HI
You are required to declare the disposal on a self assessment tax return. You should use a valuation that is just and reasonable. How you obtain this, is up to you. 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. For your convenience, there are exchange rates at
Exchange rates from HMRC in CSV and XML format and for older rates at
Exchange rates
You are free to use any of the supplied rates or one of your own choosing.