Self-employed people can choose not to. Termination of services. If you can be sacked you are probably an employee. A contract may be terminated because you are in breach of it, or your work is not up to standard. Self-employed people present invoices for payment and have terms and conditions of sale.
Employees expect a regular wage slip.
However, some self-employed people are paid retainers, expenses or stage payments that could look suspiciously like a salary. If you are entitled to statutory sick pay and statutory maternity pay, you are an employee. You are more likely to be regarded as self-employed if you take financial risks, take responsibility for investment and management, employ other people and can benefit from careful management of a project. If you use your own tools and equipment, you are more likely to be a freelance. If you disagree with your status.