The data was initially released to me by Post Office Ltd. in January 2018 as a result of a request under the Re-Use of Public Sector Information Regulations. (See the WhatDoTheyKnow thread.) The initial dataset contained several duplicate rows and also erroneous Sunday opening hours for a number of branches. These issues were fixed in an amended release made in Februrary 2018. An updated dataset was obtained by a similar FOIA/RoPSIR request in March 2019, and again (by someone else who kindly passed it on to me) in October 2019.
The file above contains details of all Post Office Branches in the UK (with the exception of some non-public sites). The details included are an ID number, the branch name, address, postcode, status (open or temporarily closed), type (directly managed branch or other), and geographic coordinates. (The ID number is the primary key in the Post Office database, so should should remain consistent over any subsequent data releases.)
The information has been licensed for re-use by Post Office Ltd. under the terms of the Open Government Licence 3.0, with the default attribution statement "Contains public sector information licensed under the Open Government Licence v3.0", and an acknowledgement that "the information may not be entirely up-to-date and that the latest network information can be found on https://www.postoffice.co.uk/branch-finder". I am told that addresses have been collected independently of AddressBase or similar products, and that the geographic coordinates are not encumbered by any third-party IP rights. So both can be used safely under the OGL.
The request was made primarily to help improve the mapping of Post Offices in OpenStreetMap. More details of the efforts to use the information to this end can be found at https://osm.mathmos.net/postoffice/. There are also some mapping notes with for those who want to help.