"Tim Roll-Pickering" wrote

The main reason is almost certainly down to the survival of the counties
in the mailing addresses. A postal county was never needed for CROYDON, so
people have had much less of a reason to include "Surrey" in their
addresses, whereas until 1996 "Surrey" was needed for KINGSTON.

(That said, BROMLEY and TWICKENHAM also didn't require counties - what's
the view there? And aren't there some DARTFORD addresses within the
Greater London boundaries?)

AIUI before postcodes a postal county was needed in all addresses, apart
from those of London and some other major cities, though it was the county
of the post town, not that of the actual address. So Tatsfield in Surrey had
a Tatsfield, WESTERHAM, Kent postal address. At some stage after postcodes
were introduced it became permissible to omit the county where the post town
was the 'driver' of the postcode - so BROMLEY, BR1 xxx, but CHISLEHURST,
Kent, BR7 5xx. Later still it became permissible to omit all counties from
addresses.

And aren't there some DARTFORD addresses within the Greater London
boundaries?

Yes - Crayford in the London Borough of Bexley has DARTFORD addresses. Most
if not all of the London Borough of Bexley is within the DA postcode area,
but apart from Crayford has SIDCUP, WELLING, BEXLEY, BEXLEYHEATH, or ERITH
postal addresses.

Peter