ive used chemical metal before but only to repair a radiator on a fiesta where the turbo decided to say hi to it.

i must admit it worked great as i thought the rad was well beyond repair. done that and stuck in some rad weld and the huge hole that was in it was sorted.

as far as using it to stick metal together i have no idea how good it is.

might sound like a ture cowboy here but how about no nails or even better tiger seal.

bloody strong stuff is tiger seal and if its just a number plate u might get away with it.
but dont just do it on my advice as i dont wanna be the one who makes the brother come knocking at ur door next week saying my number plate fell off