You should be able to take the lane so that no vehicle can come around you. Do that. The vehicles that stop even though you are stopped are idiots. Wave them through then bang on your helmet. Signal your exit from the roundabout.
The only slightly dangerous thing I've seen is with group rides. The lead rider must signal if they're going to stop for a car that's entering the roundabout and the other riders need to watch that rider rather than the entering car, then deal with cars once they are the entering bicycle. Groups have to realize that they will have to filter through the roundabout. They must not try to enter as a single vehicle.
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