Helmet cam, get the incident on video, get their plate, try to get their face on cam. Chances are small and could end up convicting you of speeding or somethingA few year back I had a situation where I traveled in a safe formation with my wife. As a car cut in between both of us I flashed my lights to point out that there was more than one bike on the road and the driver's maneuver was unsafe. In response to that the driver slammed on his breaks nearly landing me in his rear seat... well it was a close one.
I followed the driver keeping a safe distance (few car lengths) as I was figuring out what actions can I take. Took a photo of the licence plate at lights (which did not turn out as iphone does not do good photos in the dark). But figured out I can not do anything and its my word against his, so after stressing the driver a bit I left home. I hope he learnt his lesson.
I learnt mine... not to prove my point.. in the end I did not have a cage around me.
Watch the video again.... The rider is threatening to throw something at the driver at the moment when the driver tried to knock him off his bike.