Any income or gains held by a non-domicile prior to his arrival in the UK are regarded as capital, and these funds should be segregated from subsequent income, or gains, so that this capital may be brought into the UK without any tax charge arising here.
The original capital may be increased after arrival in the UK, and this could arise from various sources, including:
Income or gains taxed in the UK and subsequently taken offshore.
The distribution of capital gains arising before 5 April 2008 from an offshore Trust, or possibly the distribution of a gain arising to an offshore Trust where the trustees have elected for the April 2008 rebasing provisions, and as a result no taxable gain arises for UK tax purposes.
Gifts, or an inheritance from say a relative.
Funds arising from an exempt gain, for instance the sale of the overseas home within 9 months of it ceasing to be used as the individual’s Principal Private Residence. (18 months for disposals before 6 April 2020)