I assumed the ounces in this recipe were weight ounces (as in, 16 ounces = 1 pound), not liquid volumetric ounces. This was how recipes specify dry good amounts in Europe--and I assumed this British recipe was similar. (I suspect that this is because traditionally you could walk to the dry goods store and ask for exactly the amount of sugar or flour you need; they would weigh it out for you.)

Edit: I just noticed an example in the Royal Icing recipe, which specifies one pound of sugar.

So 4 ounces (1/4 pound) of flour would have a different volume than 4 ounces of sugar--because their densities (weight per unit volume) are different. Trying to use the ounces on your liquid cup measure could lead to rather unexpected results!