My first port of call is generally the internet. I am a member of a number of philatelic societies and many provide some excellent information regarding post marks and cancellations.
The Great Britain Philatelic Society: http://www.gbps.org.uk/ have plenty of information including listings of numeric and duplex numbers allocated to individual post offices throughout England, Scotland, Wales and Ireland.
For collectors of Rhodesian stamps The Rhodesian Study Circle: http://www.rhodesianstudycircle.org.uk/ have an excellent section listing post offices in the area in alphabetical order. It is not necessary to be a member to access this information.
The Travelling Post Office and Seapost Society: http://www.tpo-seapost.org.uk/tpo2/home.html are an excellent starting point for train and sea travel covers and post marks.
Air Mail enthusiasts should go to The British Air Mail Society website: http://www.britishairmailsociety.co.uk/ where many of the society’s earlier journals are freely available for download.
There are clearly many more society websites which provide excellent information for the marcophilist to consult and I will be spotlight some of them in future blogs.
A good publication for Great Britain enthusiasts is the Stanley Gibbons publication Collect British Postmarks. The book covers everything from the early stages of the postal system to modern postal labels. I have built up a reference library over the years and a great source for purchasing second hand and out of print publications is HH Sales (Philatelic Publications) who can be found at http://www.hhsales.co.uk/