Hi andy1,
Although HMRC provide exchange rates at:
Exchange rates from HMRC in CSV and XML format
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 can use those exchange rates, the closing rate of the London stock exchange on the day of the transaction, or from any other source, as long as it is a just and reasonable exchange rate for each acquisition and disposal.
Thank you.