Hi,
The gain would be reported in a Self Assessment tax return, in the foreign section.
Under the terms of Self Assessment, HMRC does not provide an official exchange rate and the onus is on the individual to use a just and reasonable exchange rate at the time of the gain, You would need to convert the gain to pounds sterling. HMRC does have a record of exchange rates HERE:
Exchange rates from HMRC in CSV and XML format.
You can use any of the rates it provides or you can use exchange rates from other sources, for example, the London stock exchange or those rates reported in newspapers.
Thank you.