Hi Emil and Ewa Kowalski
Private residence relief will cover the property acquired in 1980, as it is your principle private residence.
Only once you have disposed of this property can you claim that the other property is your principle private residence from that date.
If you late dispose of the property acquired in 1990, then you may have capital gains tax arising on gains from 1990 to the date it became your principle private residence.
Please see
HS283 Private Residence Relief (2024) Updated 6 April 2024
Thank you