In this essay, the speaker says that he is going to Battersea. He is in Battersea as he says this. He adds that in order to actually see Battersea, he must go to other lands. In other words, he has been in Battersea for so long that he has grown accustomed to everything about it: good and bad. In order to experience his English home with fresh perspective, he needs to leave and come back to it.
When he returns, he notes that he had been correct. He sees England again, but with fresh eyes. It is beautifully new and beautifully old to him. In this optimistic spirit, he says to an American woman that he is making a list of things that are better in England. But then, he changes his tune when he says that with this fresh perspective, he's also able to see the flaws of England.
He criticizes politicians for making everything sound fair and comfortable. But, he adds, this is "humbug." Humbug is deceiving or false talk. Chesterton uses the ivy as a metaphor for this "humbug" or false praise which glosses over actual problems.
His first point is that in order to see/understand the land and culture of a place, it helps to physically or mentally leave that place in order to look at it objectively and with a fresh perspective. There are certainly pleasing aspects of that culture but, at least politically speaking, some of this praise is actually humbug. The danger is that people allow themselves to be blinded by the comfort of this double speak; Chesterton notes earlier that he is in a "cloud of sleep" and therefore, unable to really see Battersea. When he gets out and returns, he is better able to see the good and the bad.
The effective literary device is the metaphor of the ivy. Even with an attempt at a refreshed, objective perspective, there is the danger that one will simply see the good aspects (the ivy) of the culture. But, of course, the ivy is simply a comfortable, nice-looking covering, and it just might be humbug.