Generally an everyday occurrence, not worth getting that angry about. Looked like the d'n'b driver wanted to take an opportunity to make a little progress (i.e., about 3 metres) but is massively angry that it didn't work out. Any "merge in turn" design results in this.
Hate to defend BMW tosspots but you were ready to recklessly aim for the gap (regardless of your right to filter) just as eagerly as the BMW. Both of you made rash moves. I think you should have read the situation better. Given that you were on BMW's tail for ages he should have been aware of your general presence, though I can see how he could be taken by surprise that you were by his side. Whether he was aware of the collision - not so easy to determine.
No problem approaching the roundabout, giving plenty of distance
I really don't like your music though!
Nothing personal
What car were you driving at the time?
Send that to your local police force. They'll send a letter tomthe driver.