https://www.gov.uk/using-the-road-159-to-203/roundabouts-184-to-190
186
Signals and position
When taking the first exit to the left, unless signs or markings indicate otherwise
* signal left and approach in the left-hand lane
* keep to the left on the roundabout and continue signalling left to leave.
When taking an exit to the right or going full circle, unless signs or markings indicate otherwise
* 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 taking any intermediate exit, unless signs or markings indicate otherwise
* select the appropriate lane on approach to and on the roundabout
* you should not normally need to signal on approach
* 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 there are more than three lanes at the entrance to a roundabout, use the most appropriate lane on approach and through it.