You should ask yourself, what was the main purpose for making the currency change?
You were going on holiday or visiting another country and you changed some money into their local currency. When travelling back home, the exchange rates had favourably changed. You end up with more money equivalent in comparison to the rates originally.
You may not pay tax on this gain, as there was no intention to make a gain on the purchase of the currency.
You noticed there was political uncertainty with a country and they have their own currency. You purchase some of the currency in the view of making a gain if the exchange rates were to change.
You may be liable to make a gain entry on your Tax Return under the SA108 Capital gains page. You should choose 'Other property' as the category to report these gains.
You have a currency denominated bank account containing an amount of money say in 2021. You are withdrawing the money from the account and you make a gain.
Whilst the above guide should be helpful, it is not considered advice and we recommend you contact HMRC if you have any queries that you aren't sure on.