Hi,
If you have two or more properties, you are required to inform HMRC which property is your main residence and the date this takes effect from.
Private Residence Relief is worked out using months as the basis of the calcultion. You can see guidance here:
HS283 Private Residence Relief (2024)
You can find a capital gains tax calculator here:
Tax when you sell property
From the link above you can register for a capital gains account and report and pay Capital Gains Tax within 60 days of the completion date.
Thank you.