If your contract and working practices give the impression that you are inside or caught by IR35 legislation then you will have to pay full tax and full National Insurance.
Instead of the usual salary and dividends from the profits of your company, on the money you withdraw from the company and not all of your expenses may be allowed as a deduction in the company accounts. This is because HM Revenue and Customs believes that as you not taking the financial risks or have the same level of control as a director of your own limited company, you aren’t entitled to the same corporate tax structure.