Hi,
To take up employment in the UK, you will require a National Insurance number. This is something you can apply for here:
Apply for a National Insurance number
If you commence an employment, you will need to advise your employer that you do not have a P45. Your employer should ask you to complete a starter checklist. This will help determine the corrct tax code to apply. UK Income Tax will be payable on your worldwide income from the date you arrive in the UK, unless you apply the remittance basis, then you would only be taxable on your UK income, but would not have a tax free allowance.
Any money transferred to the UK, that was earned while you were not resident in the UK, would not be taxable. Any interest that it generates would be taxable. The rules for the remittance basis will change from 6 April 2025. Guidance on those changes is still being written at this time.
There is a tax free threshold for savings interest. The amount of this interest that is tax free, depends on the level of your income. You can see guidance here:
https://www.gov.uk/apply-tax-free-interest-on-savings
Thank you.