The reason is that the question itself is a well-known urban myth. The Sovereign DOES NOT have to ask permission to enter the City of London. This arises from a misunderstanding of an occasional formal ceremony.

There is a popular historical myth that the King must ask permission to enter the "City of London" (the Square Mile), but in reality, the King has the absolute right to enter at any time. The confusion stems from the Temple Bar Ceremony, an ancient tradition that occurs when the Monarch visits the City on formal occasions. In this ceremony, the King's procession stops at the City boundary (marked by the Temple Bar memorial), where the Lord Mayor of London presents the "Pearl Sword" to the Monarch as a sign of loyalty. The King then touches the hilt and returns it, symbolically acknowledging the City’s ancient privileges while confirming his own ultimate authority. While the ceremony looks like the King is being "stopped," it is actually a ritual of greeting and subservience by the Lord Mayor. The King is the sovereign over all of London, including the City, and requires no legal permission to travel through any part of his realm. This colorful tradition simply serves to highlight the unique, centuries-old relationship between the Crown and the world's oldest financial district.