another question on roundabout lanes

hi all,
I've got a question on lane rules for roundabouts which hopefully you guys can help me out with. I was in a minor accident today and just want to check who was at fault.

Car A and car B arrive at a roundabout from the same (two lane) road and enter it at about the same time. Car A is on the outside lane, car B the inside lane. Car A does not exit the roundabout at the first exit and instead continues round, however car B does try to exit the roundabout at the first exit and the two cars collide.

The roundabout has two lanes onto it, and two lanes at the first exit. The first exit is straight on from the entry point of both cars.
Car A claimed that they did not do anything wrong as they stayed in their lane. Car B claims that they also did nothing wrong as the roundabout has a two lane exit, meaning cars on right lane should be able to use the first exit.

Anyone know who is in the right?

Cheers