So what was bad about his standard of driving that you need to report?
I do hate some of this stereotypical rubbish about people with blacked out beemers and what they are likely to be involved in, bullsh1t. It's just like saying every biker is a hairy fat fook, going on the Harley Davidson stereotype (okay so I fit that

).
I don't entirely get why you want to report the guy when you have an illegal numberplate yourself. I'm sure you do more incriminating things on the road in a day than he does