But that's the point. DRM works at all (in terms of causing inconvenience, not preventing copying, for that it will never work of course) because the people producing the data have an interest in applying the DRM.

But the people producing 3D printable gun parts are _not_ interested in applying the DRM.

If you want to draw an analogy to media, this is more like if the government mandated porn detection software on your computer which would prevent porn from being able to be displayed on your screen. Or mandating HDCP between your monitor and your computer so that your computer could implement restrictions on what you could view on the monitor.

Except that computers are extremely difficult to DIY from basic components (I mean raw chips and metal). Meanwhile I can literally buy aluminium extrusions, or even bits of wood, some stepper motors, some gears, some belts, some pulleys and some stepper drivers, an STM32 devboard and get PCBWay to make me a simple PCB, or just use a prototyping board. And at the end of it, I would have a high quality (maybe a bit slow) 3D printer. I can tell you with absolute certainty that it could print gun parts because I have personally taken a trash-tier prusa i3 mk2 clone and turned it into a machine which could probably rival the mk3 at least.

How exactly are they planning on stopping me from designing a part, slicing it, and then putting it on a DIY 3D printer?

They could maybe achieve this by restricting the sale of certain components such as hot-ends, extruder gears (although you can get away with generic gears), or stepper motors and stepper motor drivers. I just don't see it happening. Maybe they could ban open source slicers and CAD programs?

But I guess I better start stocking up on high quality stepper motors and stepper motor drivers and buy a milling machine and a lathe so I can manufacture the other parts myself. You never know when the UK government will steal another wonderful authoritarian idea from another country.