You would need to check if the trading is classed as a business or just personal gains.
If it is a business then this would be reported as self-employment income. If this is for personal gains then this would be reported for Capital Gains Tax.
You can find further guidance to check if you are trading or not here:
BIM56840 - Financial traders - instruments and shares: the badges of trade
BIM20200 - Meaning of trade: badges of trade: contents