Yes they can refuse them entry here, but that' not the issue. If i was a drone operator, I can stand on any public land and fly that drone as long as I have line of sight. There are of course exceptions, near airports restricted areas etc, but other then that its perfectly legal. Universal wont get airspace restricted, for various reasons it wont happen. They cant stop you filming there staff or there land oe whats going on, as long as like I state above your on public property following all uk laws.