There are actually a number of different ways of answering this question. Let us consider what it is that the driver says, which is also how the author chose to end this disturbing story of post-war alientation. As the driver begins to laugh, he justifies his laughter by saying:
I'm sorry. I can't help it, Mr. Thomas. There's nothing personal, but you got to admit it's funny.
One way of approaching this story and looking at its ending is by focusing on the driver's lack of ability to empathise with Mr. Thomas, who is sobbing and quite clearly distraught and offended by the driver's laughing. Does this show the way in which not only the boys have been "deadened" by the blitz and the war?
Secondly, you can link this to the way in which the driver likens the house when it was there to a "man in a top hat," which is clearly a symbol of the ruling class. This would indicate that the last line and the disrespect it conveys shows the way in which the ruling order of the British class system has been profoundly altered by the war, and the upper class no longer occupy a position of such respect and power as they once did.