How do we know you had priority on the roundabout? How do we know it wasn't just a case of you being a naff driver?
I often see drivers who fail to give priority to cyclists on roundabouts when they should.
By the OP I read it as he was already on the roundabout. Unless traffic light controlled, it's very rare to find one where traffic on the roundabout does not have priority, and priority is to the right.
I have experienced similar from clubs. I was on the way to work on a Sunday late morning in Macclesfield, the Knutsford/Chelford/Macclesfield area is popular for cycle races/events in Summer at the weekends.
https://goo.gl/maps/nYBv7GgtNqm
I was ON the roundabout, which is a typical roundabout, only to have to stop extremely quickly, as cyclists approaching from the left failed to even slow down. It was part of their race, and not a single one in the large chain reduced their speed at all. It could have been a very serious accident, but they all seemed more interested in their race than traffic that had priority. They were seriously moving, at a very impressive pace, but not one would have been able to stop.
I have only experienced this once in the 2 years I worked and commuted there, so I'm not claiming they're all guilty, far from it! But I am inclined to believe the same or similar happened with the OP's incident.