@mikey
If I were approaching along the east road and I wanted the NW exit, I would be in the right hand lane and indicating right. This is a positive indication to
(i) cars to my rear, telling them my plan, enabling them to change to an inside lane, ready for their planned exit
(ii) cars already on the inside lane, confirming I am not about to switch lanes, cutting in front of them
(iii) cars approaching at speed or waiting to enter the roundabout from the south road, showing I will be crossing their path
(iv) ditto for cars on the western road.
Switch indicator to left, passing the west exit. Barring idiocy, there should be nothing in the inside lane, so I could turn into the northwest exit, cutting across the roundabout lane to my left, in doing so.
I happen to find the usage of terms "inner" and "outer" lane ambiguous in some of the above posts because there is the generic sense of "inside lane" which is to your left, wherever you may be and there is the special sense of inner (smallest radius) middle and outer (largest radius) of a roundabout (or any curve, for that matter).
For instance, I was unable to understand how you could be trapped by someone who was exiting opposite their entrance but using middle or inner lane to do so when you wanted the same exit. That is to say I can't picture which lane you were in such that you were obstructed.