Understanding the impact of not declaring a capital gain from a foreign property sale

Hello, I am trying to understand a situation that I have recently been made aware of. In 2017 I sold a freehold land in a foreign country and I made a profit. The country does not have any capital gains tax but I did not know that I needed to declare it my self-assessment in the UK in 2018. I need to declare it now but want to understand the impact in terms of penalties and late interest charges that the UK will charge me. This has me worried, any advice would be helpful. thanks in advance.

Hi aksche2024,
If you are in receipt of foreign  income or capital gains, you meet the criteria for completing a Self Assessment tax return.
Please have a look at the guidance here: 
Make a voluntary disclosure to HMRC Updated 30 January 2024
Make a disclosure using the Worldwide Disclosure Facility
There is a penalty and interet calculator at:
Calculate interest and penalties for tax years ending 5 April 2004 to 5 April 2023.
You may have to save the URL link and open Adobe acrobat reader, then open the file from within.
Thank you.