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.