You don't have to go Ltd no, and if you did you wouldn't be "self-employed" in the technical sense, you would be an employee/director of the business.
There would be a lot more set up costs with going Ltd, plus accountants fees for auditing and filing. It is a lot simpler to start as a sole trader (self employed) as it will cost nothing. But dependant on your turnover/profit expectations there is a difference between Ltd and self employed on your tax liabilities.
Basically, if it's small amounts to start with, just go as a sole trader, as soon as it picks up and gets serious then I would go Ltd.