Formed by a merger of the Public Record Office and the Royal Commission on Historical Manuscripts, The National Archives (TNA) is the principal repository for English and Welsh records. You might like to start by browsing the titles of their 193 guides classified under 'family history'. TNA offer many, many useful Research Guides.
The National Archives (TNA)'s Discovery system is now the key to finding their holdings - see also: TNA Discovery Help.
TNA's Access to Archives (A2A) programme has been absorbed into Discovery (above).
The National Register of Archives has now been absorbed into Discovery (above). It is/was an extensive searchable database (formerly run by the Royal Commision on Historical Manuscripts, now part of TNA) which contains information on the nature and location of manuscripts and historical records that relate to British history.
Your Archives — NB this is now closed but remains available as a read-only resource — "The National Archives' online community of records users", a wiki that archive users can contribute to: "These pages are for you to contribute your knowledge of archival sources held by The National Archives and by other archives throughout the UK."
Mark Howells has provided a listing of English Record Offices and Archives on the Web.
The L.D.S. (Church of Jesus Christ of Latter-Day Saints) have now provided online search facilities for the I.G.I. (International Genealogical Index). As well as the general search facility, there is also a Custom Search page allowing more refined searches. Hugh Wallis has provided a search facility which greatly simplifies searching by batch number. See also Finding L.D.S. Batch Numbers.
The FamilySearch site has a list of Family History Centres in England. These are provided by the Church of Jesus Christ of Latter-Day Saints.
In particular, the London Family History Centre has a useful web site.
The Guildhall Library, London.