Hi,
As you have a foreign source of income, it does not matter what it is, you need to declare it in a Self Assessment tax return, SA100, in pounds sterling. There is a section in SA106 for dividends.
Self Assessment: Foreign (SA106) EnglishCymraeg
If you have had to pay tax in the foreign country on the dividends, you can claim a percentage as a foreign tax credit, if there is a tax treaty with the UK and the other country. You can see the guidance below to find out how much relief is available.
Tax treaties
Thank you.