Archives, museums, and institutions

The British Postal Museum and Archive (the public identity of the Postal Heritage Trust)
http://www.postalheritage.org.uk
Includes a searchable index to the archival collections

The British Library Philatelic Collections
http://www.bl.uk/reshelp/findhelprestype/philatelic/philateliccollections/philatelycollectionshub.html

Bath Postal Museum
http://www.bathpostalmuseum.org/

The American Philatelic Society Research Library
http://stamps.org/About-the-Library

The Post Office
http://www.postoffice.co.uk/