Where there's not much space, you can just paint the circle [1].
I think that one probably exists because people are going south and east (to the two main roads), so it's not clear that either direction should have priority. The junction looks more-or-less the same size as the crossroads to the north. (Which, incidentally, has no signs, just road markings to indicate which road has priority.)
For interest, move slightly south east to see the large roundabout between two main roads, then follow the A4540 north to see two more.
Drivers in the US are so bad that painting a circle will do nothing. The physical roundabout with curbs and landscaping near me gets driven straight through a couple of times a year. If it won't destroy your car, American drivers will disregard it when it is inconvenient.
Driving over a painted ("mini") roundabout could lead to a £50 fine if it's spotted by a police officer in Britain, although in many cases (when there's no other traffic) I think they'd just stop the driver and warn them not to do it again.
(Driving over one in a bus, lorry or other large vehicle that can't make a tight turn is allowed.)
Driving over a normal roundabout with curbs and landscaping is unheard of, there isn't a specific fine. The kerbs are usually tall enough that it risks damaging the vehicle. I think it would be considered fairly serious¹, as it shows either a serious lack of attention to the road, or a complete disregard for the rules.
¹ i.e. you get a fine and 3-6 penalty points on your driving licence, which increases your insurance cost. If you get 12 points you lose the licence and have to retake the driving test.
I think that one probably exists because people are going south and east (to the two main roads), so it's not clear that either direction should have priority. The junction looks more-or-less the same size as the crossroads to the north. (Which, incidentally, has no signs, just road markings to indicate which road has priority.)
For interest, move slightly south east to see the large roundabout between two main roads, then follow the A4540 north to see two more.
[1] https://www.google.com/maps/@52.4875739,-1.9213084,194m/data...