Hello and welcome.
Actually, cake flour is neither plain or self-rising. Plain flour is all-purpose flour and self-rising is all-purpose flour with leavening and salt added.
Here in the US, cake flour is often sold in a box similar to a cake mix box but larger. It is a softer wheat flour which is lower in protein content than AP.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan