As a Kiwi I'm surprised to hear that. Perhaps the author is speaking of some parallel New Zealand, but in the one I inhabit it's not especially hard to own a shotgun or rifle: get a license, buy your firearm.
Pistols, high-capacity semi-automatic rifles, automatic weapons are very hard or impossible to own, but guns themselves are not uncommon.
To own a pistol for instance, you need to be an active member of a gun club for 6 months before buying one, and then you need to keep it at the gun club for 6 months before bringing it home. At home you are only allowed to store it in a locked cabinet without ammo and the only time you are allowed to transport it is to and from the gun range.
In the US you have the expectation that a gun will provide "protection", i.e. you can buy a gun with the purpose of harming others. I think this is the biggest difference.
But.. City of London averages 3-4 days... https://keyassets.timeincuk.net/inspirewp/live/wp-content/up...
To put it differently: City is one of 33 regions of London (the other 32 are boroughs; City is not a borough), and one of the very smallest.
> and for a long time it had its own police force
It still does: https://www.cityoflondon.police.uk/
This seems to be true of many things. When I worked in the UK I never ended up with the national health number that you're supposed to get, because I was based in the Black Country and there's essentially an infinite backlog (as in, at my address they were no longer taking applications).
A friend applied from the address of his workplace near the City, and had one within a few weeks.
However, everybody and his dog owns a shotgun or a rifle to go hunting; it's just so that we mostly use them for hunting rather than for self defense or committing crimes...
The shorter answer is that disarming hasn't change the homicide rate with guns at all. That has remained steady and inline with violent crime and crime in general, the amount the populace is armed didn't have any impact. We are told it has but the statistics alas don't lie and we have stats for this going back quite a way. There should be clear and present dips after gun control laws or changes in trajectory but they just aren't there.
In the US if you buy a firearm online from a retailer or distributor it is required that it be shipped to a Federal Firearm Licensee, who will perform a NICS (National Instant Criminal Background Check System) check prior to the transfer.
Though not required, many private sellers will only ship firearms to an FFL so that the transfer is documented (in what is called a "bound book") and the private seller knows the buyer can legally purchase the firearm.
Some private sellers (a minority, in my experience) don't insist on having a dealer do the transfer, but that has nothing to do with where the sake takes place.