Offensive? Why, and to whom? I'm not a particular fan of it, but what's wrong with a bit of nationalistic jingoism and flag waving? It's primarily an "English" thing. I'm not English, but so what? I'm a Brit but still have national pride, many countries display their pride overtly, so what's wrong with people gathering in a spirit of national identity, singing a few songs and waving a few flags?
Absolutely nothing wrong whatsoever.