John Rowland wrote:

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.

And the Potters Bar part of Middlesex was given to Hertfordshire (which
meant that the boundary between Barnet and Potters Bar went from having
Hertfordshire on the south side only to having Hertfordshire on the north
side only).

However this one did have the addess updated, one of the few of the Greater
London changes that was followed.