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UK Post Offices by County (Ken Smith)Excellent and comprehensive set of PDF files listing post offices and what is known of their dates of opening, closing etc.
LetterStamper (Andrea Rossignoli)Detailed articles on GB postal history, including the full text of many postal Acts.
Maltese Crosses of GB and IrelandCompanion presentation to Rockoff and Jackson's Encyclopaedia of the Maltese Cross Cancellations of Great Britain and Ireland.
GB Victorian Postmarks (Russell Taylor)Detailed listings of Scottish experimental duplexes and roller cancellations.
Universal Machine Postmarks of Great Britain & Northern Ireland (Paul Carter)History and detailed checklist of these machines.
Krag Machine Postmarks of Great Britain & Northern Ireland (Paul Carter)History and detailed checklist of these machines.
Cardiff Machine Cancellations 1905-52 (Roger Gilbert)Listing in album page format.
Postal History of the "Kingdom of Carrick" (Ayrshire) (Tom Fowler)Presentation of parts of a collection.
Gloucestershire Postmarks (Colin Fountain & John Hine)Catalogue of postmarks of the county.
Bristol Postal History Catalogue (Bristol PH Group)Catalogue of postmarks of the city.
Offices under Bristol Catalogue (Bristol PH Group)Catalogue of postmarks of subsidiary offices of the city.
Somerset Postal History Catalogue (Nick Bridgwater)Catalogue of postmarks of the county.
Somerset (& Bristol) Postal History Blog (Nick Bridgwater)Blog dealing with the postal history of the county.
King George VI Postal HistoryBlog dealing with the 1937-53 period.
A is for AIRGRAPH (US National Postal Museum)Short history, part of the "Alphabetilately" exhibit.
Pacific Clipper Postal HistoryGB section of a site devoted to worldwide Clipper mail from WW2.
Early Precancels of Great BritainAs used on newspapers (mostly).
UK Meter FrankingComprehensive treatment of UK meter marks.
International Postage Meter Stamp Catalog, GBAnother useful treatment of UK meter marks.