Michael R N Dolbear wrote:

Ilford & Romford are nominally Essex for Post Code purposes and most
people assume they are in Essex but in fact they are in London Boroughs

Huh ?

The postcode system cares nothing for counties and nothing in outer
Greater London has a London postcode.

Depends what you mean by "outer Greater London". I live in Forest Gate in
Newham and we're E7 and part of outer London on the political definition
(strictly the boroughs that weren't in the Inner London Education
Authority's remit; this mostly matched the old London County Council Area
but part of Newham, namely North Woolwich, was a notable exception). *
Walthamstow is E17. The E post codes go right up to the Greater London
border and even beyond it in one direction, but stops rather short of it in
another.

Postal counties were abolished by the Royal Mail in the mid 1990s when
technology changed how addresses were read (although a series patchwork of
local government reforms at the time where some areas lost just the county
council but others lost the lord lieutenancy as well can't have been greeted
with pleasure). One can put any county they like on the mail although
"London" also being a post town does complicate things (and the
encouragement to write a post town all in UPPER CASE and the rest as normal
hasn't caught on well), and I've sold things on eBay to places in Greater
London but outside the LONDON post town with all manner of entries for
county and even none.

(* However the Office of National Statistics puts Newham, and also
Harringey, in Inner London and Greenwich in outer London.)