It's free for all resident citizens, i.e., they have to Be UK citizens and living in the UK. My wife is a British National, born and raised there, raised a family there, worked there, even qualified for a government pension like our Social Security but when she moved to America to live with me the NHS dropped her. She would go to visit her children and grandchildren for 1 month per year and she had to buy travel insurance in case she got sick while she was over there.
The US should be so diligent.