Complex sentence: Located mainly on Portsea Island, it is the United Kingdom's only island city and with a population of 205,400, it is the only city in the British Isles with a greater population density than London.
Split into simpler sentences: Located mainly on Portsea Island, it is the United Kingdom's only island city. With a population of 205,400, it is the only city in the British Isles with a greater population density than London.