Foreign currency is an asset for Capital Gains Tax purposes.
The disposal of foreign currency may therefore give rise to chargeable gains or allowable losses.
Apart from the sale of foreign currency for sterling, there is also a disposal if foreign currency is used to purchase an asset.
Please see CG78300P, which includes the treatment of foreign currency bank accounts.
CG78300P - Capital Gains Manual: Chattels and other assets: Foreign currency: contents
One further point to note is that foreign currency is a fungible asset (fungible assets are ones which it is not possible to identify individually in a holding of more than one because they are functionally identical) and thus separate acquisitions of the same currency at different times, must be pooled in the same way as shares of the same class.
CG11820 - Shares and securities: share identification rules