Don't try to hard to persuade her. Let her come and see, if she wants to stay, great. If however, she wants to return to UK and follow her dreams of becoming a police or whatever, let her, its her right...she is already mature enough to know what she wants, even if her decisions are bound to change in the future or the very fact that she is most likely to make many mistakes.

By all means try to keep her with you, but it is unfair if she has to give up her future or career choices for the sake of it.