Hi,
Yes.
You will need to change all values to GBP Sterling, to work out the loss using UK rules.
To convert to Sterling, you need to use a just and reasonable exchange rate that would be in use at the time or acquistion or disposal.
You are free to use any of the HMRC exchange rates at
Exchange rates from HMRC in CSV and XML format or you can obtain from a source of your choosing,
such as the London stock exchange.
There is a calculator at
Tax when you sell shares to help work out the loss.
The disposal of the shares and the loss should be declared in a self assesment tax return, so that you can elect to carry the loss forward.
Thank you.