The problem is stating how these match the big three criteria. Only “Explore” has a chance.
Just doing a small amount of research took me to GPO Marker Posts - About the Posts
It seems that, in the same way as UK Postboxes - https://community.wayfarer.nianticlabs.com/t/uk-postboxes-topic these also have the reigning monarchs cypher.
“From before the end of George’s reign the original design of cast iron marker posts was replaced with concrete pillars”
The majority of the “concrete” posts are from ERII and using the same criteria as postboxes would not be distinct.
If you found a “cast iron” or the rare “George V” concrete examples you probably have a chance.
Good luck.