Before they gave you the choice of venue for your trial the Magistrates would have heard an outline of the offence from the prosecution and decided, from that description, that the case was suitable for trial in the Magistrates' court.
The current maximum sentence that can be imposed by Magistrates is indeed 6 months,However, if you are found guilty following your trial, for "Either Way" offences (of which this is one) you can be sent to Crown Court for sentencing if, after hearing the full details of the offence during the trial, the Magistrates think their powers of punishment are insufficient.
If you are convicted you will be liable to pay a contribution towards prosecution costs and this will be in the region of £500-£700.