Ottilie, the average school cost in the US is $44,700 (roughly £33,300). Most cost somewhere between $35,000 and $45,000, though there are some that can cost upwards of $50,000. And that is tuition alone. I don’t know how it’s done in the UK, but the schools here nickel-and-dime you for everything (health fees, post office fees, lab fees, music lessons, etc).
Sidura, there are several well-regarded boarding schools in California, but, you’re right, they are mostly in the southern part of the state. Cate School in outside of Santa Barbara, and The Thacher School is in Ojai. Many other schools are in the Ojai Valley as well. But the vast majority of boarding schools are focused in the northeast and the mid-Atlantic.
Also, in the US, uniforms are more common at private day schools, very few boarding schools have uniforms. Some have fairly strict, formal dress codes (slacks, sports coats and ties for boys, and dresses/skirts or slacks for girls; look at Phillips Exeter Academy or Groton School), and some have no dress code at all (Phillips Academy and Cate School). Most, like mine, are somewhere in between with a “clothes in good repair” dress code (few rules overall, but they are strictly enforced).
It’s definitely not something for everyone. Kids at boarding school have to be mature and motivated. Sure, the teachers and dorm parents are great, but there is only so much that they can do. Also, keep in mind that communication is a lot easier nowadays than it was even when I was in school (I entered boarding school in 2000, and graduated in 2004).
What blade says is true–boarding school is incredibly rare. Most people thought I was insane for wanting to go. But it is also a lot more common in the northeast, where boarding schools are more common, or, at the very least, people have more knowledge of boarding schools because they are so much more prevalent.