For example, it is not illegal to sing the entire musical of Les Miserables to yourself, rather loudly on the London Underground. In fact, it might make the London Underground a much more cheery place, were carriages to normalise singing. However, there is a social stigma against loud singing on the London underground. As a matter of principle and to enforce this principle, you will not be thrown into a jail cell, but you will receive a lot of cutting stares, disgruntled faces, and maybe someone whispering and gesturing for you to keep it down. Or for particularly awful singing, a tut.