We don't exclusively use metric in the UK. We use a hybrid of the two. Most of the comments that "get on Americans' case" about it are likely coming from continental Europeans and other countries which do use metric exclusively.
I didn't say the US did... I'm explaining it in a way Americans would understand, because they use feet but unusually don't use its weight equivalent, and ask in comment sections wtf a stone is (bit of a funny role reversal considering it's usually US units people are asking wtf they mean)
It's pretty much just the UK and Ireland that use it, and even then, it's only used for weighing people, boxing, and horse racing.
A lot of people just say their weight in kg, stone is used less and less as older people die out.