There's a joint liability in my opinion. Red was in the wrong lane for the exit he intended to take. Blue is in a position that there were two lanes in to roundabout, on the roundabout and two lanes out so in principle he was making a valid maneuver (although, in my opinion it would have been more correct to be in the left lane coming on to, on the roundabout and exiting the roundabout, or to have moved from right lane to left lane on the roundabout before exiting), but was not sufficiently road aware of Red.

Primary negligence on the part of Red, with contributory negligence from Blue - I'd give it 75%/25% Red/Blue for a split, or 100% Red for a single liability.