The remittance basis and the increase in tax charge - section 91
Because a foreign chargeable gain is only liable to Capital Gains Tax in the year it is remitted to the UK the rate of Capital Gains Tax for that year applies; not the year in which the section 87 gain accrues through the matching process. But the year in which the section 87 gain accrues through the matching process is used for the purposes of calculating any tax due under TCGA92/S91. See CG38800.
If the taxpayer has unremitted foreign chargeable gains from a number of years it will be necessary to identify from which year a remittance is made. Because TCGA92/S87B treats section 87 gains as foreign chargeable gains the mixed funds rules in ITA2007/S809Q apply if the chargeable gain includes gains from more than one year, s809Q(6). You use the last-in first-out basis in s809Q(3) to identify which part of the foreign chargeable gains is remitted first. See CG38830 for a remittance from a fund that includes only gains and CG38835 for a remittance from a fund that includes income and gains.