As a sole trader, you register for Self Assessment with HMRC. Tell HMRC by 5 October after the end of the tax year in which you start, or you could get a penalty.
In a partnership, the nominated partner registers the partnership for Self Assessment and each partner registers separately as an individual. The same 5 October deadline applies.
A limited company registers with Companies House before it trades. HMRC must then be told that the company has come within the charge to Corporation Tax no later than 3 months after its first accounting period begins, which is normally when it starts to do business.
Set up as a sole trader
Register for Self Assessment, choose a business name and keep records.
Set up a limited company
Choose a name, appoint directors and register with Companies House.
Register your company for Corporation Tax
When to tell HMRC your company has started to do business, and how.