I have baked many of Nigella's recipes but I am wondering why she favours plain flour plus baking powder as opposed to using self-raising flour? It is a question I have pondered over for some time!
Nigella prefers to use a combination of plain (all-purpose) flour and baking powder rather than self-raising (self-rising) flour for practical reasons. Self-raising flour contains baking powder but as baking powder will expire after a period of time you need to use up self-raising flour more quickly than plain flour. Of course you need to check that the baking powder you add to the plain flour is also within its "use by" date, but Nigella has found this approach to be more useful for her. It also saves cupboard space as you only need to keep one bag of flour plus a small container of baking powder.
In the US self-rising flour also contains added salt which can lead to some of the recipes tasting a little too salty if this flour is used.
As a guide, Nigella uses 150g (1 cup) plain flour plus 2 teaspoons baking powder to replace self-raising flour in recipes. Also for some recipes, particularly those containing cocoa, Nigella finds it useful to use a little bicarbonate of soda (baking soda) in addition to the baking powder, for extra lift.