Children Act, Human Rights Act and MHA

Hello so i have recently heard about a situation were a 14 year old informal patient requesting to self discharge (who was told they did not meet the criteria for s5(4) was told they could be keept under s25 of the children’s act. Is this right?

Can a general hospital physically prevent an informal mental health patient from leaving under duty of care as they can’t use s5(4) and have no MHA options?

Is it legal to detain a person to a general (physical) hospital when there are no such physical needs to require hospital, in cases where a suitable mental health bed is unavailable in the long term (months)?