Hi Yin Yin Lee,
Self-assessment is a way of reporting your income and paying tax to HM Revenue and Customs. Self employment is only a part of Self Assessment.
You will need to review the statutory residence tests to determine if you are taxable in the UK for the full tax year.
If you are, then you would need to declare all of your foreign income on a self assessment tax return, including any foreign tax paid.
If you are deemed not resident for tax purposes for the full tax year, you would claim split year treatment and declare only that income arising from the date you arrived in the UK.
The statutory test can be found at RDR3 Statutory Residence Test