It looks like a close thing (I cannot see the white lines in the video), but if he was on the roundabout before you doesn't he have right of way.
"Give way to the right but also give way to traffic already on the roundabout"
I get this in the lorry when I enter a clear roundabout, to get someone enter at breakneck speed from the other side and have to brake because my trailer is in the way, they then procede to blast the horns and call me all sorts of names just because I'm on the road in front of them.
Rich.
I agree with Rich.
OK so BM was riding fast, but was also aware of the risks and what could happen, he had the good sense to ake sure that he was able to stop if needed so never came to grief. I ride like that too. But I would argue that if someone is on the roundabout well before you are, then what you are possibly doing, is catching them up, then going on to make an overtaking maneuver. If everyone entered the ro8ndabout at the same speed and at the same time. Assuming the roundabout is large enough to accommodate everyone at the same time, you would all be able to keep moving.
If you are going to ride on a roundabout at speed, that is the way to do it, by checking that you can escape. Motorists tend to drive like this all the time and complain when they have to brake, thinking that they have the right of way. There is a point where someone pulling out becomes someone who is travelling along the carriageway (or roundabout) There is also that grey area inbetween.
Common sense and caution are the order of the day.