CG13130 - Introduction and computation: occasions of charge: assets lost/destroyed/negligible value: date of deemed disposal/reacquisition
The decision in  Williams (H.M. Inspector of Taxes) v Bullivant [1983] established the general rule that the date of the deemed disposal and reacquisition is the date of the claim. Any resulting loss thus arises at the date of the claim. The date of the claim is the date on which the claim is received by HMRC.
explains the timing of the disposal and reacquisition of the asset and the conditions required to claim an earlier time.