The largest city in UK by area?

The largest city area in the UK is in fact Carlisle, Cumbria at 401.5 sq mi (1,039.97 sq km). City status applies to entire local government districts in the UK.

If the question is more specifically, which is the largest metropolitan- an area which has many settlements within close proximity- or urban area- a continuously built up area- of the UK, then the answer to either is Greater London.