In the US it is against code to have any regular appliance use more than 15 amps continuously I believe
Many commercial appliances use 5-20 plugs. Also, lots of commercial cleaning equipment seems to use 5-20, which is why you see businesses tend to only use 5-20 outlets.
I'm not aware of any kettles that use 5-20 plugs but I bet they exist somewhere.
A kettle is fine at a full circuit load for under 3 hours.
If I wanted a British kettle in the US I'd wire up a 6-20R, buy a British kettle, and swap the plug.
Fuses/breakers are ultimately about heat control. They can run "indefinitely" at their rated amperage, but can support short, higher loads.
Practically speaking, any intermittent device that _can_ run continuously or can fail to a continuous needs to be considered as such for safety purposes.
For example, a fridge should only run the compressor intermittently, but it has two obvious cases where the compressor could run indefinitely:
* An influx of heat, like filling an empty fridge with room temperature cans.
* A door being left open.
In the case of the kettle, it will likely be evaluated against it's "nominal" draw after the initial startup. If the auto-off sensor were to fail, it could run continuously at the tempurature.
An appliance manufacturer may be under different UL regulations for failure modes of a device on a specific rated circuit. I don’t know anything about that.
So you can run indefinitely at the nameplate rating in Australia, but only 3 hours in the US. And the startup current (inrush) can be much higher than the rating. Most breakers are thermo-magnetic. The magnetic part has a much higher tripping point and allows for inrush current. The thermo part trips when it gets too hot, and that'll be the current printed on the breaker.
Prior to the late 70s / early 80s so called "automatic kettles" were not necessarily all that common in the UK, and one had to manually switch them off.
I certainly recall a time or two with a kitchen full of steam due to forgetting about putting the kettle on.