In the early days of Virgin Atlantic, when it was trying to establish itself as a serious competitor to BA business class, they used to offer reduced/free tickets to entertainers if they gave a performance enroute. Julien Lloyd Weber took his cello on board and pumped it out. I don't now all the others, but I heard it was quite common. I could well imagine comedians practicing their routines during lunch. May be some older Virgins can tell us the truth.