I use the word 'city' advisedly here as officially Guildford is not a city, despite having a cathedral which once upon a time was the crucial factor.
Unlike in the U.S., a place in Britain has to be quite substantial to be called a city, even in informal parlance. My town has 90,000 people, but it is never referred to as a city, just a town.
There is a definitive list of places with city status and every so often a new place is added to the list.