It's better to not make any laws because someone might break them.
I can go to one of those countries with strong gun control right now and turn a formidable firearm if you like - do they need better mill control?
So if you do lock your house doors when you go out, you're implicitly agreeing that you do this to make it more difficult for someone to get inside your house, reducing the likelihood of theft.
Same thing with guns.
So America has zero house burglary?
Parent claims guns shouldn't be restricted since someone can make one on a lathe if they cannot buy one at a store.
Meanwhile parent almost certainly locks their house doors despite a thief with crowbar being able to bypass their lock.
Therefore, the parent's reasoning that gun control won't work because a criminal can mill their own gun is not logically sound.
I'm going to go out on a limb and claim murders are not an opportunistic crime. People are not killing because they can and can get away with it. The comparison you are trying to make dies there because the type of crime is not comparable.
Since someone can physically go and mill a gun, should that be the reason not to enact gun control? Seems like a ridiculous reason to me, but apparently is a core bit of logic for the gun crowd.
I don't dispute that some people may own illegal firearms, but its by no means an endemic problem. Somebody getting shot in the UK is guaranteed to make the news. Just the presence of a criminal with a gun will interrupt live news channels, that's how rare it is.
While I don't doubt that the knowledge of who to talk to and where to go to get a gun is present in certain UK communities, the basic fact that everybody involved in procuring, handling and importing guns is committing a criminal act certainly acts as a pretty solid prevention mechanism to stop, say, an angry 20-something man from acquiring sufficient guns and ammo to go on a shooting rampage at a college.