Cosmopolitanism is bad because you are rooted in nothing while thinking you are rooted in something that does not exist. Patriotism is not merely proto-fascism because it does not lead to fascism, and it has been a bullwark against it.
Simple patriotism isn't what I read Chesterton to be referring to here. Patriotism leaves space for cosmopolitanism, but this passage sounds an awful lot like the "blood and soil" of fascism. I don't know anything about the Chesterton, just going on what you've quoted which seems to say love England or leave it.
