Hi soton123,
If you leave the UK, you would need to review the residence tests again, to determine if split year treatment applies.
If it doesn't then the overseas income earned when you are not resident, would not be taxable in the UK.
If you were considered resident for the full tax year and split year treatment does not apply, you would need to declare your overseas income and tax deducted on a self assessment tax return.
If you move overseas, you will need to complete P85 if you lived and worked in the UK and have left the UK and may not be coming back or you’re going to work abroad full-time for at least 1 full tax year and you have not been asked to complete a self assessment tax return in the tax year you leave the UK.
Get your Income Tax right if you're leaving the UK
Thank you.