Open Question: Can a self employed work in a hotel,pub,restaurant ?

Hello,I am a Romanian and worked as a self-employed in big hotel in Uk,and I am asking how come I worked there as"self employed" , is something that they know and I don't? Worked in the same place for the same hotel for a few months I had a minimum of 40 hours/week 5.8 hour and overtimes I was answering to a manager I had to pay NIN contributions and TAX Signed an agreement not a contract Didn't have payed holly day Thank you