Can self-raising flour be used in lieu of plain flour when making a roux for a bechamel?
Since you will only be using 1.5 tablespoon of flour for more than a cup of milk, it shouldn't impact the flavor. Also as there is no salt in the Sainsbury Self-raising flour, you will not need to adjust for salt. However, the alkalinity from the Sodium Hydrogen Carbonate as it heats and breaks down into Sodium Carbonate might impact the flavor. But, the acidity of the milk and butter should counter that, so it wouldn't be noticeable.