The 12 o clock rule refers to an idea of what you would do on the approach to roundabouts. It really is a guideline more than a rule but it gives the driver a rough rule to use on approaching roundabouts.
If you are approaching a roundabout and the turning left and the exit is before 12 o clock then you approach in the left hand lane and give a left signal on approach. If you are going ahead and the exit is at 12 clock you again approach in the left hand lane with no signal on approach and signal left to leave as you pass the last exit before the one you wish to take. If your exit is past 12 o clock you would approach in the right hand lane with a right signal on approach and signal left to leave as you pass the exit before the one you wish to take.
However as said earlier it is a guide line because local knowledge and the roundabout configuration could alter this. If for instance there is no right exit on a roundabout you are approaching then they may have a the left lane for turning left and the right lane for turning right, so there is one for each direction which is different to a four exit roundabout where normally the left lane is for left and ahead, but on this occasion they may as well use the right lane for ahead. The same going if you are using this roundabout from the other direction where if there is no left exit just ahead and right, then the left lane is used to go ahead and so no signal is needed on approach and then you would just signal left to leave.
Sometimes if the exit you are taking slightly past 12 o clock and there is another exit at say 3 o clock it is not worth giving a right signal as it will mislead the oncoming driver thinking you are taking the 3 o clock exit.
Mspsl, check your mirrors, signal left, position your vehicle on the left hand lane, adjust your speed and gear by looking early at the round about for gaps and opportunities to carry on to the roundabout.