Hi, I'd like to help you with your question.
In the UK, a person can be detained if they pose a threat to themselves or someone else or they show marked deterioration in self care.
An approved mental health therapist can perform an evaluation to determine need for inpatient treatment. It then needs to be approved by up to two physicians before admittance can occur. As in the U.S., there is a limit to how long you can be detained before a hearing must be held. At the hearing, it would be determined if a longer stay is needed for care.
If you feel your father does not have grounds to commit you (you are not a danger to yourself- threatening suicide for example, or a threat to anyone else) then there is nothing he can do. He could file to have you committed, but he would have to say you are a threat then the mental health professional would need to feel there is enough evidence to enforce the commitment. Then the doctors would have to agree as well.
You may want to offer to your father that you will explore outpatient therapy if he feels strongly that you need help. That way, you are addressing his concerns but avoiding unnecessary attempts at commitment. However, if you do feel like you may hurt yourself or someone else, a stay in the hospital might help you. But in that case, you could admit yourself voluntarily and have more control over your situation.
I hope this helps,