It wouldn’t happen in Germany, and certainly not in Italy. It wouldn’t cause as much as a raised eyebrow in the US or even in Russia. Only in Britain would a political leader going to the opera stir a controversy.
The fact that the opera was at a country house in the Sussex countryside, with a black-tie dress code is part of the story, of course. That the politician in question is a Labour figure, a woman and working class probably even more so.