Companies House is the government department that is responsible for the legal incorporation/registration of limited companies in the UK. Anyone who wants to set up a limited company has to send an application to Companies House, either directly or through a company formation agent - you cannot set up a company without their approval; they are the gatekeepers of limited company formation and compliance!
Once your company has been registered, regular reports about business activities and income must be sent to Companies House. This information is used to monitor your company and ensure you are adhering to all legal obligations. Companies House makes all of this corporate information available to the public.
Her Majesty’s Revenue and Customs (HMRC), previously known as Inland Revenue, is the ‘tax man’. They collect tax and National Insurance from individuals who live and work in the UK, as well as all businesses that operate in the UK. HMRC is completely separate from Companies House, but you will have to send them financial accounts and tax returns every year and pay your business taxes to them.