One of the problems with tracking down cemeteries is that there tends not to be a standard name for many of them. Over the course of a century, the same cemetery can go by half a dozen different names. Less often, the same name can be used for more than one cemetery. And with this one, both those things are happening.
The cemetery owned by the City of Westminster, situated on the south side of Uxbridge Road, has been variously known as City of Westminster Cemetery, Mill Hill Cemetery (not to be confused with the Mill Hill Cemetery in Mill Hill) and Hanwell Cemetery (not to be confused with the other Hanwell Cemetery on the other side of Uxbridge Road, owned by the Royal Borough of Kensington and Chelsea). Westminster are currently calling it Hanwell, so I am too.
As a large, old (opened in 1854) cemetery in a prosperous part of town, you'd expect some spectacular monuments, but sadly, Hanwell lacks many. There are angels in various poses and a few granite mausoleums, but mostly the cemetery is filled with arch-shaped stones and crosses. Worth a look for Jerrard's spectacular chapel, though, which is built on an entirely different scale to most cemetery chapels.