Far from an exhaustive list, but these are the ones I tend to often use.
Great Britain Philatelic Society – Essential! I have just had a clear out of this links page and deleted many of the links that were here. Amazingly, a huge chunk of the information that you could find on those individual sites has been transferred to the GBPS site. This has to be the number one site for information on GB philately worldwide. Well done GBPS and Maurice Buxton (their webmaster). http://www.gbps.org.uk/
The Royal Philatelic Society London – Houses the Perkins Bacon archive and other bits n pieces. http://www.rpsl.org.uk/home.asp
The Mulready Group – Essential! Have access to more experts than you can shake a stick at. Now found on "Stamp Forums" stamp-forums.com/
The Missing Imprimatur (MI) site – Essential! http://www.maltesex.com/missing_imprimatur/
Penny Black Plating – from the same nice people as the site above. http://www.maltesex.com/plating/
The Nissen reconstruction – Courtesy of Spink. Indispensable for collectors of the early plates. https://spink.com/private-treaty/nissen-reconstructions
RetroReveal – Load in a good quality tif image and say goodbye to postmarks and other camouflage. Very useful. http://retroreveal.org/
British Postal Museum http://www.postalheritage.org.uk/
The British Library – with a large and important philatelic collection. http://www.bl.uk/
UK Post Offices by County https://sites.google.com/site/ukpostofficesbycounty/
My Victoria. The remarkable GB line engraved collection assembled by Chip Gliedman, all featuring the check letters CG. He also happens to be a very nice guy. http://victoria.cgpostal.com/
AP
2015-22