If you're involved in an investment business and you're operating it as a trade, profession or vocation, HMRC class you as gainfully self-employed for Class 2/4 National Insurance contributions (NICs) and Income tax purposes. Don't tick the 'Investment' box as you'll be registered for National Insurance and Self Assessment as a self-employed earner.
Only tick the 'Investment' box if you aren't operating as a trade, profession or vocation. HMRC will send you a bill once a year so you can pay Class 2 NICs voluntarily if you wish.
HMRC will register you for Self Assessment. When filling in your return you must fill in additional sections known as supplementary pages.
Follow the link below for more information about the supplementary pages. https://www.gov.uk/self-assessment-forms-and-helpsheets