Is it? The flag is black, so could be a variety of things, not necessarily even a national flag. Just a flag in a march. (Anarchism uses a black flag.)
To me, the blank (not black) nature of the flag is the point: it's about being blinded by any ideology, even one that the artist or beholder might agree with.
The guy is walking off a cliff and he is blinded by the flag. I assume it is a commentary on Brexit. It is just short of a decade since that vote. Nationalism blinded people and they did something stupid. Not dissimilar to what is going on in the US too.
And how is blindly following a flag differ between a national flag and an ideology flag?