Hi,
Yes, you can. If you do not complete Self Assessment tax returns, then your loss claim must be in writing and include evidence to support your loss claim. If you complete a tax return, you will need to amend the tax return for the tax year that the loss arises, including supporting evidence for the loss.
The conversion to pounds sterling use a just and reasonable exchange rate for the time of acquisition and disposal. Allowable costs include acquisition and disposal costs and certain improvements. You can see guidance here:
CG15180 - Expenditure: enhancement expenditure
Thank you.