Docspock, times have changed a lot, thank goodness.You're a bit behind the times.
Magistrates do get training and do get independently assessed before being allowed to be magistrates.They are drawn from a wider range of people than before,too . In the distant old days, magistrates were untrained and pretty much self-selecting and all of a type in background
Everyone who ever practised widely had horror stories of some courts, many years ago.Guilt in a lot of courts was decided by the one in the middle, the chairman,not the 'bookends', assisted by the Clerk (who ran the court as his personal fiefdom). The great trick for the advocate,was to discover the personal foibles of both, and get the clerk on your side (didn't matter how: flattery, frightening the wits out of him, obsequience, picking up his personal eccentricity and playing to it, it didn't matter). That got a lot of sway over the chairman, who might acquit against his personal judgment, but you had to play to him as well.
And the great thing about challenging police was to be in either an extremely upper class area, populated by people who snootily treated a policeman as they would a servant (might lie might not) or a really working class area (same, but they thought of him as another working class man like themselves). It was the classic Hyacinth Bucket, middle class benches who revered policemen and thought it was a sin to disbelieve a policeman because policeman never lied , who were impossible.
Happily, those days are gone.