Losses accruing on the disposal of an asset do not usually count as ‘allowable losses’, and hence may not be deducted from chargeable gains, unless they have been notified to HMRC in a quantified amount.
TCGA92/S16(2A) states a capital loss will be allowable only if it is notified within the normal time limit for claims, see TMA70/Section 43(1). The notice of loss to be an allowable loss is treated as if it were a claim for relief.
There is no specific claim form. In practise, the notice may be the inclusion of details of the loss within a person's tax return and supporting computations of gains and losses.