Every microwave I think I've ever used here in the US has some form of power setting.

The problem is that it's completely nonstandardized, so saying "power level 6" can't be applied to arbitrary other microwaves to get a comparable effect.

I think that it's rarely useful, because generally you might as well just run at full power for less time and then wait afterwards for heat to spread.

What I really wish we had would be at least semi-standard settings across microwaves. Like, instead of a time setting -- microwaves apply energy at different rates -- the base unit should be a number of watt-hours to be applied, something like that.

not popular in the UK

Trivia: the UK invented the gizmo that can output that shit-ton of power in the form of microwave radiation in a microwave. It was an absolutely critical technical development in World War II -- it let radars be vastly more powerful then they had been, and it was a "Eureka" moment, a major nonobvious breakthrough that other countries wouldn't have just gotten iteratively shortly.

https://en.wikipedia.org/wiki/Cavity_magnetron

The cavity magnetron is a high-power vacuum tube used in early radar systems and subsequently in microwave ovens and in linear particle accelerators.

The cavity magnetron was a radical improvement introduced by John Randall and Harry Boot at the University of Birmingham, England in 1940. Their first working example produced hundreds of watts at 10 cm wavelength, an unprecedented achievement. Within weeks, engineers at GEC had improved this to well over a kilowatt, and within months 25 kilowatts, over 100 kW by 1941 and pushing towards a megawatt by 1943. The high power pulses were generated from a device the size of a small book and transmitted from an antenna only centimeters long, reducing the size of practical radar systems by orders of magnitude. New radars appeared for night-fighters, anti-submarine aircraft and even the smallest escort ships, and from that point on the Allies of World War II held a lead in radar that their counterparts in Germany and Japan were never able to close. By the end of the war, practically every Allied radar was based on a magnetron.

https://en.wikipedia.org/wiki/Tizard_Mission

The Tizard Mission, officially the British Technical and Scientific Mission, was a British delegation that visited the United States during World War II to share secret research and development (R&D) work that had military applications. It received its popular name from the programme's instigator, Henry Tizard, a British scientist and chairman of the Aeronautical Research Committee which had orchestrated the development of radar.

The mission travelled to the U.S. in September 1940 during the Battle of Britain. They conveyed a number of technical and scientific secrets with the objective of securing U.S. assistance in sustaining the war effort and obtaining the industrial resources to exploit the military potential of these technologies, which Britain itself could not due to the immediate demands of other war-related production. American historian James Phinney Baxter III later said "When the members of the Tizard Mission brought one cavity magnetron to America in 1940, they carried the most valuable cargo ever brought to our shores."

Pretty standard feature in every microwave I've ever seen

Pretty standard