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.
If nobody ever uses the term in the UK, then why would its definition matter or be different?
It matters because the original title was changed to a term only popular in one country.
https://www.oxfordlearnersdictionaries.com/us/definition/eng...
There is only one definition. If someone doesn’t know the term, they can look it up. Your original comment claimed that it means different things in different countries, but that just doesn’t seem to be the case. The term seems like the appropriate English translation, even if it doesn’t exist in the UK.
As an American, this sounds like exactly like a charter school system is. Or do you mean that the word is used differently outside America in other anglophone countries? From looking up the term online, it seems like it's mainly an American term.
I didn't want to editorialize, so I used the original title.
I changed "free school" to "charter school" which may make contextually more sense.
I hope that's acceptable.