Technically, if someone speeds into a round-about and nails a slow vehicle that has already entered from the next entry point then it is a rear end collision. Therefore the faster driver is a fault. The rule is simply "give way to traffic on round-about" whether it is on your right or left. As soon as a vehicle's wheels cross the line, they are on the round-about and anyone still approaching has to give way to them. The only reason the slow car mentioned above would be charged is if the speedster simply lied and said they were on the round-about first. I'm sure the accident investigators would be able to determine who was in the right based on impact point, tyre marks and all that stuff though.
In reality though, you do well to be cautious because it is just plain dumb to purposely put your vehicle in front of a moron who doesn't understand the rules even if it's their fault.

