The old driver made a mistake. Although I didn't hear an apology, I take it he realised he'd made a mistake, and you spoke to him without losing your rag.
I don't know if you'd signalled, but signal or not, you had right of way and the driver was completely in the wrong. I think (from daily experience) that mini roundabouts are particularly dangerous and I am always ready to brake for the idiots coming on from the left.
You seemed ready, appeared to be covering your brakes and that's all we can do. We have to allow for the mistakes of others (and ourselves,) keep cool, and give the odd 'lecture' where possible. I've had a word or 2 with drivers before, and it's 50/50. They either look apologetic, or they get very defensive. Either way, say your piece and move on like you did. By 'telling the old chap off' hopefully he'll be more wary in future, and you'll have saved the next cyclist who may not be experienced enough to allow for such a bad lapse of judgement?