The National Archives - used to be the Public Record Office

Consider checking the Government site The National Archives regularly: it was the famous PRO Pubic Record Office

2012/13 Very hard to keep track of Archives pages - they have a new site again so some of my links will not work
It is all now at Discover our online collections | The National Archives

another treasure of information is at Archives Hub and also A2A http://www.a2a.org.uk/ which is a catalogue of enormous archives

before we look at their databases Kew researching yourself or Employing Researchers etc

- new databases continually appearing - keep an eye on them - service records, medals etc - these documents are usually at a cost of GBP 3.50 - some of the following have this data included in a subscription - you will need to check what they have Commercial sites of Major Interest

Use "Search the Catalogue" for keyword searches for regiments etc. and names
And the "go to reference" search on top left can be used to check census locations. LDS 1881 which often just gives a street in London without suburb - for example use reference RG11/471 (drop any leading zeros) and result is
"Registration Sub-District 1A Mile End Old Town, Western Civil Parish, Township" (often abbreviated as MEOT versus MEOT for Mile End New Town)
Another search possibility is to check Missing records from Census - very often people search and search and in fact the area they want has been lost. Enter the census reference in Series Code box and the word "missing" in the phrase box.

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The above sites can be used to search for the following but I have a separate page for other places that might be useful

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1901 Census of England and Wales Online - - this was first site with 1901 others have it now visitEnglish Census help for links

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Various Acts of Parliament of Interest to the Genealogist http://freepages.genealogy.rootsweb.com/%7Eframland/acts/actind.htm