Hi,
You add up all your gains in the year and deduct the losses. If a gain still arises, this is declared.
There is no requirement to report losses to HMRC, unless you want to apply those losses in some way. If you complete a tax return, you would claim the losses in the capital gains section, along with the action to be taken.
If you do not need to complete a tax return, then you declare the same, but in a letter, declaring the action to be taken. In both cases, you will need to provide supporting evdience of the losses.
You should note that loss relief on deeply discounted securities was largely abolished on 27 March 2023, as explained in the guidance below. There is an exception to this rule for government securities and there is also a special rule that relates to securities held before the above date.
SAIM3080 - Deeply discounted securities: taxation: losses
Thank you.