Although it is now not advised in the Highway Code to do U turns on a mini roundabout, (the DfT don't live in the real world), as ROG said, control the speed of the vehicle behind. Keep it slow. If space allows, ie. nothing to your left, it is usefull to take a slightly wider arc than you would if just turning right, although in all probability your turning circle would not be sufficiently tight to do otherwise. This, along with your slower speed, should allert a waiting driver to the fact that you are going full circle, although I wouldn't bank on it.
![Rolling Eyes :roll:](./images/smilies/icon_rolleyes.gif)