Andrew Heenan wrote:

Since the day that Tony Benn introduced postcodes, adding the 'county' has
been a waste of ink. It's utterly redundant to the sorting process and the
postman on his round.

It was actually in 1996 the requirement was dropped, long after Tony Benn.
Indeed some of the 1972 Local Government Act changes were incorporated into
the postal addresses - "Avon" was a particularly unpopular one.

And I fail to see how it helps anybody else, especially in and around
London, when several Essex, Kent and Surrey addresses (not to mention
100% of Middlesex), are actually part of Greater London.

Not all of Middlesex - Spelthorne was put into Surrey and the postal
counties never updated to that. But I agree it only adds to confusion.