Where to find the Official Foreign Rates of Exchange

I need to convert my dollar pension into sterling for my taxes. I am having problems finding this, I found a rate for August on FCDO consular services but no way of finding the rates for previous months I keep being taken to the August page. Is this the only way I can find the official rates.

Hi Denise Lada,
Our rates are here:
Exchange rates from HMRC in CSV and XML format.
Thank you. 

Thank you so much.