HMRC cannot advise on hypothetical questions.
If you wish to claim split year, this must be done in the tax return and you will then declare income received since arrival in the UK. The guidance at RDRM12150 will help you work out if split year treatment applies.
RDRM12000 - Residence: The SRT: Split year treatment: Contents
If you qualify for split year then you only report any foreign income for the UK part of the year
If you do not qualify then you will need to report all your foreign income to the UK.
Tax on foreign income