Gun powder instead of toner ink??
Gun powder isnt an explosive, its an accelerant....it burns, not explodes.
Plus, it not white. White indicates RDX, semtex, Comp 4 (C4) and other maliable explosives. They should have been "sniffed" before boarding but not in Yemen methinks.
Maybe some Muslim-sympathizers in the UK put them on the planes when they refueled? Just a thought.