Hi
There is no requirement to report losses to HMRC.
If you want to claim for losses and carry the losses forward to a future tax year, you would declare the losses in the capital gains section of the online tax return or SA108 paper supplementary page.
In the relevant shares section, you would show the disposal value, your allowable costs and the resulting the loss.
In the losses and adjustment section, you would declare that you wish to carry the losses forward.
Capital Gains summary (2024).
Capital Gains Summary notes
Thank you