Although the photograph only shows the metalic parts, this is a complete kit with backing pieces and overflow pipe.
I selected this to go with a bath with a mixer tap where all the water flows from a single point at the centre. The water hits the plug directly, causing it to rotate open. Only by holding the plug in-place until the water is at least 5cm deep can the bath be filled.
This plug may be suitable only if you have two separate taps. Even then, it can sometimes open because of water movement in the bath.