Traditionally, a place is granted city status by the reigning monarch. This is usually done on the recommendation of the government, which will take into account a number of factors, including the size of the population, the economic and cultural significance of the place, and its historical heritage.
In recent years, it has become more common for places to be granted city status without the need for a royal charter. This can be done by the government through an Order in Council.
There are currently 69 cities in the United Kingdom, including London, Edinburgh, Cardiff, and Belfast.