Do you only pay tax on crypto when you cash out UK?

I'm a bit confused about the tax rules surrounding cryptocurrencies in the UK. Do I only have to pay tax on my crypto assets when I cash them out or convert them into fiat currency? Or am I taxed on the value of my crypto holdings every year, regardless of whether I sell them? Also, how does the UK government determine the taxable value of crypto assets, given that their value can fluctuate wildly? And what about trading crypto for other crypto assets - does that trigger a tax liability? I'd appreciate it if you could clear up these points for me.