Hi mil,
Yes, plain flour may be used in place of 'all purpose flour'. They're the same thing.
The only difference between plain flour/all purpose flour and self-raising flour, is that in the self-raising, is all-purpose flour plus 1½ teaspoon baking powder and ½ teaspoon of salt per cup of flour, so you don't have to make these additions---they're already incorporated for you.
Hope you're making something yummy! ;D
Cher