How to do mini roundabouts - when to pull out:
Look at the roundabout as a whole and try to predict when cars coming from straight ahead will cause the cars on your right to stop.
The best time to go is usually as soon as the car on your right hesitates - if you wait too long then the crossing vehicle will have gone and the car on the right will move rapidly out behind it. Aim to be in the middle of the roundabout at the same time as the vehicle you are using as the block (ie. silver taxi in photo below).
When you approach a mini roundabout do so at a speed that will enable you stop behind the line, and to deal with any situation that arises: Maybe a car from the left that should give way but doesn’t. Or a car from the right that isn’t indicating to turn, but does anyway. If possible time your approach so that you don’t arrive at the roundabout at the same time as another vehicle that you may have to give way to. Look at other vehicle's speed, indicators, and road position, and where the driver is looking, before you make your decision to go.
When entering a busy mini roundabout you may have to be assertive to proceed. If you have to push your way in then use a two-stage technique. First of all move yourself carefully across the lane you have to give way to (your right) so that you are in the middle of the roundabout; then before proceeding any further make sure that traffic that has to give way to you (your left) is stopping for you.