If it's such a basic security feature, what is it actually protecting?

In a standard (non-British) home, each room has a separate (10A or 16A) breaker in the breaker panel. That breaker protects the wires from overheating (preventing cable fires), and in more modern instances might protect the human from electric shock with an RCD. Then each device may have an internal (usually self-resetting) fuse to protect itself from failure modes. That fuse is tuned to the device needs, both in how fast it triggers and how much current it allows.

What does a fuse in the cable buy you? It can't reliably protect the device, because the user might replace the cable or replace a blown fuse with one with a different rating. It can't protect wires because it's after the house wiring, a large number of functioning devices overheating your wires is at least as likely as a single malfunctioning device. The only context in which it makes sense it the one Tom Scott mentions: you have a copper shortage, so you put the entire flat or even house on one circuit with one fuse. And because you don't want to cut electricity to everything every time there's a short somewhere you put little fuses everywhere.

It's a good design for the circumstances of post-war Britain, but we aren't living in a copper shortage right now.

Not everything has a 13 amp fuse in the plug. In fact most things have a 3 amp fuse unless they are kettles or something. There's also 5 amp fuses and it is possible (tho not that common these days) to have even a 1amp fuse in the plug itself.

Surely it is better to have a lower amp fuse tailored to the device, than a high amperage fuse that's happy to send 3000 watts into your dodgy mobile phone charger without fusing.

> What does a fuse in the cable buy you? It can't reliably protect the device, because the user might replace the cable or replace a blown fuse with one with a different rating.

Ehh. While I see the logic in the rest of your argument, I don't see how being able to change the fuse means the device on the end of the fuse can not be reliably protected. If anything, being able to adjust the fuse to the needs of the device is even better, e.g. if you go from a high-load bulb to something like an LED. I'm not saying anyone runs around the house checking every fuse in every wire, but this does seem like a consideration you've not really made.

> It can't protect wires because it's after the house wiring

I don't see how it can't protect the wires, could you give an example? If a device with a 13A fuse is drawing over 3120W, the fuse will blow to protect the device itself and the house wiring. Could you elaborate on this, please?

> so you put the entire flat or even house on one circuit with one fuse.

Yeah, and we still use that system today. In most residential houses in Britain, we have a main fuse coming into the house. The rating of the fuse depends on what the house can take, e.g. 60A, 80A, or 100A. The main high amperage circuit in most homes in the UK would be the shower, which needs more than 3000W. Mine is a 6.5kW shower with 32A wiring (and the respective RCD).

I personally think fuses in plugs also serve as a common sense mechanism: if your computer monitor is drawing 13A, there's obviously something wrong. That's where the fuse comes in. It's just another protection.

So you have a 16A fuse/breaker, and a wire that will take 16A. Then you plug in your lamp with a thin wire that can only take 3A. In the UK that's fine, as you have a 3A fuse, the lamp develops a fault and starts pulling 10A, the fuse blows, job done.

Without that fuse though, the wire overheats, and you get a fire.

Looking at the fairly new soldering iron plug next to me, with a 3A fuse that wire does not look anywhere near thick enough to qualify for passing 13A.