Roy Badami wrote:

1996 is a logical date for the change. It's when the estuarial counties
created in 1974 (Avon, Humberside& Cleveland) were abolished, as far as
local government was concerned anyway.

I think that's somewhat coincidental. The Royal Mail has never really
cared if their postal counties are out of date with respect to local
government areas.

Not sure about "never" - they did implement most of the English changes in
1974 (apart from Greater Manchester, whilst they partitioned Humberside and
Hereford & Worcester; in the 1960s they ignored the changes in & around
Greater London bar moving Potters Bar) and gave a one year overlap period
before making the new couties mandatory.

I think the change is more to do with modernisation of the post office's
sorting systems - with more reliance on the postcode than other elements
of the address.

Yes - the increased use of optical recognition systems altered what was
considered necessary. I think the timing was just a coincidence, although
the Royal Mail couldn't have been enamoured with the subtle differences (for
the layman) between the 1980s reforms when the lieuteancy areas were
retained in the likes of Merseyside and Tyne & Wear and the 1990s ones when
even the lieutenancies were abolished for the likes of Avon and Humberside.
The precise arrangements for how services were divided up was even more
complicated.