Hi Swisschalet,
Mortgage costs are not a factor in calculating capital gains tax, how the property is purchased has no relevance.
The acquisition price is the price agreed at the time the contracts were signed.
All parts of the capital gains calculation must be in pounds sterling.
You can use the official exchange rate that applied on the day of acquisition, this would be found in most national newspapers.
Alternatively, you can use the official exchange rates at
Exchange rates from HMRC in CSV and XML format.
You will have the choice of using the monthly, annual and spot rate.
Archived exchange rates can be found at
Exchange rates.
There is a calculator at
Tax when you sell property, which will help you work out the gain.
Any gain will need to be reported in a self assessment tax return.
Thank you."