Baking Powder in scones
I do the same as you, Vivien. I find that mixing in the liquids in advance leads to too much gluten development and some outgassing from the baking powder, but that if the butter/flour mixture is kept cold enough it doesn't hurt to mix it the night before.
(Anyway, it takes longer for the oven to heat up than it takes to mix together the remaining ingredients and shape the scones.)