Major changes to Post Office operations, such as postal rates and new services, originally had to be specifically authorised by Act of Parliament, often in considerable detail. The system developed over time and from 1839 many routine changes — especially new postal rates — were made by the Postmaster-General and authorised by a Warrant signed by three of the Commissioners of His/Her Majesty's Treasury.
Originally these "Treasury Warrants" were published in the London Gazette, and John Copeland has gone through the Gazette website to find all up to the end of 1893. It is believed that few if any have been missed. The pages with the Warrant were extracted from these PDFs.
A separate page attempts a listing of Warrants by subject area, mostly countries. For convenience of reference, names used have as far as possible been updated to modern usage, even where this is something of a historical handwave — e.g. "Chili" in the original warrants is shown as "Chile", "Buenos Ayres" replaced by "Argentina", etc.
The language of the Warrants is generally very similar to that used in Acts of Parliament, so can be somewhat longwinded! In the early days especially, each Warrant typically began with half a page of legal boilerplate laying out the authority on which it was issued.
The Warrants did not get subject headings until 1888, nor titles until 1891. For earlier Warrants brief details have been given of the subject matter. (Note they sometimes contain provisions that apply to items posted within a colony, or to transit by British vessels.) Under the Rules Publication Act 1893 Gazette announcements were not required from 1st January 1894, so after this date it ceases to be a useful source for them.
Please notify John via if you spot any errors or omissions (e.g. missing or misidentified warrants, misdescriptions of the subject, or missing dates of revocation).