If you are claiming split year, then you only report income subject to tax in the UK part of the year. In general terms, income is declared when it is received therefore meaning you would report employment from abroad if it is paid in the UK part of the year, despite it being due for an earlier period.
Please see further guidance regarding employent income and residency:
EIM40001 - The scope of the charge to tax on general earnings
RDRM10245 - Residence: Coming to the United Kingdom