Calling this an increase is slightly disingenuous, considering that since 2003 there's been a downward trend that terminates to less than the rate at the time of the ban
And you'd also be ignoring this statistic, that shows that it was in fact effective at dramatically reducing firearm related homicides. The ban did what it was aiming to. Yes, there are other methods of killing, but all of them are more easily preventable. If anything I'd say that part of the reason the UK is doing so poorly in homicide prevention is that police officers have been kneecapped
Plus, part of the reason for regulating firearms more harshly is to prevent suicides and accidental injuries/deaths. Even if homicide rates stay the same, I'd consider restrictions/bans effective if they reduce those other forms of death