In Roman judiciary those are usually part of the law as written. So instead of having a law about "unresonable behaviour", you have a law explicitely stating that "making noise above X db at night is illegal" or "drinking in the street is illegal" etc. So the idea of what constitutes intent to supply is based on quantity, anything below X grams is personal use etc.
And you can always add aggravating circumstances that "promote" a crime, so you can have such thing as intent to supply counts having drugs and X amount of money on you, or X amount of drugs and leaflets saying you sell etc. In other words you can explicitely state the kind of things judiciary precedent would probably take into account ahead of time.
> The very old precedents are presumably precedents that make so much sense that nobody has successfully challenged them.
Or the higher courts have not taken a case that challanges them. I am not sure about the UK but in the US, the supreme court pretty much picks their cases which means they can arguably allow for dangerous precedent to stay as long as needed by avoiding cases they know would present a resonable chance of overturning. Or equally dangerous oversee cases that maliciously try to overturn positive precedent.
Also the process is slow, tedious and many times expensive. Going back to Townshed v Townshed, it is a case where a will was overturned because a man freed his slaves and his family said that was proof he was mentally unwell to change his will. This, again, is still being cited when overturning wills or when contesting changes late in life. I cannot possibly imagine a more obviously outdated precedent than a judge thinking freeing slaves means you are insane, and yet...
