exchange rate

I bought a US stock (in US dollars) on 10 March 2021 and sold out (in US dollars) on 20 Dec 2023. I need to exchange the purchase price and disposal price to pounds to calculate the capital gain. I want to ask can I use the following currency exchange rates (from HRMC websites) to calculate: a) To use Average exchange rate (2020 Dec) to calculate purchase price on 10 March 2021 b) To use Spot exchange rate (2023 Dec) to calculate disposal price on 20 Dec 2023

Hi,
As a UK resident you are taxed on the arising basis on your worldwide income and capital gains. If you dispose of foreign assets, you are required to report this in a Self Assessment tax return.  
The calculation for capital gains is in pounds sterling at all stages. Under the terms of Self Assessment, we do not provide an official exchange rate and the onus is on the individual to use a just and reasonable exchange rate for each acquisition and disposal. For your convenience, there are exchange rates here:
Exchange rates from HMRC in CSV and XML format
and for older rates here:
Foreign exchange rates and spot rates: 1 January 1989 to 31 March 2009
You are free to use any of the supplied rates or one of your own choosing. The gains are declared on SA108 and if foreign tax is paid on the gain, you also declare on SA106 and can choose to claim a foreign tax credit of up to 100% of the foreign tax paid. This will prevent you being taxed twice.
Thank you.