Well yeah.
But if both are rated for 6 or 8 Amps and can only supply 240 or 120V, you are bound to that.
Thus the volts are important as well.

2 kW @ 120 V is 16.7 A, which exceeds the 15 A limit on most household wiring in North America. To be able to achieve that you’d need to get a 20 A rated circuit installed by an electrician which means pulling out and replacing the wiring with a heavier gauge.

The advantage of 240 V rating in the UK is that you can draw more power with less current, so you don’t need the wiring to be so heavy for a high power appliance like a kettle.

That's exactly the point. Home connections are limited to a specific amperage (15 or 16) pretty much all across the world - certainly not beyond that anywhere. Voltage remains the only difference: 120 vs 240. With 120V at 15A you can only go up to 1800W which does take a long time to boil water.

Yes, you could have multiple phases to get to 240V in 120V countries to boil water faster but these would be special outlets and not meant for a kettle that you could buy from a Walmart.