Whilst not formally trained, there are many on the bench with a great deal of experience dealing with cases on a day to day basis. The practical Public Order, Theft and Motoring knowledge of many experienced magistrates will be far in excess of many qualified lawyers who have never ventured into the criminal court.
Even so, I don't think any Magistrate (or District Judge) would look to rule on the validity of a Byelaw and any defence would most likely fail. This leads to grounds of appeal by way of case stated to the High Court and I see no reason for this not to be entertained. However - you would need someone facing a Byelaw offence willing to plead not guilty simply to test this. Good luck with that! Unless
@ForTheLoveOf is prepared to get themselves in court to test their own position.