Hi,
Your overseas gains calculation should be completed using pounds sterling.
You will need to convert the purchase price and costs to pounds sterling, using the exchange rate at the time of purchase. You will also need to convert the disposal value and costs to pounds sterling, as well as any foreign tax paid. The official exchange rates can be found here:
Exchange rates from HMRC in CSV and XML format
The foreign gain is added to the UK gain and declared on SA108.
In SA106, you would complete page F6, box 33 and boxes 37 to 40. Do not include these amounts in box 2 on page F1. You can then calculate the Foreign Tax Credit Relief and enter in box 2.
Thank you.