That's true, but they are a "code" that a large majority of drivers usually follow. This is the most important thing here imo: you are obviously sharing the road with fellow drivers, and safe driving includes avoiding doing things that are unpredictable or unexpected to those drivers. Using the left lane to take on exit to your right is not an expected action in the UK, and it's one that will get you in trouble sooner or later imo (as well as likely failing one's test as others have mentioned). A contrary example is that if I drove US freeways and rotaries in a manner that I feel is a safe way to do so, I know that my chances of being in an accident would multiply considerably. So I drive them in the manner that fellow drivers expect them to be driven. When in Rome, do as the Romans do, I guess...
Okay, first of all, when you say "Using the left lane to take an exit to your right" -- you make that sound like a regular junction, in which case it would be complete madness.
But we're talking about very, very large roundabouts, in which your "right turn" is not a straighforward one, like driving at 90 degrees across a normal four way junction, but instead is going to actually become quite literally a left turn by the time you have entered the roundabout, driven all the way around it, and need to be in actually the left lane OF the roundabout by the time you are right next to your turn.....which you then turn left on.....
We're talking about a giant merry go round, not a regular right turn in which you can only be in the right lane to take it without an accident.
On a large roundabout with multiple lanes and even traffic light on it at several intervals, unless a left lane does indeed abort very quickly into it's own dedicated exit, as long as it or a middle lane goes all the way around, there IS NO danger of that confusing other drivers because ---- they're usually on that lane too, doing exactly what you're doing.
Sorry, but in my personal observation very complicated roundabouts usually have everyone just doing their best and the Highway Code is not exactly spread out on everyone's knees, being read religiously in real life situations like this.
I wound up dealing with THREE of these doozies for weeks on end, for what must have been dozens of times by now, and I'm glad to inform you, I never caused one moment of confusion to other drivers. I learned which lanes are good to go for all purposes and I used them. And none of them happened to be the right hand lane.
I've been driving for twenty years in the US and I'm not in the habit of continuing with anything I quickly learn is a dumb move, no matter which country.