The Cambridge English Dictionary defines 'city' as 'a large town' or 'any town in the UK which has a cathedral'. This is the common meaning of the word throughout the world, and in most countries the size of a town is the deciding factor over whether it has city status. Large towns are automatically considered cities. This is perfectly logical and sensible, however this is not the case in the UK.
Despite having a parliamentary democracy, many decisions within the UK are not open to the public, but instead controlled by the monarchy or parts of the British government which answer to the monarchy, irrespective of which political party is in office. The decision to grant a town city status is one such decision. Hence in the UK city status is not granted automatically just because a town becomes very large, or because of a cathedral. A town must be granted city status by the British monarch.
According to the part of the British government known as the Department for Constitutional Affairs:
"City status is a rare mark of distinction granted by the Sovereign and conferred by Letters Patent. It is granted by personal Command of The Queen, on the advice of Her Ministers. It is for Her Majesty The Queen to decide when a competition for city status should be held. Competitions are usually held on occasions such as important Royal anniversaries."
Because of this peculiarity there is a discrepancy between the common meaning of the word 'city' and the 'official' meaning.