Also, much as it might seem straightforward to have a London centred zonal
system spreading ever outwards, there will have to be a limit somewhere -
and it might as well be the Greater London boundary as anywhere.

Paul

Although it isn't the Greater London boundary in quite a few cases
(e.g. Epping and Elstree).

Chiltern Railways have a neat diagram on their trains showing both the
London and Centrocard zones.

I can see the London Zonal system and PAYG eventually spreading to the
inner suburban limits (e.g Hertford, Dartford, Slough, Shenfield) but
not further.