Exchange rate calculation on overseas property sale

I am in the process of selling a property in France for less Euros than when I bought it 18 years ago. As such, having had the capital gains assessed in France, I do not have any to pay there. However, taking into account Euro/GBP exchange rates, for the years of buying and selling the house, it is possible that I may need to pay CG tax in the UK. The amount will depend on what exchange rate I use. Do I have to find the Bank of England spot rates on both the day of purchase and the day of sale, can I use the exchange rate I got when I changed the purchase monies or is another rate applicable? It is far from clear on the HMRC as to what exchange rate to use.

Hi cjp,
You can use any of the rates in the tax year applicable to when bought and sold. HMRC rates are at HMRC currency exchange average rates - GOV.UK (trade-tariff.service.gov.uk)  and Exchange rates from HMRC in CSV and XML format.
Thank you.

Hi HRMC team,
(1) Sold my property in Hong Kong in August 2023 (overseas property, in HK dollars). Later on, the money was converted to GBP, and transferred to the UK at different time intervals from September to December 2023. - For exchange rate, am I free to choose between the monthly rate, average rate or spot rate applicable in the tax year to calculate the selling price in GBP?
(2) The same property was brought in February 2013 in HK dollars. - When calculating the cost of ownership, am I also free to choose the monthly rate, average rate or spot rate applicable in the 2013 tax year to be the exchange rate?
Thanks and regards, Maggie

Hi,
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. 
You are free to use any of the three rate HMRC provides, or you can use a rate obtained elsewhere, such as a newspaper or the London Stock Exchange.  
For your convenience, there are exchange rates at: 
Exchange rates from HMRC in CSV and XML format
For older rates at: 
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.
Thank you.