Sole trader is the simplest form of business to start where you carry on business on your own account. You are liable to income tax and Class 4 National Insurance on your profits. You can employ people including your spouse for work done.
Your business format is not set in stone forever and you can change between them. It is fairly simple for a sole trader to take on a partner and become a partnership and for a partnership to become a Limited Company. There are however more complications with changing from a Limited Company to a sole trader or partnership.