The Rules of the Road say "give way to traffic coming from the right or already on the roundabout".
I find this is somewhat ambiguous. If a vehicle on a junction to your right is approaching but hasn't yet reached their give way lines but you have reached yours, do you have to give way to them or not?
My view is that if you can pass your give way lines before the vehicle to your right can pass theirs, then you are technically on the roundabout and have priority. The circle of the roundabout is the major road and the junctions are minor roads. "Give way to traffic coming from the right" is there because the roundabout is one way and we go clockwise around it.
What say you?