In message , Roland
Perry writes

The first people who needed to know distances from London were the Post
Office, because they charged by distance (before the penny post).

When the Postmaster General commissioned John Cary to survey the roads
between post towns in England and Wales in 1790, Cary adopted the system
already in use on milestones (and used by earlier map-makers) by
measuring from various locations on the then outskirts of London.

For instance, distances along the road to Richmond started from Hyde
Park Corner, those along the Great Essex Road started at Whitechapel
Church, those on the Great North Road at Hick's Hall in Clerkenwell and
so forth. There are still a couple of milestones on the Upper Richmond
Road recording the distance to Hyde Park Corner.

There may have come a time when road distances were measured from
Charing Cross instead, but I have several maps from the early days of
motoring which confirm that (in effect) St Pauls was the original place.

I've also seen Charing Cross mentioned (although not by Cary). Many 18th
and early-19th century maps of London include concentric circles to
indicate distances from St Paul's, but I suspect this is more to do with
the fact that the cathedral was the highest building in London at that
time, and thus the most obvious landmark to use.

I suspect that St Paul's had a revival as the "centre" of London in the
early days of motoring, since it was as close as anywhere to being at
the starts of several main trunk roads under the 1920 classification
scheme.

--
Paul Terry