This explanation was put forth by an old schoolmate of mine: (I shall paraphrase)
Pussy is late getting home, when questioned invents a tale about going off to London to see her highness.
OK, What didja see? Another invention is put forth. To justify his absence he thinks of the only way he could
be of use to the queen and so the mouse chasing.
OK Gents! Do both these interpretations fit the mold??