Just point out that "releasing five albums of songs on her own label" means she puts them together in her bedroom (or perhaps the school's music room as class projects) and gives them to mates (or nobody), not that she's a well established pop star.
In any case, there is no rule that the BGT audition needs to be the first time you've set foot on stage in front of an audience, most acts will have some experience (such as the comedian doing the holiday park circuit for e.g.)
