What is involved with legally changing residence or custody in the UK if a teenager has recently joined her mum Can the teen possibly stay with her mum?

In the UK, legally changing residence or custody involves applying to the court for a Child Arrangement Order, which outlines where the child will live. If a teenager has moved in with her mum and both parents agree, they can informally arrange for her to stay, but it's advisable to formalize the arrangement to avoid future disputes. If the father contests the arrangement, the court will consider the teenager's wishes, especially if she is of sufficient age and maturity. Ultimately, the court's decision will prioritize the child's best interests.