In the UK, it is only a taxable event when the coin is exchanged to a "readily convertible asset", i.e. Bitcoin, Mainnet ETH.
... for now
We've seen lots of the crypto exchanges fail to make exchanges for hours to days when things get hot
Official stock exchanges have a standard practice of halting trading in stocks when unusual events happen. Sometimes this cools the market and things get back to normal, sometimes the thing has gone to zero when
If Tether crashes to $0.01, I'd be a bit surprised if it didn't stop being convertible for a significant time.