My understanding is that there are tow different types of 'roundabouts'. I recall from my learners that they are called either a mini-circle, or a midi-circle. The former is a slightly raised, possible to drive over (though not legally), painted platform tapered towards the road. The latter is a more solid, more raised and bigger, often bricked with plants and other decorasies in the middle, that will cause permanent damage to your vehicle. At the former you give right of way to the person already in the circle, at the latter you must wait for all traffic approaching from the left to pass before entering the circle yourself, and if you're in the left lane, that is a compulsory left turn or you indicate your intention to turn left then at the very next opportunity to turn left you MUST turn left, if you intend on turning right after a 270 degree turn, then you should indicate your intention to turn left after the second opportunity to turn left.
If i'm wrong, ive been doing it wrong for years. But I think thats accurate.