Hi,
In the UK, by default, income and Capital Gains Tax is calculated using the 'arising basis' on world-wide income and gains. Some individuals, can elect to use the remittance basis, to declare income that is not remitted to the UK and so is not included for tax calculation purposes. Unremitted income, must be declared on a Self Assessment Tax Return in the tax year that the income arises (SA109) as it will be taxable in a later tax year if remitted to the UK in that later tax year.
Have a look at section 9 below for more information.
Residence, domicile and the remittance basis: RDR1
Thank you.