Thanks to Jim Lancaster for the following information. It was in response to a question that appeared on the Manchester & Lancashire FHS Forum – I have edited it slightly.

Catholic nuns (there are Anglican and other nuns of whom I know nothing) are awkward to find because they ‘disappear’.  They took a name in religion and generally were not recorded.  Priests, on the other hand, are usually recorded in the annual Catholic Directory, etc.

There are two databases listing nuns.  The first relates to nuns before
1800.  This lists English nuns that entered convents in exile – in Europe.
It is only towards the end of the 1700s that convents were established in
England.  This list is the result of an academic project at Queen Mary
College, University of London,  The project is called “Who Were The Nuns”
and there is a web-site at http://wwtn.history.qmul.ac.uk/  (NOTE the odd
address, not www, )  There is a search engine that will work on surname
alone and will provide a list of matches and clicking on each name will give
a brief outline of their life.

The second database relates to post-1800 and is probably the one you will
need.  This is being developed by the Catholic FHS and its site
(http://www.catholic-history.org.uk/cfhs/ ) has the following note –

This is an index of approximately 14,000 nuns who were in the English
Province of their Order. It is arranged alphabetically by the surname of
each nun and usually gives date of birth, names of parents, religious name,
dates of profession, date and place of death and name of Order. There is no
charge for searching in this index at the moment.
The records are being updated to incorporate all additions since 1996 and
the updated index will then be published. This is a long term project and
there is no planned date at the moment.

There is a contact link at the top of the Services page, or you can write to
Mrs M. Butler, 6 Windcroft Close, Enfield, Middlesex  EN2 7BJ.  Whilst the
note on the web-site states there is no charge, that Society’s journal
suggests a donation of £5.  The list is not complete for a variety of
reasons.  Some congregations have disappeared over time and their records
lost, others have been reluctant to participate in the past and their
cooperation is being sought.