This is the same principle we saw in action at Virginia Tech, which already had what the anti-gun nuts advocate: a total ban on guns. This merely guaranteed that no one except the murderer would be armed and that he would therefore be effectively unstoppable. If the campus population had had the same statistical distribution of gun-carriers as the general population of Virginia, he would have been stopped quickly, and most of his victims would still be alive today.
Gun bans disarm only people who obey laws. I don't know what it will take for the anti-gun fanatics to get this basic fact through their heads. If I were a sociopath planning a shooting spree or any other kind of gun crime, I would certainly carry it out in a place like Britain or Virginia Tech where I knew my victims would be defenseless against me, not somewhere where I knew there was a good chance any one of them might also be armed.