Your contract will often state whether you are an employee, worker or self-employed. However, this is not the only factor to consider. The label applied to the relationship may not be legally accurate. The substance of the relationship in practice is more important in determining your status. This is what a tribunal will look at if you make a claim to the employment tribunal in which your status is crucial.
Each case will depend on its own facts and circumstances but the tribunal might look at:
the relative bargaining power of the parties when entering the contract
whether you can choose to work (or not) and when to work
whether you can send a substitute to do the work
whether you are in business on your own account
the level of control your employer has over your work (and how and when it is performed)
whether you do, or can, work for someone else
provision of a uniform or equipment by your employer
who calculates pay
notice required for time off
whose responsibility it is to pay tax and National Insurance contributions