Flour in the UK comes in plain or self raising, you can add baking powder to plain flour if needed but the self-raising flour has something in it to make the cake rise so you would rarely need to add a raising agent to that.
Look at or for a recipe that states which you need as cakes like sponges will be made with self-raising flour but a tea-bread type cake may be made with plain flour.
cake flour is a lighter flour that will rise without the use of baking powder. Cake flour is a mix of all purpose flour and cornflour aka cornstarch. To make your own cake flour take a one cup sized measuring cup and add 2 level tablespoons of cornflour then fill up the rest of the cup with all-purpose flour and level off. Then dump the cup of flour into a sieve and sift. If the recipe calls for more than just 1 cup of cake flour then all you need to do is repeat the same steps for another cup or half it for half of a cup of cakeflour.