Tax has to be voluntary because they know that if they wrote it into the legal codes, it would be in violation of Section 1 of the UK Modern Slavery Act 2015, which defines and criminalizes slavery, servitude, and forced or compulsory labour.
βThere is no code or law instructing any working citizen in the private sector to pay tax on the compensation for his labourβ @Erik Cason @Jeff Booth@SatmoJoe this is something that bitcoiners need to be aware of.