Would I be most tax-efficient trading as a sole trader or a limited company? 

There is no straightforward answer to this question.  It depends on your circumstances; how much profit are you generating?  Do you have any other income?  From that information, we can complete a tax comparison that shows which entity would be most tax-efficient for you. 

There are many more things to consider with running a limited company, for example, the money in the company is the company’s and not yours so any money withdrawn from the company, you as the director, will be liable for tax, another thing to consider is there are more submissions required each year such as a confirmation statement, accounts to Companies House, a company tax return to HMRC and you would need a registered office.  It isn’t just an income tax return being submitted once a year to HMRC.