Cost base for foreign shares acquired over several years in same company

Hi, I've purchased shares over several years within the same company (Australian) and am now wanting to sell. Because the shares were bought at different times, do I use an average of the annual HMRC foreign exchange rates to work out the cost base? The same way you work out the average cost of the shares by dividing total cost of shares by total number of shares? E.g. In 2020 I acquired 300 shares - the HMRC AUD to GBP average exchange rate for 2020 is 0.5339 AUD. In 2021 I acquired another 200 shares - the HMRC AUD to GBP average exchange rate for 2021 0.5474 AUD. When I calculate the cost base in GBP for CGT purposes, should the exchange rate for purchase be 0.5407 (the average of the two above?)

Hi,

You can see guidance here:

HS284 Shares and Capital Gains Tax (2023)

Thank you.