Self assessment tax return deadlines
By the 31st of January you must file your online tax return for the previous tax year and pay any tax due. Miss this by just one day and you're already looking at a £100 fine. You should get yourself registered as self-employed as soon as possible after you start trading. The deadline for getting yourself registered is the 5th of October in the year that you started your self-employment. Miss that, and you're risking penalties based on the potential lost tax.