jurisdiction.
Most defendants pleading guilty try to have the matter dealt with by magistrates. This is because of the lower limit of punishment they can be imposed. Among those pleading not guilty a number choose a jury trial. This could be because the extra delay in that process means that there is a stronger likelihood of witnesses not turning up or not recalling events quite so well. There is a philosophy among habitual criminals that delay can only further their cause. Nonetheless, statistics show that more defendants are acquitted after trial by magistrates than by jury.