Britain being filled with Brits, what would happen is that once the show was over, half the nation got up from the couch to turn on the kettle for a cup of tea. Those electric kettles are quite demanding, especially if millions of them turn on at the same time.
So every time there was a major event, dedicated people are monitoring the grid frequency, power plants on standby and foreign contracts at the ready when needed, just to increase capacity at the right time. You can't just schedule this stuff, because if a football match runs over its allotted time, you may suddenly add power to the grid without any load, causing all kinds of problems like the grid frequency increasing and making digital clocks run ahead. There is a temporary demand of gigawatts of power that rises within five minutes and lasts until the kettles are done.
The YouTube video provides an example of 600MW of power being requested from France... for the end of an episode of Eastenders.
I always knew the British like their tea and that there's some kind of planning going on for events like street lights turning on, but the combination of the two is a great example of complex behaviour that's easy to overlook.