Hi ACTS,
HMRC considers foreign currency to be an asset and any gain from the exchange of currency is chargeable to capital gains.
If your total gain exceeds the annual exempt allowance, you have until 31 December after the end of the tax year to declare the gain using the realtime transaction service at
Report and pay your Capital Gains Tax.
Reporting after 31 December must be in a self assessment tax return, which should be in the possession of HMRC before 31 January, when any outstanding tax should also be paid.
Thank you.