Drove in front of someone doing a u-turn at a roundabout.

My right was clear, my mistake was expecting the driver coming onto the roundabout from the left to exit on my side of the roundabout....

I gave them enough time to drive past the centre and drove onto the roundabout to go straight ahead and they did a full u-turn, they had to brake, (from maybe 10mph) no contact.

I feel like garbage. :(