Given that homicides in both the US and UK have been on a similar decline, and if the UK did not ban firearms in the first place. I would assume that they would follow the same downward trend they are now or see no change. A tool is a tool be it a gun or a knife it requires a human to use them for good or ill. Murder is already illegal, so is -blam!-, robbery and a host of other things yet people still commit those crimes.