English units were the measurement standards used across the British Empire until 1826; it’s the system that immediately preceded and (independently) developed into today’s imperial units and US customary units.
I never really thought about why the UK and US diverged slightly, but (obviously, in retrospect) the simple reason that imperial and US customary units differ in some significant ways is that both systems were developed after the US became independent, based on the in-use-at-that-time English units system.
The Wikipedia diagram showing the relationship between lengths in the English unit system is a fun reference:
And of course, no discussion on this topic can ignore this striking map of metric adoption around the world:
