PAYE is basically your tax to the HMRC, NI is for your pension when you're older and to cover costs to the NHS if you get sick. If you are a sole trader these will normally be included on your self-assessment. You don’t pay through self-assessment if you’re one of the following:
- an examiner, moderator, invigilator or person who set exam questions.
- running a business’s involving land or property.
- a minister of religion who does not receive a salary or stipend.
- living abroad and paying voluntary Class 2 contributions.
- a person who makes investments - but not as a business and without getting a fee or commission.
- a non-UK resident who is self-employed in the UK.
- working abroad.