Transfers between different foreign currencies in a UK bank account

I would like to know, please, whether, capital gains tax is payable when I (an individual taxpayer, not a business) switch from one foreign currency to another, both currencies held at a UK bank, or only when either of those foreign currencies is sold for sterling.

Hi,
Currency other than sterling is a chargeable asset and its disposal can give rise to a chargeable gain or an allowable loss:
Capital Gains Manual
A gain on the disposal of foreign currency acquired by individuals for the personal expenditure outside the United Kingdom of themselves and their family or dependents is not a chargeable gain.
This includes expenditure on the provision or maintenance of a residence outside the United Kingdom:
CG78315 - Foreign currency: personal expenditure of individuals
Thank you.