If you’re self-employed or running a small business, you’ll need to send a report of your annual earnings to HMRC. It includes the sources of your earnings plus, it is your responsibility to calculate the tax to be paid – so, no wonder why it is called self-assessment tax return.
HM Revenue & Customs (HMRC) may give assistance in calculating tax but wouldn’t guide you in organizing your affairs by minimizing your tax. So if you want to maximize revenue and minimize tax paid you must consult a professional.