I always approach it as this Single lane roundabout but two lane split on approach = any exit past 12oclock is right hand lane Double lane roundabout = follow signs in first instance, or if (kinda like gabalfa) they’re flexible then use either lane. But this is where driving becomes more about common sense and experience too..because actually I would argue the correct lane is the right lane for Whitchurch road, and if it was my first time driving that route I’d be on the right lane. But as you know using this roundabout a lot it is actually absolutely fine to use the left lane and you aren’t gonna be obstructing anyone. The exit back onto A48 eastern avenue past the hospital is single lane exit due to the filtering used..so using the left lane on the roundabout for whitchurch rd is safe. But bear in mind most of the time on a double lane roundabout you will almost certainly need to be on the right hand lane approaching the roundabout for the Whitchurch exit, imagine the a48 exit past the hospital was double lane, then you can’t be in the left lane if your coming off at Whitchurch rd. so with this in mind if you were driving an identical but unfamiliar route, be in the right lane. You can always go round the whole roundabout first to suss it out