By default you are not self-employed when you work for agencies as a supply teacher - but you can do it as self-employed and you will come out of it much better because you can deduct expenses. I have done it this way for 2 years now. Tell the agencies that you are self-employed and ask them not to deduct any tax, NI etc. They might ask you to draw up and sign a declaration saying you will be responsible for your own tax and NI and you release the agency from any obligation re. this. They might also ask you for some proof that you are self-employed, e.g. your UTC. After that just invoice them and they will pay you the full amount, e.g. if you agreed £120/day, you will get the full sum. Most agencies will have had some experience of this by now because they often also send sports coaches and dance instructors and the like to schools and these people tend to be self-employed, too. Another advantage is that if you are registered with several agencies but not as self-employed, they will all deduct tax and NI randomly, since they won't know about each other. Before I went self-employed I had to have a round each year with HMRC, who did not understand why 4-5 companies deducted tax from me, sometimes two on the same day (am and pm in two schools via two agencies) and there were always over/underpayments. Once you are self-employed, you coordinate everything: you get the full amount from each agency and do your tax return online and there are no mixups.