You were brake-checked because you have an attitude problem, one that causes risk. Even if other people make mistakes that is not an excuse to make the situation much worse, which in my opinion is what you did here. The driver will most likely remember you not as someone aggrieved over their positioning, but someone out for trouble full stop.
The dash cam driver is too aggressive. Read the road..you're approaching a roundabout so there's a good chance you need to slow down and be especially aware of cars 'getting into lane' earlier, rather than later. Why do you keep your hand on the horn so long..what do you think you're going to achieve with it? Slow down and breathe deeply.
You got video mate? Any description of the alleged offender(s)? Doing this kind of thing is clearly not on - and if they are doing it then clearly they need catching. I hope it has been reported to the police. Thanks for the heads up..keep a look-out, folks.
Show be towed away. Clearly a severe danger to all road users.