Hi limbo24,
If you choose to use the remittance basis, to remove your Singapore income from UK tax calculations, then you only need to declare that income which is not being remitted in your tax return in boxes 28 to 40.
If split year treatment applies, you would only be declaring that Singapore income that arises from 21 Nov 2022, when you arrived in the UK.
You can claim a tax credit for any Singapore tax paid.
Please take a look at the guidance on remittance basis at:
Guidance note for residence, domicile and the remittance basis: RDR1 Updated 25 April 2023
Thank you.