Capital gains on property passed by spouse

My dad passed in 2018. Since 2022 we have been going through the Polish courts to get a property that he owned a share in (50%) passed on to his family (my mother and siblings) and then sell that share. This is now complete. We are wondering whether there is capital gains tax due. The Polish courts have decided that the proceeds of the sale must all transfer to my mum, so she is the sole benefactor. If capital gains tax is due, as this is a sale of 2nd property (my mum owns a house in the UK). At what point is it due, any value increase since my dad died in 2018 and ultimately the property passed to my mum? Since 2024 when the Polish courts actually decided the property belongs to her? Since my dad owned his shares (1995)? If it's in 2018 for instance, how would we determine value increase if property was not valued at the time? Thank you

Hi,
It is based on increase in value since your father passed in 2018.
Thankyou.