Rude driver, pulled across a roundabout without looking, responded with a middle finger when he heard the horn warning him of oncoming traffic forced to brake.
Well, you know the score. The arrows painted on the road are only advisory, so merc is not breaking the law and appeared to be moving lanes cautiously. If anything you were poorly positioned, having to jump from the right lane to the left lane to exit. It could even be argued you were cutting the merc up. Have a nice day and drive more safely next time.
Bit of boot polish, a plunger and some gorilla glue and it'll be dandy.