Unless you're auditioning for the next series of Great British Bake-off, or you are Mary Berry herself, there really is no need to keep both plain and self-raising flour in your pantry!!
Not only does that take up twice the amount of strage space, and cost twice as much, but unless you're baking all day every day the self-raising flour will probably lose it magical raising power before you've used it all. And no raising agent leads to a very soggy-bottomed cake!!
Where recipes demand self-raising, use plain flour instead, adding 2tsp of baking powder to each cup of flour. Sift the two together to make sure the raising agent is evenly distributed.
Half the space, half the price, just as delicious!!
