Actually - the ahead lane is the one that does not have a default, and should always be taken in line with road markings
To turn left is normally the left lane
To turn right or turn back on yourself are normally the right lane
For ahead there is no default, in the picture shown in the Highway Code straight ahead is the left lane, but it clearly states that for ahead always check signs and markings
On my regular routes I have a dual carriageway roundabout with the first two lanes being correct for ahead - if using the second lane then you should exit into the second lane, and there is also a town dual carriageway roundabout localy which says to use all lanes when turning right (in other words stick to whichever lane you were in on approach)
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.