I wrote this on PH in reply to someone who has mis-hap on a roundabout...
there no fixed rule in Highway Code - apart rules 185/186 which both tell drivers to be aware of everyone on the roundabout und to be aware they may not indicate properly..
Rule 186 tell you to signal left und approach in left hand lane if taking first exit If turning right to be in right hand lane und signal right. You keep to the right until you need to change lanes und you put on left indicator AFTER passing exit before the one you need.

You advised to follow the road markings - but to choose the appropriate lane on approach und to watch out for twazaks und pretzels all the time

"DSA Driving Standards- Essential Skills" spell out the MSM/PSL on pages 170-173. They say to indicate "left" on approach if you intend to take the first left exit - to keep to lane whilst on roundabout und maintain theleft signal whilst within the roundabout.
If straight on .. no signal needed on approach. They say if you cannot use the left hand lane because of some blockage - to use the next one. Keep to it, use mirrors.. all mirrors , but to signal left after you pass the exit before the one you want.
If turning right - indicate right on approach. Use the right hand lane.. keep to this lane und maintain signal on roundabout. CHECK all mirrors - esspecially the one on the passenger side und indicate left as you pass the exit before the one you want. UND TO always be aware of traffic to your left or sitting in potential SMIDSY blind spot
We hear lots from my in-laws who live in the Worsley area of Manchester. They use J13 und they see all sorts of cut-ins from right hand traffic into left hand traffic at this narrow entry to M60 there.
It matter of courtesy to tell another what you are doing.
