Have you dabbled in cryptoassets such as Bitcoin and Litecoin? Buying and selling amounts of any cryptoasset is treated just like dealing in shares or bonds, so if you make a large gain you need to declare that on your tax return.
A gain or loss is made whenever a cryptoasset is exchanged for another one, or given away, or surrendered for payment made in a legal currency, such as US dollars or pounds sterling.
Transactions in cryptoassets made by UK-resident taxpayers must be calculated and reported in sterling, which means both the buy and sell value has to be stated in sterling. Thus, the gain will incorporate an amount of exchange gain or loss as well as the change in value of the underlying asset.
Where the volume of transactions is very large, you may be considered to be trading in the cryptoassets, and your profits or losses should be subject to Income Tax. However, HMRC will resist that approach.
HMRC has written to cryptocurrency exchanges that do business in the UK, asking for customer data and transaction histories. If you have used a UK firm to acquire cryptocurrencies, HMRC may already have details of all your cryptoasset transactions.