OP, why on earth did you have a child with this man when the relationship was so terrible from the beginning?!
I can assure you, you have rights to your daughter and he cannot just take her away from you. He is saying that he can as another form of power over you, to continue to instil fear in to you so you remain subjective to him. He counts on your fear and ignorance of the legal system to abuse you, so honestly, well done for reaching out.
You can certainly seek help from charities, and I would advise that you do. His abuse has conditioned you in to thinking you deserve this and what he has done is not actually abuse - it most certainly is. How he is with his son is not your concern, your concern is how he is with you and your daughter.
I can assure you, most likely you will awarded full custody, but it's time you sought help from places like Women's Aid and the Citizen's Advice Bureau as they will be able to help you legally and also provide you with wealth of information and assistance overall.
You are not obligated to tolerate this behaviour, would you wish for your daughter to suffer silently like this in the future? No? Then do not accept it for yourself either, there is a far better life out there for you and it's waiting.
This man will continue to manipulate you and stop you in your tracks if he thinks you are going to leave, it's time to begin to prepare your escape plan by seeking help and assistance. If you can stay elsewhere safely for a while, I would suggest you do so and get a good support system around you with friends and family if possible.
I wish you the best OP, you have rights, do not let him for one second make you believe that you do not.
I volunteer with a charity in my spare time that helps get women back in to work, we provide clothes, shoes, bags and interview skills. When the time is right, feel free to get in touch, I can direct you to the charity so that you can seek their help in finding a job. However, first step is understanding your rights and planning your next move, work will come once you're more settled.