The "Home Made Instant Pancake Mix" calls for 3 tablespoons baking powder (that's roughly 43 grams) for 600 grams of flour. I wonder if that is really correct or a British baking powder is somehow different? Because usually around 1 teaspoon is enough for 150 grams of flour. So it is really nearly twice as much in the recipe?
And a side question, would you suggest using a buttermilk instead of milk, since there is no acidity in the recipe and there is also baking soda in the ingredients?
Nigella's Home Made Instant Pancake Mix (from NIGELLA EXPRESS) mixes a large amount of dry ingredients that can then be measured out to make pancakes in smaller portions. The mixture contains 3 tablespoons (9 teaspoons) of baking powder and 2 teaspoons bicarbonate of soda (baking soda) to give extra-fluffy pancakes. We would normally use 2 teaspoons of baking powder to 150g flour to give self-raising flour, so it is only slightly more than that amount and the addition of the extra bicarbonate of soda helps to give further lift.
As the mixture contains baking powder there is enough acid to activate the bicarbonate of soda by using normal milk. You could use buttermilk, but you may want to reduce the leavening agents slightly in this case as the extra acidity will make the leavening agents act more quickly.