Thank you for that information which made me look at its official history:
"History did not cease with the dissolution of the medieval monastery on 16 January 1540. The same year Henry VIII erected Westminster into a cathedral church with a bishop (Thomas Thirlby), a dean and twelve...
Within London, the "City of London' has a cathedral whereas Southwark has a cathedral and is not a city while Westminster is a city without a cathedral unless you count the Roman Catholic one.
I always thought it had four cathedrals, the two you mentioned plus Anfield and Goodison Park.
Seriously though don't forget that some towns etc. have Greek and Russian Orthodox Cathedrals.