As reported, a heavily outnumbered teenager was confronted by a group of four men in Thames Ditton, one of which threatened him with a metal bar. The men involved clearly do not care about the Prevention of Crime Act, and arm themselves in public regardless. The law-abiding person, who may need to defend himself is the only one disadvantaged by the law in this situation.
Last year in Forest Groove, Oregon, a 55 year old man was able to scare away 3 muggers by discharging his handgun which he had a permit to carry. If he had carried any type of weapon for self-defence in the UK he would be guilty of breaking various Acts of parliament and would likely be charged with a more serious offence than the muggers themselves.