In article , Roland Perry
writes
It was London Transport which recalculated its route measurements to
Kilometres back in 1972 .
Ironically they chose Ongar as the 0 datum

I think they chose Ongar because it was the furthest east.

Actually Upminster is slightly further east (5561xx versus 5550xx on the
OS grid), but it's further by rail from the first common point (Mile
End).

I was recently explaining the Ongar system to someone and thought about
this. There were other possible zeros, such as Mantles Wood Junction,
High Barnet, or Morden, but Ongar has the advantage of being a terminus
with clear buffer stops on a line that had no plausible likelihood of
being extended.

--
Clive D.W. Feather