Criminals still have guns in London. However, low level criminals, a bigger threat in my mind than large scale criminals, have to make their own bullets. I say low level are a higher threat because upper echelon criminals do not want superfluous attention. So they won't go around robbing people at gun point. Whereas a low level criminal is going to rob a bank at gun point, not really thinking of any consequence, and will shoot. That's a whole debate in itself, but lets just say, for the sake of argument, that the previous argument is given and accepted.
Most people, or at least most that I know do not know how to make their own bullets. So when they attempt to the bullets usually are pretty useless, and probably won't kill a person. I highly doubt your average "gang banger" could make bullets for his ak. The only friends of mine that can make bullets happen to be rednecks who use their guns SOLELY for hunting/protection, and seldom resort to using their guns unless someone else already has a gun intending to use it against them. So I propose, why not make bullets illegal to distribute to civilians? In my sample set its a win win. I don't know though, it could e a terrible idea.