I can see the theoretical argument that legalising guns might mean more control over firearms than criminalising it, thus driving it underground.
It's one valid argument but in practice I think it's outweighed by the dangers of legalising firearms. to go back to the drugs analogy, alcohol is legal here and causes more problems than any other drug. If you could buy a hit of heroin at your corner store i'm pretty sure there'd be a bigger hard drugs problem, even if it would be safer to get hold of for those determined to do it.
However i'm more in favour of legalising some drugs than guns. Cannabis for instance i think should be legalised (but that's just me being selfish)
You mainly harm yourself with misusing hard drugs, but misusing guns that's not the case.
Another point is that if you completely ban guns and a cop catches someone with some then there is no debate about whether he should maybe let them keep it, a decision he might regret later. Total banning means there's not question. There is some gun crime in UK but proportionally it is still a pretty small problem I think(don't have any actual stats for that but that's my impression)
Anyway we don't need to look at theory - there are some pretty big experiments testing the theory and from recent results it's looking increasingly like legalising is a bad idea.