On 22/03/2020 09:27, Roland Perry wrote:
Edits pending

My opinion, based on quite a lot of research, being: having earlier
specified mileages to various gateways like Marble Arch, the AA decided
for its maps to consolidate on Trafalgar Square. This has become a meme
about "Charing Cross" (the original one only having a plaque) and an
unspoken implication that it has always been the case (long before the
AA)

The Post Office one can be as well. The mystery about the Charing
Cross/AA one is that as far as I can tell no-one has ever been able to
expand on what it says on that fairly recent and vague plaque.

I think I'll do a FOI on Westminster City Council, and see what they
have to say.

I don't understand what all the "mystery" is about.

The BBC website has a nice item on this which gives a perfectly
reasonable explanation of why a very distinctive landmark was at placed
this location and subsequently made a convenient choice as the "centre"
of London.
http://www.bbc.co.uk/london/content/..._feature.shtml

What exactly are you trying to discover?