I just skim read this whole thread. Here's my take on it.
The OP
has been a victim of crime regardless of the chances of a successful prosecution (see CPS guide below). There are two reasons to get this crime recorded:
1) So the police and government can track statistics on crime and focus on solving crime that we as a society tell them is a problem. If nobody tells the police that crime is happening they definitely won't make any plans to prevent or police it.
2) If this driver goes on to commit more offences, lets say one of which is causing the death of another person, the CPS might be able to use your prior report to assist prosecuting that driver.
In all serious offences there are potential early indicators of a serious offender. The minor offence information can be used for crime prevention. (eg someone looking at child porn might lead them onto sexual abuse of a child).
If you look at the CPS Guide:
Road Traffic Offences: Guidance on Prosecuting Cases of Bad Driving
(search for 'cyclist' in that page),
.... we cyclists get a couple of specific mentions because of public opinion.
My personal choice is to report people where I feel they have been driving dangerously (intentionally or not), or if they have maliciously tried to scare me where I have not provoked them. Where there are no independent witnesses I will not ask the police to pursue, just to record the information. I don't go around reporting everything, I try to filter out harmless mistakes and misjudgements, but sometimes those mistakes are too close to ignore.
If I can, I have a civil word with a driver. If they look like they are genuinely sorry and I feel like I have cured the problem or made them stop and think. If they are a nasty piece of work I do report it.
This approach works in rural areas, but perhaps not in London where it seems like a dog-eat-dog road life with cheeky self-advantageous manoeuvres buzzing around you all the time.