City is "special", and City has its own police force, which is why they don't venture further out - City of London Police does not generally have any business venturing out of City unless cooperating with the Metropolitan Police on something or following a suspect.

City certainly is it's own little undemocratic (sure, you can vote, but so can nominees of corporations located in City), police-state fiefdom.

(for the non-Lononders, City of London is one of the financial districts of London, on the site of the historical site of the original settlement of London, and makes up a tiny portion of London; it's got its own police force, and a government that's entirely unlike anything else in the UK)