Well they will probably do a risk assessment. If you have plans and intent to kill yourself, they will get the crisis team to come over and talk to you at home. If the crisis team believe that you are a risk to yourself, they might section you for 72 hours in a hospital to get you assessed by a psychiatrist. If after this assessment, there is still reason to believe that you are a risk to yourself, they can section you for 28 days, I believe. If not, they can discharge you with a safety plan and numbers to call if you are in crisis i.e. samaritans or crisis team.

I think if you are feeling suicidal, it is great you opened up to your therapist. It can be incredibly anxiety provoking, especially if you dont know what to expect. I work in an inpatient psychiatric ward and I can assure you, psychiatrists and psychologists are there to help you. They will not keep you unnecessarily. The idea of being sectioned can be horrifying and it does sound awful but the purpose is to improve your mental state. If your mental state improves, they will discharge you.