Many town names or spelling variants have been added from J. H. Daniel's 1898 work. Another source has been the 1872 "List Of Post Towns In Great Britain & Ireland".
Hammer Numbers are found appended to the 'Office' as in "London + F S". Rather than list the individual hammers, a "#" is substituted, thus; "London + F S #".
Many thanks to those who have added a Post Town name or two to this list, but especially to Ian Gibbons of the Mulready Group, who in addition to contributing hundreds of additions, has extracted many an unclear impression's true identity and verified the existence of the Post Office in question.
Another major contributor is Jon Baker, bringing in a great many Edward VII cancels.
You might have noted that the town cancels shown are of the Squared Circle Type. Three are the earliest usages known for that cancel and one is the latest. Earlier or later dates may yet be found. These are a fascinating study in themselves, one of the reasons this search progam was written.
The 'bible' for GB Squared Circles is Stanley Cohen's work,