Do I need more than his own words?
Since WWII it is frowned upon by a great many people.
Yes, I know. Patriotism is bad. Turns people into Nazis. Never mind the patriotism of those who fought the Nazis; that's irrelevant.
I find it very sad that this seems to be the lesson Europe took from WWII. If it happens again, there may not be sufficient people who care enough to fight.
If you take his words out of the proper context... Think about what he's referencing here:
In the 1980s, English clubs were banned from playing in Europe because they had caused too many deaths.
This is what he's talking about here. He's contrasting this event in Trafalgar Square with that ugly image. To me it's pretty clear.