If you are not resident in the UK, but have UK savings and investments (such as interest or alternative finance receipts from banks or building societies, unit trusts, National Savings and Investments, or dividends from UK companies), the income is taxable in the UK.
HS300 provides information on how this is done and includes a working sheet to assist in calculating the tax.
Non-residents savings and investment income (Self Assessment helpsheet HS300)
If you are not resident in the UK, then you are not taxable in the UK, on your non UK income or gains.
If there is a double taxation agreement between your country of residence and the UK, then you may find that salaries, wages and other remunerations, as well as pensions including state pension arising in the UK, are not taxable in the UK, but are taxable in your country of residence. In all double taxation agreements the UK has with other countries, income and gains arising from property in the UK, are taxable in the UK. A tax return must be completed every year.