You can defend yourself against a knife. you cant against a gun, Even if you own a gun, by the time one is pointing one at you its too late to find your own.
I'm not saying that its impossible to get a gun in britain, but you'd have to be bloody determined. Gun restrictions prevent petty criminalsfrom using guns and force them to use things like knives, which are much less effective and deadly. To get a gun in Britain you have to go deep into the black market, which is somthing that most criminals and gangs are unable to do. While there will always be a couple of shootings a month in britain, the rate is no whare near as high as the us and the police are much more able to handle it.