I’m absolutely sure there’s no compulsion to be a limited company. Indeed, it may well be simpler to be a sole trader and many locums work on that basis.
I work through a limited company because there are slight tax advantages – it reduces my National Insurance liability. With recent changes in the tax on dividends the advantage of doing this (factoring in accountancy fees etc) is now only slight. Also be aware that HMRC don’t particularly like this way of working and depending on your IR35 interpretation (Google it…. It’s a can of worms) many people in my position are looking over their shoulder and wondering when we will be forced to change.
I suggest you find an account you trust and get proper professional advice. It’s worth it.