Earlier quoted context omitted.
"Charter school" doesn't mean the same thing in different English speaking countries.
The English Wikipedia page for “charter school” specifically talks about friskola as an example of one. https://en.m.wikipedia.org/wiki/Charter_school#Sweden
I'm don't think translating "firskola" to UK English as "charter school" would be correct, just like no one uses the term "charter school" to refer to institutions in the UK.