As they were not for eating (by people who counted), I doubt the best flour was used. Master Wulfric's trencher recipe uses a mix of 2 1/2 cups white flour and 1 1/2 cups wheat flour.
I'll need to look through household accounts to get a better idea of types of flour. AS they needed to look good, rather than taste good, it would not surprise me to find that trencher were made with rye or other less desirable flours.