MIG wrote:

You seem to have implied that something was required after LONDON
until 1996, but it definitely wasn't.

Ah, I forgot the big post towns.

But I have often used "London" as the county when sending stuff to
Richmond-upon-Thames and Sutton (well actually Cheam but that's another can
of worms...) without a problem. Businesses in Ilford could easily start
giving their address as "Ilford, London" without waiting in vain for the
Royal Mail to turn IG1 into E20.