-look at how the character speaks rather than how you think british people speak. if they call people "pet", use it. if they don't, don't. it sounds simple, but people often forget this.
-as she's a teenager, do a little research into the school system so you don't get her referring to her year as a grade.
-do research into margaret thatcher and miners because she'll likely knou ow all about that if she lives in a mining community.
-you don't need to know every corner of british culture, but if you don't know any tv shows outside of doctor who, it's worth looking up some trashy ones because teenagers love them.
-don't show off your knowledge if it's not relevant.
-also remember that even british people will understand a large amount of american lingo and culture (especially the sitcom friends because that's a staple of any british teenage diet).