Re: Why a 3D printed gun?
In the UK you are allowed to own shotguns, long rifles, some classes of field artillery and ammunition, and you can hold explosives with a good reason.
What your not allowed can mostly be quickly summed up as pistols, automatic weapons and bolt action rifles in .50 caliber. And any of those without a good lawful reason for owning them such as being a member of a club that shoots them.