Where to start.....
Sadly, the C-17s that the RAF already own can do all the tactical things the A400 is supposed to do (and more
.... wrong.
Had they gone with CFM-56s in a C-17 style nacelle the money spent to develop the engine, gearbox, props, etc could have gone to designing in the tactical capabilities from the start. And it would have had equal or likely better tactical capabilities than what the turboprops could provide. This seems like a poorly managed program for the get-go.
....The customer specified/demanded the choice of powerplant.