Free Traffic Legal Advice

Hello all,

I got into a incident with another driver (no collision) where he attempted to speed past me in a town to jump in front of the merging, I sped up to stop him from doing that causing him to brake and merge behind me (I know I was in the wrong for this, it was  stupid petty thing to do, which I absolutely regret). Whilst waiting at the lights, he wound his window down and stated "I'm trying to decide what I'm going to charge you on, dangerous driving or the fact that your brake light is out, I've got something to drop off and come back to you in 10 minutes" I carried onto my destination (as I was in moving traffic) and off he went.

The question is, what is the legitimacy of the charge threat? If it was a police vehicle, then it was unmarked and he didn't identify himself as an office (no warrant card etc) and only said the above statement. It absolutely terrified me (rightly so)which I think was his intention. I am aware that I was in the wrong here and feel wretched for it, but I'm genuinely scared that this will result in criminal charges.

Any thoughts would be appreciated.