You always leave a gap do you? The evidence says otherwise! You try to undertake when, by your own admission, his/her actions are predictable. You followed the driver around the roundabout to the point where his/her intention is fairly likely to be to leave at that exit, but your own spectacular failure to read the situation is supressed by your moronic instinct to attempt an impatient undertake.
Strange how you were hugging the back of the coach at the start of the video. Clearly there was more to the story than you are letting on. What was the history? Looks like he is getting revenge for something.
The fact you then tailgate the van for a considerable distance isn't exactly exemplary driving on your part.
Then finally, using the right lane for straight on is perfectly legitimate, no indicating needed. He was ahead of the car in the left lane and there is no evidence of forcing his way in.
Terrible at parking