Good post by the OP. In all the kerfuffle and moral indignation about RLJs, this is the unsexy but greater issue for me in terms of cycling safety.
Contrary to my (unscientific) views on RLJ, I see many more cars (and a greater proportion of cars) entering roundabouts unsafely than I do bicycles. In fact, I rarely see a bicycle do this.
It's not just the majestic sweep onto an occupied roundabout that disturbs me, it's the guessing game about where a vehicle intends to leave the junction. Indicator use by some drivers seems random at best.
The above are a mild inconvenience for other drivers, but a major bugbear for cyclists.
This is brought home to me when cycling with my children. I notice that my 'verbal advice' to them becomes a stream of unintelligible and sometimes contradictory squawks as we approach busy roundabouts. Not helpful, but that's how I am.
Last year I was amused watching my daughter (a learner driver) trying to roll smoothly onto roundabouts. The thing that ground the most enamel off her teeth in those days was drivers sailing across roundabouts with not a thought for their indicator or mirrors.
Just to give a BBC-like balance to this almost-rant, I am frustrated both when cycling and when driving by cyclists who like to sart on the outside of a roundabout and apex-clip the central island to give themselve a shorter route over the junction. This happens rarely, but it terrifies drivers.