Hi,
If a tax return is required, you would declare the losses in the capital gain section of the online return or SA108 if on paper (
Self Assessment tax return formss).
You will need to attach supporting evidence of the losses.
If a tax return is not required you can report the losses in writing and include supporting evidence.
You cannot claim losses that you have not declared to HMRC.
The losses can be carried forward and set against a future gain.
Thank you.