London is the capital and the largest city of the United Kingdom with the population of 7.8 million. I'd also like to point out that the official name of the UK is the United Kingdom of Great Britain and Northern Ireland. I have never heard someone using that name unless it's for some kind of official document, though.
However, London is located on the River Thames and it has been a major settlement for two millennia, its history going back to its founding by the Romans, who called it Londinium. London contains four World Heritage Sites: the Tower of London; Kew Garden; the site comprising the Palace of Westminster, Westminsted Abbey, and St. Margaret's Church; and the historic settlement of Greenwich. Other famous sites are: the London Eye, Tower Bridge, Trafalgar Square, Hyde Park and it's also known for red double-deckers.
London is somewhere near the top of my list of the cities that I have to visit. This painted postcard shows the Houses of Parliament. I really like it. Thanks, Reyiel!