Is powdered mustard in a tin (Colmans) not just ready mixed mudtard before it's ready mixed?
No. The ready-mix has other stuff in it as filler, colouring, preservative. Powder is now 100% mustard flour, although apparently in the olden days it was cut with cornflour to maintain a consistent price (1d for the small oval tin) from year to year regardless of fluctuations in the mustard harvest.
More significantly in terms of taste, it also includes acetic acid. If you mix it yourself you can tune the taste by using plain water, or varying amount of vinegar of your choice.
No. The ready-mix has other stuff in it as filler, colouring, preservative. Powder is now 100% mustard flour, although apparently in the olden days it was cut with cornflour to maintain a consistent price (1d for the small oval tin) from year to year regardless of fluctuations in the mustard harvest.