In message , at 17:42:01 on Sat, 14
Sep 2019, John Williamson remarked:
On 14/09/2019 16:38, Roland Perry wrote:
That's precisely what we are discussing, but in the absence of any
evidence of who/what made it official (and when).

This is the best explanation I've seen.:-

"The custom of considering the location of the old Charing Cross to be
the arbitrary centre of London seems to have arisen in the late 18th or
early 19th century. Laws and rules were often written from that period
specifying that everything within a certain distance of Charing Cross
was to be considered part of London. In 1864 the new Charing Cross
railway station opened on the Strand, just adjacent to the new
Trafalgar Square, and the South Eastern Railway commissioned a new
Cross to stand in the station forecourt - a few hundred yards from the
site of the medieval original. London’s black-cab taxi drivers treat
this new Cross as the centre of the city: their famously rigorous
“Knowledge” training requires them to commit to memory every street
and point of interest within six miles of the station forecourt."

The original Charing Cross was on the site of the current statue of
Charles I, but the cross in the station forecourt dates from the 1860s,
so the cabbie's idea of the central point is from 1864 at the earliest,
when the station opened to traffic. The official centre is apparently
the plaque marking the site of the original Charing Cross, not the
statue.I was mistaken earlier.

I reckon treating Charles as the centre just arose out of "custom and
practice" as the Government grew and moved into Whitehall,

Moved from where?

and distances from the centre of Government needed to be specified for
various reasons. One example of this in the 1970s was when I worked for
BR in Watford, and the "London allowance" which would have increased my
salary by about 10% was only available as far out as the South side of
the road our office block occupied the North side of.

The London Stone, originally sited in the middle of what is now Cannon
Street, has also been considered to be the "centre of London", and was
traditionally a place to seal a binding bargain up until at least the
middle ages.

Most of what you say has already been explored earlier in the thread.
The "London Allowance" however is a surprise, and I'd have expected it
to be based on boroughs, some of whose boundaries are indeed down the
middle of streets. Chorleywood, not far from Watford, being one example.
--
Roland Perry