Unfortunately, we are unable to answer your questions, as the answer will depend on your personal circumstances.
Please find the guidance at RDR3:
RDR3 Statutory Residence Test
and take the residency tests.
If you are classed as resident and split year treatment applies, you would not need to declare your foreign employment prior to moving to the UK.
If you are UK resident and split year treatment does not apply, you will need to declare the foreign employment. You will be able to claim a foreign tax credit relief if you paid foreign tax.