If you contribute to an ISA whilst non-UK resident, you are in breach of the ISA rules and should notify your ISA manager, as you are likely to have made a declaration confirming that you were resident at the time of the contribution.
Any contributions made during a non-resident period will be considered ‘void’ and the status of this element of your account should be retrospectively changed meaning that the interest realised would be fully liable to UK tax, subject to the Personal Savings Allowance, personal allowance or any relief available under any applicable double tax treaty.
In the overseas part of the tax year (i.e. after the date when you have ‘split’ the tax year), you will remain liable to income tax in the UK on your UK investment income, which will be taxable domestically at your marginal tax rate, subject to the Personal Savings and Dividend Allowances.
If you are resident in another country, the provisions of the double tax treaty between the UK and your country of residence may potentially restrict the UK tax due.
HMRC’s country-by-country guidance on the UK treaties can be found here.
Where your property is held jointly the share of your rental income is usually split equally between you and your spouse.
However, if in fact your spouse is beneficially entitled to a higher portion of the income then you can make a joint election for the income to be split to reflect your spouse’s actual beneficial entitlement to the rental income. You would do this by completing HMRC Form 17, which can be found here.
You will need to first ensure that your spouse’s beneficial interest in the property does support this split - for example, by completing a declaration of trust to this effect.