I'm a freelancer who has taken legal advice on the issue of being freelance, and working for one company where they virtually dictate what you do is not freelance, never has been and never will be.
I set out some of the IR35 criteria right at the beginning of this thread and that applies, if you control what you do/when you do it/how much you charge, then you are probably freelance, cleary stated on the HMRC website although in different language.
Working for one company on a contract that is not controlled by you is not freelance, and could result in a fine and unpaid ni for the employer and large fines plus unpaid tax/ni for the freelancer/employee.
Best advice is to seek the guidance of an accountant, before going freelance, and continuing to take their counsel.