Hi,
If you are resident in the UK and not domiciled, then you can elect to claim the remittance basis. This needs to be done via a self assessment tax return, where you will declare the amount of unremitted income (box 34 of SA109) and capital gains (box 35 of SA109). You will also need to provide a breakdown of the unremitted income and capital gains (box 40 of SA109). You cannot use the online HMRC tax return to claim the remittance basis, although you can purchase commercial software to allow an online submission, using your government gateway user ID and password. As long as you do not remit the income or capital gains in a future tax year, it will not be taxed. Have a look at the guidance on the remittance basis at section 9 of RDR1:
Residence, domicile and the remittance basis: RDR1
Thank you.