Reducing power to a kettle
You could buy an inline dimmer switch intended for lamps. An example would be the 300W-rated Lutron TT-300H-WH, about $12 US. Make sure it's rated for at least the watts on your kettle nameplate. The one shown below allows you to plug it into the wall and plug the kettle into the back of the dimmer plug. 



These devices are very simple- consisting of a triac, diac (sometimes combined) a variable resistor and a capacitor. Sometimes there's a cursory effort to reduce EMI with an inductor and another capacitor. The device will get warm. 



I would be a bit worried that the kettle will not auto shut off at less than full power. Our Hamilton Beach 40865C (photo from Amazon), for example, does not shut off if the lid is not fully closed so I'm pretty sure it would not shut off under dimmed conditions. The likely consequence of this would be a dead kettle if you let it go dry, but it's always possible it could represent a fire hazard. 



In the old days we just had to pay attention (you plugged the chrome kettle into the wall and made sure to unplug it before it boiled dry- there was not even a switch). But people become dependent on safety features (think of cross-traffic cameras, pedestrian warnings, lane departure warnings, adaptive cruise control etc. on many modern cars).