I have no evidence for this, and no doubt people will correct me; but this is all very off putting.
Victims don't carry around weapons to defend themselves right now, if they were able to get hold of a gun that wouldn't change. Essentially the only thing that changes is on the attacker's side; great, so now you've added guns to that equation.
Well, if you do that, then I guess you also need to arm the police. Most police force in the UK is not firearmed (the UK has a strong notion that the police are civilians in uniform), but if you're giving more tools to attackers, the police will need to respond in kind.
So effectively, by adding guns, you've done nothing to prevent acid attacks and you made it so that the police making a mistake can end a life.
Edit: I misinterpreted the parent comment!
For example, handguns and other concealable weapons were almost non-existent in Australia when I was living there. As a side effect, there were a lot of fistfights; far more than I've ever seen at equivalent bars/pubs in the US. That sucked, but I still found it preferable to being shot/mugged at gunpoint.
Likewise, acid attacks are terrible, and there are possibly more of them since guns and knives are less accessible. Doesn't mean that overall things are worse because of the ban.
If the attackers are caught and or the knife is found it can contain dna/fingerprints from either party to link the crime to the suspect.
Acid however by it's nature is "clean" in this regard.