The River Thames flows through southern England. It is the longest river entirely in England and the second longest in the United Kingdom, best known because its lower reaches flow through central London, making it an ideal avenue for trade and transportation. As such, it's filled with ships, boats, barges and punts, so fish are scarce. It's also home to many corpses, as well as the rosemary tin Nathaniel threatened to bind Bartimaeus into with a spell of indefinite confinement to discourage any betrayal.
Annually, the British Fleet sails through the Thames, though as of late it has been lacking in vessels. Spirit activity is low, as water isn't particularly conducive to the essence of djinn etc, especially when as polluted as in the Thames. In summertime, there's a "great summer Stink", when the extent of pollution in the water becomes more apparent and watermen sell clothes pegs for noses at low tide.