The ones I've seen are really more of a half-hearted traffic-calming measure than proper roundabouts. The intersections I can think of, they're in residential neighborhoods and there's just some new island in the middle of a four-way stop. It might be impossible to widen these into proper roundabouts due to the houses up to the property lines?

Placing new STOP signs in the UK required written permission from the secretary of state until some years ago. They're still quite uncommon here, typically reserved for more risky junctions.

We most commonly use the "Give Way" sign at junctions instead, indicating that other traffic has priority.