There is no central authority in the UK to define a single set of counties and their boundaries. Our advice is to leave county names out of UK addresses completely. The county name is not needed as part of an address in the UK, and Royal Mail sorting machines ignore it. However, we do provide a choice of "traditional", "former postal" and "ceremonial" county names for customers who require this. Traditional and former postal counties are available in Postcode Lookup. Address Auto-Complete also provides ceremonial counties.
There are various different sets of county names in circulation, including:
Postal (now known as "former postal counties")
For sorting and delivering mail, England was divided into 48 postal counties. They were dropped by Royal Mail in 1996 and In 2010 the regulator advised they cease supply of county data, so they were formally abandoned.
— Abandoned by Royal Mail
— Major towns don't have a postal county
Traditional (also known as the "historic", "former" or "ancient" county names)
These continue to form the basis of modern local government areas in many parts of the country away from the main urban areas, although the newly created areas sometimes have considerably altered boundaries from the historic counties on which they are based
— Have not officially existed since 1889
— Confusing for areas inside London
Metropolitan and Non-Metropolitan
There are currently a total of 82 metropolitan and non-metropolitan counties, which exist for the purposes of local government (outside Greater London).
— Mainly for government and administrative purposes.
Ceremonial (also informally known as "geographic counties")
Historically "lieutenancy areas", these now mostly match local authority areas as they were in 1974. They are the best match with the name most people identify as their county name.
— Still not a perfect match with how people identify their county.
Please see Wikipedia for further details.