'Section 20' is an 'either way' offence, which means that the case can be heard entirely in a magistrates' court or the magistrates can decline jurisdiction (usually because they believe that their sentencing powers are unlikely to be great enough) and send the case to the Crown court.
The maximum penalty in a magistrates' court is 6 months imprisonment and/or a �5000 fine. The maximum penalty before a Crown court is 5 years imprisonment and/or an unlimited fine.
The vast majority of Section 20 offences result in a custodial sentence (but a non-custodial sentence isn't unheard of). A first time offender pleading guilty might escape prison but it's probably still more likely that he'd face a sentence of perhaps 6 to 12 months (with only half of that time actually being spent 'inside').
Chris