As a caravan owner and motorcycle training instructor I also disagree with tj_4 in certain respects.
Here is what the HC says:
162: Signals and position, unless signs or markings indicate otherwise.
When taking the first exit
* signal left and approach in the left-hand lane
* keep to the left on the roundabout and continue signalling left to leave.
When taking any intermediate exit
* select the appropriate lane on approach to and on the roundabout, signalling as necessary
* stay in this lane until you need to alter course to exit the roundabout
* signal left after you have passed the exit before the one you want.
When taking the last exit or going full circle
* signal right and approach in the right-hand lane
* keep to the right on the roundabout until you need to change lanes to exit the roundabout
* signal left after you have passed the exit before the one you want.
When there are more than three lanes at the entrance to a roundabout, use the most appropriate lane on approach and through it.
163: In all cases watch out for and give plenty of room to
* pedestrians who may be crossing the approach and exit roads
* traffic crossing in front of you on the roundabout, especially vehicles intending to leave by the next exit
* traffic which may be straddling lanes or positioned incorrectly
* motorcyclists
* cyclists and horse riders who may stay in the left-hand lane and signal right if they intend to continue round the roundabout
* long vehicles (including those towing trailers) which might have to take a different course approaching or on the roundabout because of their length. Watch out for their signals.
In ALL cases the use of signals is of paramount importance, because whilst YOU may know where your going, nobody else does UNLESS you signal your intentions.
In the scenario stated at the beginning of this thread, it is quite obvious that the poster knew he was in the wrong lane, and instead of going off the roundabout and coming back on to it in the correct position for turning to the right, he chose to exit to the right, but from the totally wrong approach lane, hence causing a moving hazard to ALL other road users.