Electric showers are very common in the UK - although the heater is in a wall-mounted unit, with the head on a hose, rather than the heater being in the head itself.
The heating element has three layers - innermost, a wire that gets hot. Around that, a ceramic insulator. And outermost, a metal cover to protect the ceramic insulator. Then the heating element is mounted in a small water tank. And that, along with some other components, is put into a suitably waterproof outer plastic box. The insulation is of course all tested at the factory.
The tank, heating element outer, and all the pipework in the general vicinity is then grounded, and the power supply passes through an RCD (our equivalent to a GFCI). There are also two or three temperature sensors, and a water pressure sensor.
Installers also have to have an up-to-date electrical safety license - so for they know better than to (for example) compromise the waterproofing of the enclosure by adding extra holes to make the wiring simpler.
In some countries, electric showers use a metal coil as a heating element, similar to a spring, located directly in the showerhead. When you turn on the faucet, water fills the showerhead and touches the metal, effectively bringing the positive and negative wires into contact. The coil then heats the water as it flows out. Occasionally, the coil can oxidize or break if there's not enough water running to keep it cool. When that happens, most people replace it themselves.
A lot of things involving electricity in places with type-G plugs seem more dangerous if you don't know/assume that almost everything is grounded. Took me a while to realize that hairdrier he mentioned in the blogpost likely didn't have a ground connection in the plug.
In the case of a toaster with a grounded frame, the most likely route for a short is straight to earth which will trip the RCD on that set of plugs.
Toasters (usually?) don't have RCD (what they call GFCI in the UK) in the US. Maybe your circuit will have a GFCI, but it depends where you put your toaster. Do all circuits have RCD in the UK?
Most buildings that were built or have had substantial renovation work done will have RCDs installed, yes.
While you can have a separate RCD for each circuit, and the newest installations do, it's more common to have two RCDs in the panel/fuse box/consumer unit, with each circuit on one or the other. This arrangement is known as a 'split load consumer unit'. The intention of having two instead of one is that plugging in faulty electronics at night not plunge the entire house into darkness. These RCDs usually have a trip current of 30mA, making them less sensitive than American GFCIs which trip around 5mA.
i'm pretty sure that the electric showers i've used didn't have the sealed magnesia insulating layer you're describing, nor were they connected to an rcd/gfci
i do agree that the measures you're describing would make them much safer
The heating element has three layers - innermost, a wire that gets hot. Around that, a ceramic insulator. And outermost, a metal cover to protect the ceramic insulator. Then the heating element is mounted in a small water tank. And that, along with some other components, is put into a suitably waterproof outer plastic box. The insulation is of course all tested at the factory.
The tank, heating element outer, and all the pipework in the general vicinity is then grounded, and the power supply passes through an RCD (our equivalent to a GFCI). There are also two or three temperature sensors, and a water pressure sensor.
Installers also have to have an up-to-date electrical safety license - so for they know better than to (for example) compromise the waterproofing of the enclosure by adding extra holes to make the wiring simpler.