The rules surrounding the Signal Crayfish are indeed contradictory . . . .
It is illegal to take them without a valid licence from the EA.
It is also illegal to return them to the water they came from . . . .
Therefore you cannot use them as a bait and the only (legal?) option remaining is to crush them on the bank and leave them there for nature to take care of them.
I don't see anything against having a Kelly kettle with boiling water though so you can drop them into that and eat them (really good eating) tha also saves the hassle of making sarnies for a day on the river