Do I only pay tax if I sell for Fiat? (UK)

Say I bought 1 ETH a couple years ago.. Now I sell it for around the same price (maybe a couple hundred more) and I buy BTC with the money then send that to my wallet to hodl and don’t cash any out for GBP.. do I need to pay tax on anything? Or do I only pay tax if I eventually sell in the future and I’ve made a profit?