Nigella has mentioned that it is easy to make self raising flour, but is it as strong and as good as bought SRF? It certainly seems cheaper to make your own.
Nigella now usually uses plain flour plus baking powder (or a combination of baking powder and bicarbonate of soda/baking soda) in her recipes as it is easier just to have one bag of flour in the house, plus the amount of leavening in the recipes can be tweaked to give the best results. In the US self-rising flour also contains salt and we feel it is preferable to regulate the amount of salt in the recipe according to taste by using all-purpose flour plus leavening. Nigella usually suggests using 2 teaspoons of baking powder to each 150g/1 cup plus 2 tablespoons of plain (all-purpose) flour to make self-raising flour.
For the UK most self-raising flour has a protein content of between 9.5% and 10%. The protein content defines how "strong" the flour is and for cakes and biscuits you want a lower protein content. UK plain flours have protein levels that are the same and US all-purpose flours usually have a protein level of between 8% and 10%. So they should both be as good as self-raising flour. We would avoid using bread flour as its protein content is usually between 12% and 13% and this can make cakes and cookies too tough and chewy.