U.K. residents: difference between "roundabout" and "circus"?

Essentially, none.

Roundabout is commonly used - you know what we mean by this; circular road markings, or a physical feature creating the circle such as an island in the middle of the road or a monument (e.g. Piccadilly Circus has the Shaftesbury Memorial and the statue of Anteros in the center), that the traffic circles around as a means of managing traffic at an intersection.

Circus is defined as a round open space at a street junction, essentially a roundabout, it's an older term and it's more commonly used to refer to larger spaces - e.g. if it was just a meter wide road marking it'd not be called a 'circus'.

ever heard of traffic island.

I have never heard of a road marking called a circus... They are all roundabouts as far as I know. Circuses in my life experience have only been the type with clowns etc.