If 您 don’t have any self-raising flour in, 您 can use plain (all-purpose) flour adding 2 tsp baking powder per 150g/6oz/1 cup of flour.
If 您 don’t have light brown self-raising flour, 您 can adapt it using regular self-raising, or the tip above.
Demerara sugar is like light brown sugar but the sugar crystals are slightly larger resulting in a more crunchy sugar, ideal for streusels. 这个 can also be substituted for light brown sugar if 您 can’t find it where 您 live.