Highway Code Rule 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".
http://www.highwaycode.gov.uk/17.htm#162
You should never turn left off a roundabout, from the right-hand lane, unless their are signs which indicate that this is permitted. I know of two roundabouts where, because most traffic is turning left, this allowed but, in general, it's extremely rare.
If you find yourself stuck in the right-hand lane when you want to turn left, you should simply do a full circuit of the roundabout.
Chris