In the UK, using bitcoin is a taxable transaction, and acquiring bitcoin requires you to do so through regulated exchanges with full know your customer checks. So the government know you have bitcoin, they know your wallet address and they can see that you made a transaction, that you should be declaring and paying tax on.

I don't want a tax evasion charge hanging over my head and I can't be bothered declaring crypto transactions on my tax return, particularly when I'm using to buy medication that I may not be legally allowed to buy.