...to avoid the playing of God Save the Queen.
In the mid '70s, the RAF Valley Astra was open to all comers, including the locals. I was there once and the usual short clip of HM was shown to accompany the National Anthem. A group of locals didn't stand up, whereupon the projector stopped, the lights came on and 'Jones the Movie' came out of his booth and yelled something in Welsh at them. Then he went back, the projector started up again - this time everyone stood up.
One of our number was Welsh; we asked him what Jones had said, "OI! You lot, stand up or f*** off!", he told us. They chose the first option!