If claiming split year treatment, you need to register for self assessment and complete the residence section of the return.
If you qualify for split year then you only report any foreign income for the UK part of the year RDRM12000 - Residence: The SRT: Split year treatment:
If you do not qualify then you will need to report all your foreign income to the UK Tax on foreign income
The guidance at RDRM12150 - Residence: The SRT: Split year treatment: Case 4: Starting to have a home in the UK only
will help you work out if split year treatment applies.