CG15800 advises that there are some exceptions to the general rule that losses cannot be carried back.
These include when there are losses of the tax year in which individual dies that cannot be deducted from gains of the year (TCGA92/S62(2)), see CG30430 onwards.
A terminal loss claimed on the termination or expiry of a mineral lease may be carried back and deducted from certain chargeable gains accruing in earlier years (TCGA92/S202(7)), see CG71751
Subject to conditions, an election may be made for a loss accruing on a disposal of certain rights to deferred unascertainable consideration to be deducted from chargeable gains accruing in previous years (TCGA92/S279A(1)), see CG15080+.
CG15800 - Losses: allowable losses
Based on this guidance, you are unable to carry back the losses.