Don't forget that many (most? all?) of the US spelling variants are due to Webster, who tried to "fix" spellings in his dictionary.
He also tried to fix the bible, changing words like cherogryllus (rock hyrax) into rabbit.
However, whatever the unit system your car was made with, it is worth keeping both sets of sockets. This is because a worn nut in one system can often be gripped by a slightly smaller socket from the other system.
As for the English as she is spoken* in England, it is a mix of Old Britton and Celtic, Latin, old Norse, mixed assorted Germanic, and French. Pretty well everybody who invaded Britain.
Apply to that a set of grammar rules invented by scholars in the 17/18th centuries to try and make it more like 'pure' Latin.
On top of that is that everybody who lives more than 10 miles away from anybody in England speaks with a different accent. The BBC tried to set up a "Standard English" accent, but it seems they have given up on that.
*This is a shout out to a famous book on speaking English written by a non-native English speaker...