Possibly, tax obligations are explained here

https://www.gov.uk/government/publications/revenue-and-customs-brief-9-2014-bitcoin-and-other-cryptocurrencies/revenue-and-customs-brief-9-2014-bitcoin-and-other-cryptocurrencies

How much are you thinking on cashing out? Unless it's serious money, you're unlikely to be found out. I think you could argue that profits have been speculative too, meaning it would be be similar to a gambling win.