A worker's employment status, that is whether they are employed or self-employed, is not a matter of choice. Whether someone is employed or self-employed depends upon the terms and conditions of the relevant engagement. The tax and National Insurance contributions (NICs) rules do, however, contain some special rules that apply to certain categories of worker in certain circumstances. See section on special cases.
If you work for someone else, it is important to know whether you are working for that person in an employed capacity or in a self-employed capacity as an independent contractor. If you are an engager, it is your responsibility to correctly determine the employment status of your workers.
A worker's employment status will determine the charge to tax on income from that employment or self-employment. It will also determine the class of NICs, which are to be paid.