Ok, firstly the 'cynical' head. You need to make sure it gets crimed. Once something is crimed, it 'exists' and the force looks bad if they don't 'solve' it. So, how do you get it crimed? Well, presuming no assaults, you are looking at the Public Order Act 1986. Now, if you want the full, unabridged and entirely correct version of the law, go google it. If you want the basic, easy to explain version, this is it:
Section 5 P.O.: Someone acts in a way (words or behaviour) that may cause someone harassment, alarm or distress. Doesn't have to be directed. Lowest rung of the P.O. ladder and usually the one people get charged with, even when the situation better fits the below versions... (thank CPS for that, not the police, it's easier to prove...)
Section 4a: As above, but they direct it at a particular person. I.E. Motorist calls you a '**** of a cyclist, whose mother should go **** his ********* ****, ******', without actually getting out of his car or active threatening you. A directed insult.
Section 4: Has an element of a threat in it with an element of immediacy, I.e. he's gets out of the car shouting "I'm going to ******* knock your head off"
There are higher sections, but I'll focus on these. Call up, make sure a log is created (get the reference number), make sure they put onto the log the words and actions used against you, and how it made you feel. This makes it harder for the log to be written off later saying "Caller wasn't intimidated.... blah blah blah".
When the officer comes out to deal with it, don't let them leave without giving you a crime number. Conversly though, don't jump down their throat for it either - if you get someone who was going to deal with it correctly, this might put their back up. Let the officer deal with you, answer their questions honestly, but if they begin to leave without criming it, politely ask them for the crime number.
Home Office Crime Recording Rules state that if a crime is reported to the police, we HAVE to crime it. No choice on that. We DO have discretion to deal with certain crimes as we wish, so there is no requirement to HAVE to do anything, but we MUST crime it. Once that crime number exists, pressure then falls to 'solve' it rather than leave it an undetected crime.
So part one, get a crime number.
I'm blathering on, bear with me, part two in a short bit and it'll reference what can be done, and what evidence we need...