Capital gain in foreign account

I have a question: I move to UK in Nov 2022 on BNO VISA. I still have my bank account in Hong Kong and I report all my bank interest incurr in my Hong Kong bank account. My bank account also have some Hong Kong stocks which I kept for a very long time (more than 20 years) and I never sell any of those stocks/shares since I move to UK (so there is no capital gain). My big question is: I don't have the record of how much $ I bought 20 years ago. Then in case if I need to sell my stock any time in future, how can I report on my self assessment? Which figure I can refer if I really need to sell?

Hi,
Please have a look at the guidance at CG59510: (CG59510 - Shares and Securities: Valuation of shares and securities: Valuation of quoted shares: definition of market value) CG59514: (CG59514 - Valuation: quoted shares: further points) for quoted shares and CG59540 for unquoted shares:
(CG59540 - Shares and Securities: Valuation of shares and securities: Valuation of unquoted shares: Valuation: unquoted shares: market value rule).
Thank you.