From
myLondonIf you are thinking about singing your heart out on the Tube after a good night out, you may want to think twice - unless you wouldn’t mind a penalty.
We all know the London Underground to have its array of musicians and buskers as we commute around the city, but it is illegal for passengers themselves to sing.
Under TFL▸ railway byelaws “no person on the railway shall to the annoyance of any person shall sing” unless you have been given written permission from the Operator. This unknown rule applies to all chanting, singing and performing on the train or platform.
The article goes on to describe busking on the underground, and mentions several famous names who started there.