Cam driver, you are the dangerous driver who should be off the road. You are the one who threw your toys out the pram over the minor inconvenience of not being able to cut in. What was wrong with "Oops, wrong lane, I'll just go round the roundabout and get back on course." But no, you forcibly cut in and are then clearly hell bent on antagonising the other driver, following him and harassing him. If you showed this to the police it would be self incriminating. Yes, the other guy could have shown more courtesy but it was not his right to. And it was not your right to force your way into the other lane.
I overtook this guy in my sinclair C5.