The National Archives catalogue of records held in repositories worldwide: http://discovery.nationalarchives.gov.uk/
List of records repositories in the United Kingdom: http://discovery.nationalarchives.gov.uk/find-an-archive
The British Records Association: http://www.britishrecordsassociation.org.uk/
The Business Archives Council: http://www.businessarchivescouncil.org.uk/
The Archives and Records Association: http://www.archives.org.uk/