Hi L,
No matter how much or little your overseas income / gains are, you must declare them in a Self Assessment tax return, even if you do not have to pay UK tax on the overseas income / gains.
If yu are UK resident for less than 16 years, but not Uk domiciled, you may qualify for 'Exemption for small amounts of foreign income'.
Have a look at section 9 of Exemption for small amounts of foreign income, to see if you meet the criteria.
If you need to complete a tax return, you can download a paper version, including supplementary pages at:
Self Assessment tax return forms
Alternatively, if you are able to register for a person tax account, you will be able to complete your tax return online.
Personal tax account: sign in or set up
Thank you.