You might wish to think twice about becoming self-employed in March. If you have any trading activity, then there will be a requirement to file an income tax return for the year ended 5 April 2018. If you delay starting until 6 April, then your first tax return will be for the next year ended 5 April 2019.
Talk it over with an accountant if you are not sure. Perhaps there is a good reason to start in March after all, but make sure that you have some clear ideas.
This advice applies to sole traders and partnerships. It does not apply to companies which start in March which are taxed under a different system.