Dull Men's Club

I'm seeing that average UK socket circuits are 32A, which is nuts, I'm jealous. So that's it right there, 32A (I know (or hope) the kettle isn't pulling 32A) * 240v is a ridiculous amount of power, obviously more than any kettle would ever pull in a million years. My heat pumps and dryer are the only thing on the double breakers pulling 30A. Couldn't imagine a teakettle.

So the reason you can't just "add more amps" is US wiring standards. Most houses have 15 or 20A circuits. This puts an upper limit on the amount of wattage a single circuit can pull of either 120x15= 1800W or 120x20=2400W. This is going to be the biggest bottleneck, since going above this rating will either trip your breaker or light the cables on fire in your walls.

Beyond that, most plugs in homes are NEMA 5-15 outlets, which also limits the output of a single plug to 15A. If a manufacturer wanted to use a NEMA 5-20 plug to get that extra 5 amps, you'd need a different receptacle and thicker wiring to safely use it. Since most people don't have 5-20 plugs, there isn't really a reason for manufacturers to make them.

All of this is why pretty much every electric kettle made to work with the US electrical system is slower than ones made for 240V systems. Also, they all take about the same amount of time to heat a specific volume of water, so cheap ones are going to do just as good of a job as expensive ones.

All of these same limitations apply to space heaters as well, since they are essentially doing the same thing.

e=i*r. the coils are usually quartz coated nichrome alloy, which has a resistance based off length and diameter. so to get more amps, you just need more volts. as long as the circuit is basic (no electronics) that's just fine. however, most are rated for 110-240 or so volts, so it is only realistic in the us, and would require replacing the plug.