Which city is the United Kingdom's only island city?

Portsmouth is a port city in Hampshire, England, mainly on Portsea Island, 70 miles (110 km) south-west of London and 19 miles (31 km) south-east of Southampton. It is the United Kingdom's only island city and has a population of 205,400. The city forms part of the South Hampshire built-up area, which also covers Southampton and the towns of Havant, Waterlooville, Eastleigh, Fareham, and Gosport.

The city's history can be traced to Roman times. A significant naval port for centuries, Portsmouth has the world's oldest dry dock and was England's first line of defence during the French invasion in 1545. Special Palmerston Forts were built in 1859 in anticipation of another invasion from continental Europe. By the early-19th century, Portsmouth was the most heavily fortified city in the world, and was considered "the world's greatest naval port" at the height of the British Empire throughout Pax Britannica. The world's first mass production line was set up in the city, making it the most industrialised site in the world. During the Second World War, the city was a pivotal embarkation point for the D-Day landings and was bombed extensively in the Portsmouth Blitz, which resulted in the deaths of 930 people.