How would you recommend writing a British girl? I know idioms and some specific words are important but my novel is not in english yet and I'd like to know if there are some distinguish things in the behavior I could write that could help the readers understand that my character is British (except stereotypes such as drinking tea of course)
Well, to start at the top, “British” generally refers to someone from Great Britain, though may also refer to someone from the British Isles. Great Britain has more than one country in it:
The first thing you’ll have to do is decide where she’ll actually be from. The second thing is that you’ll have to decide if the character can communicate effectively with the other characters. (By that, I mean “does she speak the same language”). Also, tea is a big part of their culture, so it would be really weird for her to not drink it.
After you decide those two things, all that’s left is research. I’ll give you some sources to look into, but because your ask wasn’t terribly specific, that’s all I can really do.
You can also find more with google using quick keyword searches. These were found using “habits British people have”. Once you decide where you want her to be from, you can do much more specific searches.
As for books, there’s always How to Speak Brit, which you should remember to check your local library for before deciding if you want to purchase.