Hi Finlay R,
The individual will need to review the statutory residence guidance at RDR3 and take the statutory residence tests:
RDR3 Statutory Residence Test
To confirm whether they are considered to be resident in the UK for the whole tax year.
If they are, then then need to look at the guidance on split year treatment, to determine if this applies as well.
If split year treatment applies, then they need to complete form SA109 with their tax return to claim split year treatment.
They do not declare their overseas income in the tax return, other than as a free hand note in the additonal information box.
If split year treatment does not apply, they need to complete SA103 to declare their US earnings and if tax is deducted in the USA, they also need to complete SA106, to declare the tax.
All figures in the tax return are in pounds sterling only.
Thank you.