Ok, I promised to come back. May I just say what a relief to see how many of you disagree on here: clearly DH and I are not stupid for feeling the answer’s not obvious. Not that I was planning to LTB over this, but still.
Obviously, DH and I do both agree with the many PP who have said it’s best to pause, eye the other guy, and proceed with caution: that’s what we’ve told DS, adding what one of you so helpfully said about not hesitating TOO long during the actual driving test.
But, if it comes to figuring out the actual rule, I seem to be with the minority on this thread. I say C (going straight) has priority over A (turning right).
TBH, I’m amazed at how many PP have claimed that “yield to the right” is relevant (Highway Code rule 185). Neither car is “on the right” of the other before entering the roundabout; they’re nose to nose. Priority to the right applies only on entering the roundabout, not to what happens once you’re in it and certainly not to where you plan to exit it. (If you think about how you actually drive on a larger roundabout, we never do, indeed can't, give priority to the right once in it.)
More to the point, as they enter our roundabout you can think of the cars as following one another nose to tail on a circle, or as side to side heading opposite directions, in which case they will pass each other right side to right side. Either way, neither car has right-side priority. For some reason my DH cannot see this, as it seems many of you don’t?
So I feel the key is that turn. My view is that 1. it’s at best irrelevant that A plans to turn right, and 2. at worst the turn actually means C must have priority. Because:
1: As many of you pointed out, in theory, if both cars go around the nose of the circle instead of straight across, as required in rule 188, there's no conflict: C should have exited the roundabout before A starts to turn. In practice, as we all agree, in a mini roundabout when A starts turning towards his/her right, the cars are still on top of each other, so A will probably rear-end C. That in itself seems like a clue: basically, you rear-end anyone, let alone on a roundabout, you’re definitely in the wrong. But also
2: Nothing in the Highway Code gives priority in a roundabout based on where you plan to exit. Searching for other clues, I turn to rule 180: when you are turning right at a junction (and don’t forget a roundabout IS a junction), you must yield and wait for vehicles going straight on. This, I notice, is basically what PoshPineapple, former driving instructor, said at 13:45. It doesn’t come up in a normal roundabout because in a normal, large roundabout you are not really turning right to exit but veering off to your left.
But, as I started out saying, while I still think I’m right and DH is wrong, I see that many, many of you disagree with me. And clearly the rule is so confused there’s no sense playing High Noon with other drivers.
The foreign part of me thinks it’s weird the rules aren’t spelled out more clearly to avoid accidents, lawsuits, and vendettas; the British part of me thinks it’s nice that we still rely on common sense and courtesy among drivers.