I think that one particular way in which Greene's short story can be seen as a political allegory would reside in how there is no structure or order for the characters in the narrative. There are no literal structures that govern or house human actions. The bombing of London has reduced everything to rubble. This operates on a spiritual level, as well. The kids only know destruction, as that is their only way to form solidarity. Even Mr. Thomas finds that he lives in a world where there is no order or structure. When he finds his home destroyed, the lorry driver cannot stop laughing because "it's nothing personal." The hollowed out meeting place for the gang only serves to accentuate a world in which there is nothing to guide human actions or human impulses.
It is here in which Greene's story can operate as a political allegory. Greene brings out the political dimensions to a world shattered by war, and one in which there is nihilism evident. In this setting, the work operates as an allegory to remind the reader of the reality in which there is no structure within which humans live. It is a setting in which human impulses are not guided by the tenets of construction, but rather in which the atmosphere of desolation governs how humans interact with one another.