Yea that whole thing with Iberia is nuts, but not too crazy. Seeing as how Britain probably attacked the Gauls a lot and the Julii not even resembling a kind neighbor the Gallic armies probably just got wiped out in a couple of decisive battles and the remaining forces were just spread too thin to cause much resistance. Then with what was left of the Gallic army being nowhere near them, the Iberians couldn't help but move in and take over a bit.