Do not book your trip if you just want census information.
You did not mention what year you are looking for, so I will give you some general information I have found as well as some resources.
Censuses were taken every ten years in the U.K.I have used the LDS microfilms for 1891 with success.They have at least indexes to the 1841 through 1891 census.Drop in at your local LDS family history library.There should be a binder listing the on site resources.The staff is usually very helpful in identifying available resources.If they do not have a copy on site you can fill in a request to have it ordered in from the central library in Salt Lake City.There is a copy of the 1881 UK census on CD.I have a copy of this CD set and can make a search if you request.Additionally, They have a good web page for family history research http://www.familysearch.org/http://www.familysearch.org/In addition to Census information there is also advice on tracing your family tree which should help when you get stuck.
Gordon Beavington has made a family project out of indexing the 1851 census for Gloucestershire.He has a web page at http://www.mycensuses.com/http://www.mycensuses.com/.I have a copy of his CD and could look up an item if you wish.The CD's are not expensive and Gordon also makes some information available on-line through his web page.
For a primer on Gloucestershire and Bristol check out the GENUKI (GENealogy in the UK and Ireland) web site http://www.genuki.org.uk/http://www.genuki.org.uk/.Follow the links through England, Region: Gloucestershire (which has relevant information), then to Bristol.
A great general resource is Cyndi Howell's terrific list http://www.cyndislist.com/http://www.cyndislist.com/Take a look at the Census page, and the "Hit a Brick Wall" page.