It's determined based on several factors like whether you set your own schedule, whether you use your own equipment, whether you have a manager, whether you hav

It's determined based on several factors like whether you set your own schedule, whether you use your own equipment, whether you have a manager, whether you have autonomy to complete your work how you see fit. There are no hard-and-fast rules though, and from my experiences, people completely ignore it (in software contracting). See https://www.gov.uk/employment-status/selfemployed-contractor https://www.gov.uk/employment-status/selfemployed-contractor