This socket tester only works for European grounded sockets, not for the UK 3 pin sockets.
With this tester for sockets you can test very easily if your grounded outlet works, so if it is connected to the ground via the earthing loop of your home.
You can easily identify a grounded socket by the metal strips on the side (Dutch) or the metal pin that sticks out (Belgian). If you do not see metal, your outlet is not grounded. Then it makes no sense to use this tester.