Hi,
If you are not UK resident and dispose of a UK property, you use UK Capital Gains rules to calculate any gain.
Using UK rules allows for private residence relief for periods where the property was you main residence.
If a gain arises, you have 60 days to report and pay the Capital Gain.
There is guidance and a calculator at:
Tax when you sell property: What you pay it on - GOV.UK to work out your gain.
The links after the calculator allow you to register for a Capital Gains account, where you can report and pay the tax due.
As a non UK resident you are required to declare the disposal, even if there is no gain.
Thank you.