If you've been charged with 'assault on a constable in the execution of his duty', it's a 'summary only' offence, which can only be heard in the magistrates' court. The maximum penalty is 6 months imprisonment and/or a �5000 fine.
If you've been charged with 'assault with intent to resist arrest', it's an 'either way' offence. It can be heard in the magistrates' court, where the maximum penalty is 6 months imprisonment and/or a �5000 fine. However, the magistrates can decline jurisdiction and send the case to the Crown Court if they believe that (if you're convicted) their sentencing powers would be insufficient. You can also elect to have the case referred to the Crown court if you wish it to be heard before a jury. The maximum penalty in a Crown court is 2 years imprisonment and/or an unlimited fine.
Chris